topic paper

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C H A P T E R

1 Concepts of Radiologic Science OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Describe the characteristics of matter and energy 2. Identify the various forms of energy 3. Defi ne electromagnetic radiation and specifi cally ionizing radiation 4. State the relative intensity of ionizing radiation from various sources 5. Relate the accidental discovery of x-rays by Roentgen 6. Discuss examples of human injury caused by radiation 7. List the concepts of basic radiation protection

OUTLINE Nature of Our Surroundings Matter and Energy Sources of Ionizing Radiation Discovery of X-rays Development of Modern Radiology Reports of Radiation Injury Basic Radiation Protection The Diagnostic Imaging Team

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C H A P T E R 1 Concepts of Radiologic Science 3

NATURE OF OUR SURROUNDINGS In a physical analysis, all things can be classifi ed as mat- ter or energy. Matter is anything that occupies space and has mass. It is the material substance of which physical objects are composed. All matter is composed of fundamental building blocks called atoms, which are arranged in various complex ways. These atomic arrangements are considered at great length in Chapter 3.

A primary, distinguishing characteristic of matter is mass, the quantity of matter contained in any physical

object. We generally use the term weight when describing the mass of an object, and for our purposes, we may con- sider mass and weight to be the same. Remember, how- ever, that in the strictest sense they are not the same. Mass is actually described by its energy equivalence, whereas weight is the force exerted on a body under the infl uence of gravity.

Mass is measured in kilograms (kg). For example, on Earth, a 200-lb (91-kg) man weighs more than a 120-lb (55-kg) woman. This occurs because of the mutual at- traction, called gravity, between the Earth's mass and the mass of the man or woman. On the moon, the man and the woman would weigh only about one-sixth what they weigh on Earth because the mass of the moon is much less than that of the Earth. However, the mass of the man and the woman remains unchanged at 91 kg and 55 kg, respectively.

MATTER AND ENERGY Matter is anything that occupies space. It is the material substance with mass of which physical objects are com- posed. The fundamental, complex building blocks of matter are atoms and molecules. The kilogram, the sci- entifi c unit of mass, is unrelated to gravitational effects. The prefi x kilo stands for 1000; a kilogram (kg) is equal to 1000 grams (g).

Mass is the quantity of matter as described by its energy equivalence.

HIS CHAPTER explores the basic concepts of the science and technology of x-ray imaging. These include the study of matter, energy, the

electromagnetic spectrum, and ionizing radiation. The production and use of ionizing radiation as a diagnostic tool serve as the basis for radiography. Radiologic technologists who deal specifi cally with x-ray imaging are radiographers. Radiographers have a great responsibility in performing x-ray ex- aminations in accordance with established radiation protection standards for the safety of patients and medical personnel. Radiography is a career choice with great, yet diverse, opportunities. Welcome to the fi eld of medical imaging!

T

A Penguin Tale by Benjamin Archer

In the vast and beautiful expanse of the Antarctic region, there was once a great, isolated iceberg fl oating in the serene sea. Because of its location and accessibility, the great iceberg became a Mecca for penguins from the entire area. As more and more penguins fl ocked to their new home and began to cover the slopes of the ice fi eld, the iceberg began to sink further and further into the sea. Penguins kept climbing on, forcing others off the iceberg and back into the ocean. Soon, the iceberg became nearly submerged owing to the sheer number of penguins that attempted to take up residence there.

Moral: The PENGUIN represents an important fact or bit of information that we must learn to understand a subject. The brain, similar to the iceberg, can retain only so much information before it becomes overload- ed. When this happens, concepts begin to become dis- lodged, like penguins from the sinking iceberg. So, the key to learning is to reserve space for true “penguins” to fi ll the valuable and limited confi nes of our brains. Thus, key points in this book are highlighted and referred to as “PENGUINS.”

PART I Radiologic Physics4

Although mass, the quantity of matter, remains un- changed regardless of its state, it can be transformed from one size, shape, and form to another. Consider a 1-kg block of ice, in which shape changes as the block of ice melts into a puddle of water. If the puddle is allowed to dry, the water apparently disappears entirely. We know, however, that the ice is transformed from a solid state to a liquid state, and that liquid water becomes water vapor suspended in air. If we could gather all the molecules that make up the ice, the water, and the water vapor and measure their masses, we would fi nd that each form has the same mass.

Similar to matter, energy can exist in several forms. In the International System (SI), energy is measured in joules (J). In radiology, the unit electron volt (eV) is often used.

Potential energy is the ability to do work by virtue of position. A guillotine blade held aloft by a rope and pulley is an example of an object that possesses potential energy ( Figure 1-1 ). If the rope is cut, the blade will descend and do its ghastly task. Work was required to get the blade to its high position, and because of this position, the blade is said to possess potential energy. Other examples of objects that possess potential energy

include a roller coaster on top of the incline and the stretched spring of an open screen door.

Kinetic energy is the energy of motion. It is possessed by all matter in motion: a moving automobile, a turning windmill wheel, a falling guillotine blade. These systems can all do work because of their motion.

Chemical energy is the energy released by a chemical reaction. An important example of this type of energy is that which is provided to our bodies through chemi- cal reactions involving the foods we eat. At the molecu- lar level, this area of science is called biochemistry. The energy released when dynamite explodes is a more dra- matic example of chemical energy.

Electrical energy represents the work that can be done when an electron moves through an electric po- tential difference (voltage). The most familiar form of electrical energy is normal household electricity, which involves the movement of electrons through a copper wire by an electric potential difference of 110 volts (V). All electric apparatus, such as motors, heaters, and blowers, function through the use of electrical energy.

Thermal energy (heat) is the energy of motion at the molecular level. It is the kinetic energy of molecules and is closely related to temperature. The faster the molecules of a substance are vibrating, the more thermal energy the substance has and the higher is its temperature.

Nuclear energy is the energy that is contained within the nucleus of an atom. We control the release and use of this type of energy in electric nuclear power plants. An example of the uncontrolled release of nuclear en- ergy is the atomic bomb.

Electromagnetic energy is perhaps the least familiar form of energy. It is the most important for our pur- poses, however, because it is the type of energy that is used in an x-ray. In addition to x-rays, electromagnetic energy includes radio waves, microwaves, and ultravio- let, infrared, and visible light.

Just as matter can be transformed from one size, shape, and form to another, so energy can be transformed from one type to another. In radiology, for example, electrical energy in the x-ray imaging system is used to produce electromagnetic energy (the x-ray), which then is con- verted to chemical energy in the radiographic fi lm.

Reconsider now the statement that all things can be classifi ed as matter or energy. Look around you and think of absolutely anything, and you should be convinced of this statement. You should be able to classify anything as matter, energy, or both. Frequently, matter and energy ex- ist side by side — a moving automobile has mass and kinetic energy; boiling water has mass and thermal energy; the Leaning Tower of Pisa has mass and potential energy.

Perhaps the strangest property associated with matter and energy is that they are interchangeable, a charac- teristic fi rst described by Albert Einstein in his famous theory of relativity. Einstein's mass-energy equivalence equation is a cornerstone of that theory.

FIGURE 1-1 The blade of a guillotine offers a dramatic ex- ample of both potential and kinetic energy. When the blade is pulled to its maximum height and is locked into place, it has potential energy. When the blade is allowed to fall, the poten- tial energy is released as kinetic energy.

Energy is the ability to do work.

C H A P T E R 1 Concepts of Radiologic Science 5

This mass-energy equivalence serves as the basis for the atomic bomb, nuclear power plants, and certain nuclear medicine imaging techniques.

Energy emitted and transferred through space is called radiation. When a piano string vibrates, it is said to radiate sound; the sound is a form of radia- tion. Ripples or waves radiate from the point where a pebble is dropped into a still pond. Visible light, a form of electromagnetic energy, is radiated by the sun and often is called electromagnetic radiation. In fact, electromagnetic energy that travels through space is usually referred to as electromagnetic radia- tion or, simply, radiation.

Matter that intercepts radiation and absorbs part or all of it is said to be exposed or irradiated. Spending a day at the beach exposes you to ultraviolet light. Ultraviolet light is the type of radiation that causes sunburn. During a radiographic examination, the patient is exposed to x-rays. The patient is said to be irradiated.

Ionizing radiation is a special type of radiation that includes x-rays. Ionizing radiation is any type of radi- ation that is capable of removing an orbital electron

from the atom with which it interacts ( Figure 1-2 ). This type of interaction between radiation and matter is called ionization. Ionization occurs when an x-ray passes close to an orbital electron of an atom and trans- fers suffi cient energy to the electron to remove it from the atom. The ionizing radiation may interact with and ionize additional atoms. The orbital electron and the atom from which it was separated are called an ion pair. The electron is a negative ion and the remaining atom is a positive ion.

Thus any type of energy that is capable of ionizing mat- ter is known as ionizing radiation. X-rays, gamma rays, and ultraviolet light are the only forms of electromagnetic radiation with suffi cient energy to ionize. Some fast- moving particles (particles with high kinetic energy) are also capable of ionization. Examples of particle-type ionizing radiation are alpha and beta particles (see Chapter 3). Although alpha and beta particles are some- times called rays, this designation is incorrect.

SOURCES OF IONIZING RADIATION Many types of radiation are harmless, but ionizing radiation can injure humans. We are exposed to many sources of ionizing radiation ( Figure 1-3 ). These sources can be divided into two main categories: natural envi- ronmental radiation and man-made radiation.

Natural environmental radiation results in an annual dose of approximately 300 millirem (mrem) (3 millisievert [mSv]). Man-made radiation results in approximately

Negative ion (free electron)

Positive ion (remaining atom)

Target atom

e–

Ionizing radiation

Ion pair

FIGURE 1-2 Ionization is the removal of an electron from an atom. The ejected electron and the resultant positively charged atom together are called an ion pair.

Radon 198 mrem (1.98 mSv)

Cosmic 29 mrem

(0.29 mSv)

Internal 39 mrem

(0.39 mSv) Medical x-rays 39 mrem

(0.39 mSv) Nuclear medicine

14 mrem (0.14 mSv) Consumer products 10 mrem (0.1 mSv)

Occupational Nuclear power

Industrial 2 mrem

(0.020 mSv)

Natural 295 mrem (2.95 mSv)

Man-made 65 mrem (0.65 mSv)

Terrestrial 29 mrem

(0.29 mSv)

FIGURE 1-3 The contribution of various sources to the aver- age U.S. population radiation dose, 1990.

Ionization is the removal of an electron from an atom.

Radiation is the transfer of energy.

Mass-Energy

E = mc 2 where E is energy, m is mass, and c is the speed of light in a vacuum.

PART I Radiologic Physics6

60 mrem (0.6 mSv). An mrem is 1/1000 of a rem. The rem is the unit of r adiation e quivalent m an. It is used to express radiation exposure of populations (see Chapter 2).

Natural environmental radiation consists of three components: cosmic rays, terrestrial radiation, and in- ternally deposited radionuclides. Cosmic rays are par- ticulate and electromagnetic radiation emitted by the sun and stars. On Earth, the intensity of cosmic ra- diation increases with altitude and latitude. Terrestrial radiation results from deposits of uranium, thorium, and other radionuclides in the Earth. The intensity is highly dependent on the geology of the local area. Internally deposited radionuclides, mainly potassium-40 ( 40 K), are natural metabolites. They have always been with us and contribute an equal dose to each of us.

The largest source of natural environmental radiation is radon. Radon is a radioactive gas that is produced by the natural radioactive decay of uranium, which is present in trace quantities in the Earth. All Earth-based materials, such as concrete, bricks, and gypsum wall- board, contain radon. Radon emits alpha particles, which are not penetrating, and therefore contributes a radiation dose only to the lung.

Collectively, these sources of natural environmen- tal radiation result in approximately 2 to 10 micro- roentgens (µR)/hr at waist level in the United States ( Figure 1-4 ). This equals an annual exposure of approx- imately 20 milliroentgen (mR)/yr (0.2 milligray [mGy]/ yr) along the Gulf Coast and Florida to 90 mR/yr (0.9 mGy/yr) or higher in the Rocky Mountain region.

Remember, however, that humans have existed for several hundred thousand years in the presence of this natural environmental radiation level. Human evolution undoubtedly has been infl uenced by natural environ- mental radiation. Some geneticists contend that evolu- tion is infl uenced primarily by ionizing radiation. If this is so, then we must indeed be concerned with control of unnecessary radiation exposure because over the past century, with increasing medical applications of radia- tion, the average annual exposure of our population to radiation has increased signifi cantly.

Diagnostic x-rays constitute the largest man-made source of ionizing radiation (39 mrem/yr) (0.39 mSv/yr). This estimate was made in 1990 by the National Coun- cil on Radiation Protection and Measurements (NCRP). More recent estimates put this source at nearly 320 mrem/yr (3.2 mSv/yr), with increases due principally to the increasing use of multislice spiral computed tomog- raphy (MSCT) and high-level fl uoroscopy.

The benefi ts derived from the application of x-rays in medicine are indisputable; however, such applica- tions must be made with prudence and with care taken to reduce unnecessary exposure of patients and person- nel. This responsibility falls primarily on the radiologic technologist because the technologist usually controls the operation of the x-ray imaging system during a radiologic examination.

The currently accepted approximate annual dose resulting from medical applications of ionizing radia- tion is 50 mrem (0.5 mSv). In contrast to the natural

FIGURE 1-4 Radiation exposure at waist level throughout the United States. (Courtesy U.S. Geological Survey.)

C H A P T E R 1 Concepts of Radiologic Science 7

environmental radiation dose, this level takes into account people who are not receiving a radiologic ex- amination and those undergoing several within a year.

The medical radiation exposure for some in our pop- ulation will be zero, but for others, it may be quite high. Although this average level is comparable with natural environmental radiation levels, it is actually a rather small amount of radiation. One could question, there- fore, why it is necessary to be concerned about radiation control and radiation safety in radiology.

Question: What percentage of our annual average radiation dose is due to diagnostic x-rays? (see Figure 1-3 )

Answer: 39 mrad 360 mrad

= 0.108 @ 11%

Other sources of man-made radiation include nucle- ar power generation, research applications, industrial sources, and consumer items. Nuclear power stations and other industrial applications contribute very little to our radiation dose. Consumer products such as watch dials, exit signs, smoke detectors, camping lantern man- tles, and airport surveillance systems contribute a few millirems to our annual radiation dose.

DISCOVERY OF X-RAYS X-rays were not developed; they were discovered, and quite by accident. During the 1870s and 1880s, many university physics laboratories were investigating the conduction of cathode rays, or electrons, through a large, partially evacuated glass tube known as a Crookes tube. Sir William Crookes was an English- man from a rather humble background who was a self-taught genius.

The tube that bears his name was the forerunner of modern fl uorescent lamps and x-ray tubes. There were many different types of Crookes tubes; most of them were capable of producing x-rays. Wilhelm Roentgen was experimenting with a type of Crookes tube when he discovered x-rays ( Figure 1-5 ).

On November 8, 1895, Roentgen was working in his physics laboratory at Würzburg University in Germany. He had darkened his laboratory and completely enclosed his Crookes tube with black photographic paper so he could better visualize the effects of the cathode rays in the tube. A plate coated with barium platinocyanide, a fl uorescent material, happened to be lying on a bench top several feet from the Crookes tube.

No visible light escaped from the Crookes tube be- cause of the black paper that enclosed it, but Roentgen noted that the barium platinocyanide glowed. The in- tensity of the glow increased as the plate was brought closer to the tube; consequently, there was little doubt about the origin of the stimulus of the glow. This glow is called fl uorescence.

Roentgen's immediate approach to investigating this “X-light,” as he called it, was to interpose various materials — wood, aluminum, his hand! — between the Crookes tube and the fl uorescing plate. The “X” was for unknown! He feverishly continued these investiga- tions for several weeks.

Roentgen's initial investigations were extremely thorough, and he was able to report his experimental results to the scientifi c community before the end of 1895. For this work, in 1901 he received the fi rst No- bel Prize in physics. Roentgen recognized the value of his discovery to medicine. He produced and published the fi rst medical x-ray image in early 1896. It was an image of his wife's hand ( Figure 1-6 ). Figure 1-7 is a photograph of what is reported to be the fi rst x-ray examination in the United States, conducted in early February 1896, in the physics laboratory at Dartmouth College.

The discovery of x-rays is characterized by many amazing features, and this causes it to rank high among the events in human history. First, the discovery was accidental. Second, probably no fewer than a dozen contemporaries of Roentgen had previously observed x-radiation, but none of these other physicists had rec- ognized its signifi cance or investigated it. Third, Roent- gen followed his discovery with such scientifi c vigor that within little more than a month, he had described x-radiation with nearly all the properties we recognize today.

FIGURE 1-5 The type of Crookes tube Roentgen used when he discovered x-rays. Cathode rays (electrons) leav- ing the cathode are attracted by high voltage to the anode, where they produce x-rays and fl uorescent light. (Courtesy Gary Leach, Memorial Hermann Hospital.)

PART I Radiologic Physics8

DEVELOPMENT OF MODERN RADIOLOGY There are two general types of x-ray examinations: radiography and fl uoroscopy. Radiography uses x-ray fi lm and usually an x-ray tube mounted from the ceiling on a track that allows the tube to be moved in any di- rection. Such examinations provide the radiologist with fi xed images.

Fluoroscopy is usually conducted with an x-ray tube located under the examination table. The radiologist is provided with moving images on a television monitor or fl at panel display. There are many variations of these two basic types of examinations, but in general, x-ray equip- ment is similar.

X-ray voltages are measured in kilovolt peak (kVp). One kilovolt (kV) is equal to 1000 V of electric potential. X-ray currents are measured in milliampere (mA), where the ampere (A) is a measure of electric current. The prefi x milli stands for 1/1000 or 0.001.

Question: The usual x-ray source-to-image receptor distance (SID) is 1 meter. How many millimeters is that?

Answer: 1 mm = 1/1000 m or 10 − 3 , therefore 1000 mm = 1 m.

Today, voltage and current are supplied to an x-ray tube through rather complicated electric circuits, but in Roentgen's time, only simple static generators were available. These units could provide currents of only a few milliamperes and voltages to 50 kVp. Today, 1000 mA and 150 kVp are commonly used.

Radiographic procedures that involve equipment with these limitations of electric current and potential often required exposure times of 30 minutes or longer for a satisfactory examination. Long exposure time results in image blur. One development that helped reduce this exposure time was the use of a fl uorescent intensifying screen in conjunction with the glass photographic plates.

Michael Pupin is said to have demonstrated the use of a radiographic intensifying screen in 1896, but only many years later did it receive adequate recognition and use. Radiographs during Roentgen's time were made by exposing a glass plate with a layer of photographic emulsion coated on one side.

Charles L. Leonard found that by exposing two glass x-ray plates with the emulsion surfaces together, exposure time was halved and the image was considerably en- hanced. This demonstration of double-emulsion radiog- raphy was conducted in 1904, but double-emulsion fi lm did not become commercially available until 1918.

Much of the high-quality glass used in radiography came from Belgium and other European countries. This supply was interrupted during World War I; therefore radiologists began to make use of fi lm rather than glass plates.

The demands of the army for increased radiologic services made necessary a substitute for the glass plate. The substitute was cellulose nitrate, and it quickly be- came apparent that the substitute was better than the original glass plate.

The fl uoroscope was developed in 1898 by the Amer- ican inventor Thomas A. Edison ( Figure 1-8 ). Edison's original fl uorescent material was barium platinocyanide, a widely used laboratory material. He investigated the fl uorescent properties of more than 1800 other materi- als, including zinc cadmium sulfi de and calcium tung- state — two materials in use today.

There is no telling what additional inventions Edison might have developed had he continued his x-ray research, but he abandoned it when his assistant and long-time friend, Clarence Dally, suffered a severe x-ray burn that eventually required amputation of both arms. Dally died in 1904 and is counted as the fi rst x-ray fatality in the United States.

Two devices designed to reduce the exposure of pa- tients to x-rays and thereby minimize the possibility of x-ray burn were introduced before the turn of the 20th

FIGURE 1-6 The hand shown in this radiograph belongs to Mrs. Roentgen. This fi rst indication of the possible medical ap- plications of x-rays was made within a few days of the discovery. (Courtesy Deutsches Roentgen Museum.)

To provide an x-ray beam that is satisfactory for imaging, you must supply the x-ray tube with a high voltage and a suffi cient electric current.

C H A P T E R 1 Concepts of Radiologic Science 9

century by a Boston dentist, William Rollins. Rollins used x-rays to image teeth and found that restricting the x-ray beam with a sheet of lead with a hole in the center, a diaphragm, and inserting a leather or aluminum fi lter improved the diagnostic quality of radiographs.

This fi rst application of collimation and fi ltration was followed very slowly by general adoption of these tech- niques. It was later recognized that these devices reduce the hazard associated with x-rays.

Two developments that occurred at approximately the same time transformed the use of x-rays from a novelty in the hands of a few physicists into a valuable, large-scale medical specialty. In 1907, H.C. Snook in- troduced a substitute high-voltage power supply, an interrupterless transformer, for the static machines and induction coils then in use.

Although the Snook transformer was far superior to these other devices, its capability greatly exceeded the capability of the Crookes tube. It was not until the in- troduction of the Coolidge tube that the Snook trans- former was widely adopted.

The type of Crookes tube that Roentgen used in 1895 had existed for a number of years. Although some modifi cations were made by x-ray workers, it remained essentially unchanged into the second decade of the 20th century.

After considerable clinical testing, William D. Cool- idge unveiled his hot-cathode x-ray tube to the medi- cal community in 1913. It was immediately recognized as far superior to the Crookes tube. It was a vacu- um tube that allowed x-ray intensity and energy to be selected separately and with great accuracy. This

FIGURE 1-8 Thomas Edison is seen viewing the hand of his unfortunate assistant, Clarence Dally, through a fl uoroscope of his own design. Dally's hand rests on the box that contains the x-ray tube.

FIGURE 1-7 This photograph records the fi rst medical x-ray examination in the United States. A young patient, Eddie McCarthy of Hanover, New Hampshire, broke his wrist while skating on the Connecticut River and submitted to having it photographed by the “X-light.” With him are (left to right) Professor E.B. Frost, Dartmouth College, and his brother, Dr. G.D. Frost, Medical Director, Mary Hitchcock Hospital. The apparatus was assembled by Professor F.G. Austin in his physics laboratory at Reed Hall, Dartmouth College, on February 3, 1896. (Cour- tesy Mary Hitchcock Hospital.)

PART I Radiologic Physics10

had not been possible with gas-fi lled tubes, which made standards for techniques diffi cult to obtain. X-ray tubes in use today are refi nements of the Coolidge tube.

The era of modern radiography is dated from the match- ing of the Coolidge tube with the Snook transformer; only then did acceptable kVp and mA levels become

possible. Few developments since that time have had such a major infl uence on diagnostic imaging.

In 1913, Gustav Bucky (German) invented the station- ary grid (“Glitterblende”); 2 months later, he applied for a second patent for a moving grid. In 1915, H. Potter (American), probably unaware of Bucky's patent because of the First World War, also invented a moving grid. To his credit, Potter recognized Bucky's work, and the Pot- ter-Bucky grid was introduced in 1921.

In 1946, the light amplifi er tube was demonstrated at Bell Telephone Laboratories. This device was adapted for fl uoroscopy by 1950. Today, image-intensifi ed fl uo- roscopy is universal.

Radiology emerged as a medical specialty because of the Snook transformer and the Coolidge x-ray tube.

BOX 1-1 Important Dates in the Development of Modern Radiology

DATE EVENT 1895 Roentgen discovers x-rays. 1896 First medical applications of x-rays in diagnosis and therapy are made. 1900 The American Roentgen Society, the fi rst American radiology organization, is founded. 1901 Roentgen receives the fi rst Nobel Prize in physics. 1905 Einstein introduces his theory of relativity and the famous equation E = mc 2 . 1907 The Snook interrupterless transformer is introduced. 1913 Bohr theorizes his model of the atom, featuring a nucleus and planetary electrons. 1913 The Coolidge hot-fi lament x-ray tube is developed. 1917 The cellulose nitrate fi lm base is widely adopted. 1920 Several investigators demonstrate the use of soluble iodine compounds as contrast media. 1920 The American Society of Radiologic Technologists (ASRT) is founded. 1921 The Potter-Bucky grid is introduced. 1922 Compton describes the scattering of x-rays. 1923 Cellulose acetate “safety” x-ray fi lm is introduced (Eastman Kodak). 1925 The First International Congress of Radiology is convened in London. 1928 The roentgen is defi ned as the unit of x-ray intensity. 1929 Forssmann demonstrates cardiac catheterization … ... on himself! 1929 The rotating anode x-ray tube is introduced. 1930 Tomographic devices are shown by several independent investigators. 1932 Blue tint is added to x-ray fi lm (Dupont). 1932 The U.S. Committee on X-ray and Radium Protection (now the NCRP) issues fi rst dose limits. 1942 Morgan exhibits an electronic phototiming device. 1942 First automatic fi lm processor (Pako) is introduced. 1948 Coltman develops the fi rst fl uoroscopic image intensifi er. 1951 Multidirectional tomography (polytomography) is introduced. 1953 The rad is offi cially adopted as the unit of absorbed dose. 1956 Xeroradiography is demonstrated. 1956 First automatic roller transport fi lm processing (Eastman Kodak) is introduced. 1960 Polyester base fi lm is introduced (Dupont). 1963 Kuhl and Edwards demonstrate single-photon emission computed tomography (SPECT). 1965 Ninety-second rapid processor is introduced (Eastman Kodak). 1966 Diagnostic ultrasound enters routine use. 1972 Single-emulsion fi lm and one-screen mammography become available (Dupont). 1973 Hounsfi eld completes development of fi rst computed tomography (CT) imaging system (EMI, Ltd.). 1973 Damadian and Lauterbur produce fi rst magnetic resonance image (MRI). 1974 Rare Earth radiographic intensifying screens are introduced. 1977 Mistretta demonstrates digital subtraction fl uoroscopy. 1979 The Nobel Prize in Physiology or Medicine is awarded to Allan Cormack and Godfrey Hounsfi eld for CT. 1980 First commercial superconducting MRI system is introduced. 1981 Slot scan chest radiography is demonstrated by Barnes.

C H A P T E R 1 Concepts of Radiologic Science 11

Each recent decade has seen remarkable improve- ments in medical imaging. Diagnostic ultrasound ap- peared in the 1960s, as did the gamma camera; positron emission tomography (PET) and x-ray computed tomog- raphy (CT) were developed in the 1970s. Magnetic reso- nance imaging (MRI) became an accepted modality in the 1980s, and now, magnetoencephalography (MEG) is being investigated. Box 1-1 chronologically summa- rizes some of the more important developments.

REPORTS OF RADIATION INJURY The fi rst x-ray fatality in the United States occurred in 1904. Unfortunately, radiation injuries occurred rather frequently in the early years. These injuries usually took the form of skin damage (sometimes severe), loss of hair, and anemia. Physicians and, more commonly, patients were injured, primarily because the low energy of ra- diation then available resulted in the necessity for long exposure times to obtain an acceptable radiograph.

By about 1910, these acute injuries began to be con- trolled as the biologic effects of x-rays were scientifi cally investigated and reported. With the introduction of the Coolidge tube and the Snook transformer, the frequency of reports of injuries to superfi cial tissues decreased.

Years later, it was discovered that blood disorders such as aplastic anemia and leukemia were occurring in radiolo- gists at a much higher rate than in others. Because of these observations, protective devices and apparel, such as lead gloves and aprons, were developed for use by radiologists. X-ray workers were routinely observed for any effects

of their occupational exposure and were provided with personnel radiation monitoring devices. This attention to radiation safety in radiology has been effective.

BASIC RADIATION PROTECTION Today, the emphasis on radiation control in diagnostic radiology has shifted back to protection of the patient. Current studies suggest that even the low doses of x-radiation used in routine diagnostic procedures may result in a small incidence of latent harmful effects. It is also well established that the human fetus is sensitive to x-radiation early in pregnancy.

It is hoped that this introduction has emphasized the importance of providing adequate protection for both radiologic technologist and patient. As you progress through your training in radiologic technol- ogy, you will quickly learn how to operate your x-ray imaging systems safely, with minimal radiation ex- posures, by following standard radiation protection procedures.

One caution is in order early in your training — After you have worked with x-ray imaging systems, you will become so familiar with your work environment that you may become complacent about radiation control. Do not allow yourself to develop this attitude because it can lead to unnecessary radiation exposure. Radiation

Because of effective radiation protection practices, radiology is now considered a safe occupation.

DATE EVENT 1981 The International System of Units (SI) is adopted by the International Commission on Radiation Units and

Measurements (ICRU). 1982 Picture archiving and communications system (PACS) becomes available. 1983 First tabular grain fi lm emulsion (Eastman Kodak) is developed. 1984 Laser-stimulable phosphors for computed radiography appear (Fuji). 1988 A superconducting quantum interference device (SQUID) for magnetoencephalography (MEG) is fi rst used. 1990 Last xeromammography system is produced. 1990 Spiral CT is introduced (Toshiba). 1991 Twin-slice CT is developed (Elscint). 1992 Mammography Quality Standard Acts (MQSA) is passed. 1996 Digital radiography that uses thin-fi lm transistors (TFTs) is developed. 1997 Charge-coupled device (CCD) digital radiography is introduced by Swissray. 1997 Amorphous selenium fl at panel image receptor is demonstrated by Rowlands. 1998 Multislice CT is introduced (General Electric). 1998 Amorphous silicon-CsI image receptor is demonstrated for digital radiography. 2000 The fi rst direct digital mammographic imaging system is made available (General Electric). 2002 Sixteen-slice spiral CT is introduced. 2002 Positron emission tomography (PET) is placed into routine clinical service. 2003 The Nobel in Physiology or Medicine is awarded to Paul Lauterbur and Sir Peter Mansfi eld for MRI. 2004 Sixty-four – slice spiral CT is introduced. 2005 Dual-source CT is announced (Siemens). 2006 Two hundred fi fty six – slice spiral CT is introduced (Toshiba).

BOX 1-1 Important Dates in the Development of Modern Radiology—cont'd

PART I Radiologic Physics12

protection must be an important consideration during each x-ray procedure. Box 1-2 reports the Ten Com- mandments of Radiation Protection.

Minimizing radiation exposure to technologist and pa- tient is easy if the radiographic and fl uoroscopic imaging systems designed for this purpose are recognized and understood. A brief description of some of the primary radiation protection devices follows.

Filtration Metal fi lters, usually aluminum or copper, are inserted into the x-ray tube housing so that low- energy x-rays are absorbed before they reach the patient. These x-rays have little diagnostic value.

Collimation Collimation restricts the useful x-ray beam to that part of the body to be imaged and thereby spares adjacent tissue from unnecessary exposure. Collimators take many different forms. Adjustable light-locating collima- tors are the most frequently used collimating devices. Collimation also reduces scatter radiation and thus improves image contrast.

Intensifying Screens Today, most x-ray fi lms are exposed in a cassette, with radiographic intensifying screens on both sides of the fi lm. Examinations conducted with radiographic inten- sifying screens reduce exposure of the patient to x-rays by more than 95% compared with examinations con- ducted without radiographic intensifying screens.

Protective Apparel Lead-impregnated material is used to make aprons and gloves worn by radiologists and radiologic tech- nologists during fl uoroscopy and some radiographic procedures.

Gonadal Shielding The same lead-impregnated material used in aprons and gloves is used to fabricate gonadal shields. Gonadal shields should be used with all persons of childbear- ing age when the gonads are in or near the useful x-ray beam and when use of such shielding will not interfere with the diagnostic value of the examination.

Protective Barriers The radiographic control console is always located behind a protective barrier. Often, the barrier is lead- lined and is equipped with a leaded-glass window. Un- der normal circumstances, personnel remain behind the barrier during radiographic examination. Figure 1-9 is a rendering of a radiographic/fl uoroscopic examination room. Many radiation safety features are illustrated.

Other procedures should be followed. Abdomi- nal/pelvic x-ray examinations of expectant mothers should not be conducted during the fi rst trimester unless absolutely necessary. Every effort should be made to ensure that an examination will not have to be repeated because of technical error. Repeat examinations subject the patient to twice the necessary radiation.

When shielding patients for x-ray examination, one should consider the medical management of the patient. Except for screening mammography, examination of asymptomatic patients is not indicated.

Patients who require assistance during examination should never be held by x-ray personnel. Mechanical immobilization devices should be used. When necessary, a member of the patient's family should provide the nec- essary assistance.

THE DIAGNOSTIC IMAGING TEAM To become part of this exciting profession, a student must complete the prescribed academic courses, obtain clinical experience, and pass the national certifica- tion examination given by the American Registry of Radiologic Technologists (ARRT). Both academic ex- pertise and clinical skills are required of radiographers ( Box 1-3 ).

Always practice ALARA: Keep radiation exposures A s L ow A s R easonably A chievable.

BOX 1-2 The Ten Commandments of Radiation Protection

1. Understand and apply the cardinal principles of radiation control: time, distance, and shielding.

2. Do not allow familiarity to result in false security. 3. Never stand in the primary beam. 4. Always wear protective apparel when not behind a

protective barrier. 5. Always wear an occupational radiation monitor

and position it outside the protective apron at the collar.

6. Never hold a patient during radiographic examina- tion. Use mechanical restraining devices when pos- sible. Otherwise, have parents or friends hold the patient.

7. The person who is holding the patient must always wear a protective apron and, if possible, protective gloves.

8. Use gonadal shields on all people of childbearing age when such use will not interfere with the exami- nation.

9. Examination of the pelvis and lower abdomen of a pregnant patient should be avoided whenever pos- sible, especially during the fi rst trimester.

10. Always collimate to the smallest fi eld size appropri- ate for the examination.

C H A P T E R 1 Concepts of Radiologic Science 13

BOX 1-3 Task Inventory for Radiography as Required for Examination by the American Registry of Radiologic Technologists

PATIENT CARE 1. Confi rm patient's identity. 2. Evaluate patient's ability to understand and comply with requirements for the requested examination. 3. Explain and confi rm patient's preparation (e.g., dietary restrictions, preparatory medications) before performing radio

graphic/fl uoroscopic examinations. 4. Examine radiographic requisition to verify accuracy and completeness of information (e.g., patient history, clinical

diagnosis). 5. Sequence imaging procedures to avoid effects of residual contrast material on future exams. 6. Maintain responsibility for medical equipment attached to patients (e.g., IVs, oxygen) during radiographic procedures. 7. Provide for patient safety, comfort, and modesty. 8. Communicate scheduling delays to waiting patients. 9. Verify or obtain patient consent as necessary (e.g., with contrast studies). 10. Explain procedure instructions to patient or patient's family. 11. Practice standard precautions. 12. Follow appropriate procedures when in contact with patient in isolation. 13. Select immobilization devices, when indicated, to prevent patient movement. 14. Use proper body mechanics and/or mechanical transfer devices when assisting patient. 15. Before administration of a contrast agent, gather information to determine appropriate dosage, and to discern whether

patient is at increased risk for an adverse reaction. 16. Confi rm type of contrast media to be used and prepare for administration. 17. Use sterile or aseptic technique when indicated. 18. Perform venipuncture. 19. Administer IV contrast media. 20. Observe patient after administration of contrast media to detect adverse reactions. 21. Obtain vital signs. 22. Recognize need for prompt medical attention and administer emergency care. 23. Explain postprocedural instructions to patient or patient's family. 24. Maintain confi dentiality of patient's information. 25. Document required information (e.g., radiographic requisitions, radiographs) on patient's medical record.

A C

BD

G

E F

FIGURE 1-9 The general purpose radiographic/fl uoroscopic (R&F) imaging system includes an overhead radiographic tube (A) and a fl uoroscopic examining table (B) with an x-ray tube under the table. Some of the more common radiation protection devices are the lead curtain (C), the Bucky slot cover (D), leaded apron and gloves (E), and the protective viewing window (F). The location of the image intensifi er (G) and of associated imaging equipment is shown.

Continued

PART I Radiologic Physics14

RADIATION PROTECTION 26. Clean, disinfect, or sterilize facilities and equipment, and dispose of contaminated items in preparation for next

examination. 27. Evaluate the need for and use of protective shielding. 28. Take appropriate precautions to minimize radiation exposure to patient. 29. Question female patient of childbearing age about possible pregnancy, and take appropriate action (i.e., document

response, contact physician). 30. Restrict beam to limit exposure area, improve image quality, and reduce radiation dose. 31. Set kVp, mA, and time or automatic exposure system to achieve optimum image quality, safe operating conditions, and

minimum radiation dose. 32. Prevent all unnecessary persons from remaining in area during x-ray exposure. 33. Take appropriate precaution to minimize occupational radiation exposure. 34. Wear a personnel monitoring device while on duty. 35. Evaluate individual occupational exposure reports to determine whether values for the reporting period are within

established limits.

EQUIPMENT OPERATION 36. Prepare and operate radiographic unit and accessories. 37. Prepare and operate fl uoroscopy unit and accessories. 38. Prepare and operate specialized units. 39. Prepare and operate digital imaging devices.

IMAGE PRODUCTION 40. Remove from patient or table all radiopaque materials that could interfere with the radiographic image. 41. Select appropriate fi lm-screen combination. 42. Select appropriate equipment and accessories (e.g., grid, compensating fi lters, shielding) for the examination requested. 43. Use radiopaque markers to indicate anatomical side, position, or other relevant information (e.g., time, upright,

decubitus, postvoid). 44. Explain breathing instructions before beginning the exposure. 45. Position patient to demonstrate the desired anatomy with body landmarks. 46. Using calipers and technique charts, determine appropriate exposure factors. 47. Modify exposure factors for circumstances such as involuntary motion, casts and splints, pathologic conditions, or

the patient's inability to cooperate. 48. Process exposed image. 49. Reload cassettes and magazines by selecting fi lm of proper size and type. 50. Prepare digital/computed image receptor for exposure. 51. Verify accuracy of patient identifi cation on radiograph. 52. Evaluate radiographs for diagnostic quality. 53. Determine corrective measures that should be used if radiograph is not of diagnostic quality, and take appropriate

action. 54. Store and handle fi lm/cassette in a manner that will reduce the possibility of artifact production.

EQUIPMENT MAINTENANCE 55. Recognize and report malfunctions in the radiographic or fl uoroscopic unit and accessories. 56. Perform basic evaluations of radiographic equipment and accessories. 57. Recognize and report malfunctions in processing equipment. 58. Perform basic evaluations of processing equipment and accessories.

RADIOGRAPHIC PROCEDURES 59. Position patient, x-ray tube, and image receptor to produce diagnostic images of the following: • Thorax • Abdomen and GI studies • Urologic studies • Spine and pelvis • Cranium • Extremities • Other: arthrography, myelography, venography … .

BOX 1-3 Task Inventory for Radiography as Required for Examination by the American Registry of Radiologic Technologists—cont'd

C H A P T E R 1 Concepts of Radiologic Science 15

SUMMARY Radiology offers a career in many areas of medical im- aging, and it requires a modest knowledge of medicine, biology, and physics (radiologic science). This fi rst chap- ter weaves the history and development of radiography with an introduction to medical physics.

Medical physics includes the study of matter, energy, and the electromagnetic spectrum of which x-radiation is a part. The production of x-radiation and its safe, diagnostic use serve as the basis of radiology. As well as emphasizing the importance of radiation safety, this chapter presents a detailed list of clinical and patient care skills required of the radiographer.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. Energy b. Einstein's mass–energy equivalence equation c. Ionizing radiation d. The mrad e. The average level of natural environmental

radiation f. The Coolidge tube g. Fluoroscopy h. Collimation i. The term applied to the chemistry of the body j. Barium platinocyanide 2. Match the following dates with the appropriate

event: a. 1901 b. 1907 c. 1913 d. 1895

1. Roentgen discovers x-rays 2. Roentgen wins fi rst Nobel Prize

in physics 3. The Snook transformer is

developed 4. The Coolidge hot-cathode

x-ray tube is introduced

3. Describe how weight is different from mass. 4. Name four examples of electromagnetic

radiation. 5. How is x-ray interaction different from that seen in

other types of electromagnetic radiation? 6. What is the purpose of x-ray beam fi ltration? 7. Describe the process that results in the formation

of a negative ion and a positive ion. 8. What percentage of average radiation exposure to

a human is due to medical x-rays? 9. Why was the discovery of x-rays such an amazing

event in human history? 10. Why is radiography now considered a radiation-

safe occupation? 11. The acronym ALARA stands for what? 12. Name devices designed to minimize radiation

exposure to the patient and the operator. 13. Briefl y describe the history of x-ray fi lm. 14. What are the three natural sources of whole-body

radiation exposure? 15. What naturally occurring radiation source is

responsible for dosing to lung? 16. How would you defi ne the term “radiation”? 17. What are cathode rays? 18. Place the following in chronologic order of appear-

ance: a. Digital fl uoroscopy b. American Society of Radiologic Technologists

(ASRT) c. Computed tomography (CT) d. Radiographic grids e. Automatic fi lm processing 19. List fi ve clinical skills required by the ARRT. 20. List fi ve personal skills required by the ARRT.

The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

500

C H A P T E R

32 Human Biology OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Discuss the cell theory of human biology 2. List and describe the molecular composition of the human body 3. Explain the parts and function of the human cell 4. Describe the processes of mitosis and meiosis 5. Evaluate the radiosensitivity of tissues and organs OUTLINE Human Radiation Response Composition of the Body Cell Theory

Molecular Composition The Human Cell

Cell Function Cell Proliferation Mitosis Meiosis

Tissues and Organs

C H A P T E R 32 Human Biology 501

HUMAN RADIATION RESPONSE The effect of x-rays on humans is the result of interac- tions at the atomic level (see Chapter 10). These atomic interactions take the form of ionization or excitation of orbital electrons and result in the deposition of energy in tissue.

Deposited energy can produce a molecular change, the consequences of which can be measurable if the molecule involved is critical. Figure 32-1 summarizes the sequence of events between radiation exposure and resultant human injury.

When an atom is ionized, its chemical binding prop- erties change. If the atom is a constituent of a large molecule, ionization may result in breakage of the molecule or relocation of the atom within the molecule. The abnormal molecule may in time function impro- perly or cease to function, which can result in serious impairment or death of the cell.

At each stage in the sequence, it is possible to repair radiation damage and recover.

T IS KNOWN beyond the shadow of a doubt that x-rays are harmful. If suffi ciently intense, x-rays can cause skin burns, cataracts, cancer, leukemia, and other harmful effects. What is not known for cer- tain is the degree of effect, if any, after exposure to diagnostic levels of x-radiation.

The benefi ts derived from diagnostic applications of x-rays are enormous. It is the job of the radio- logic technologist, the radiologist, and the medical physicist to produce high-quality x-ray images with minimal radiation exposure. This approach results in the greatest benefi t with the lowest risk to patients and radiation workers. This is the practice known as ALARA — “as low as reasonably achievable.”

This chapter examines the concepts of human biology and discusses the known radiosensitivity of tissues, organs, and cells.

I

cell death

Ionization and

excitation Molecular alteration

Biochemical lesions

Recovery from sublethal damage

Latent period

Selection and repair

Point mutations

Biochemical lesions

cell death Cell

death

Cell death

Radiation exposure

By direct effect

By interaction with water

Enzymatic repair

Immediate effects: • Death • Organ dysfunction • Tissue damage

Late somatic effects: • Leukemia • Cancer • Tissue damage

Genetic damage

FIGURE 32-1 The sequence of events after radiation exposure of humans can lead to several radiation responses. At nearly every step, mechanisms for recovery and repair are available.

PART VI Radiobiology502

This process is reversible. Ionized atoms can become neutral again by attracting a free electron. Molecules can be mended by repair enzymes. Cell and tissues can regenerate and recover from radiation injury.

If the radiation response occurs within minutes or days after the radiation exposure, it is classifi ed as an early effect of radiation. On the other hand, if the human injury is not observed for months or years, it is called a late effect of radiation.

A general classifi cation scheme of possible early and late human responses to radiation is shown in Box 32-1 . In addition, many other radiation responses have been experimentally observed in animals. Most human responses have been observed to occur after exposure to rather large radiation doses. However, we are cautious and assume that even small doses are harmful.

Table 32-1 lists some of the human population groups in which many of these radiation responses have been observed.

The ultimate goal of radiobiologic research is to accu- rately describe the effects of radiation on humans so that radiation can be used more safely in diagnosis and more effectively in therapy. Most radiobiologic research seeks to develop dose-response relationships so the effects of planned doses can be predicted and the response to accidental exposure managed.

COMPOSITION OF THE BODY At its most basic level, the human body is composed of atoms; radiation interacts at the atomic level. The atomic composition of the body determines the charac- ter and degree of the radiation interaction that occurs. The molecular and tissue composition defi nes the nature of the radiation response. Box 32-2 summarizes the atomic composition of the body and shows that more than 85% of the body consists of hydrogen and oxygen.

CELL THEORY Radiation interaction at the atomic level results in molecular change, which can produce a cell that is defi cient in terms of normal growth and metabolism. Robert Hooke, the English schoolmaster, fi rst named the cell as the biologic building block in 1665. Shortly

BOX 32-1 Human Responses to Ionizing Radiation

EARLY EFFECTS OF RADIATION ON HUMANS 1. Acute radiation syndrome

a. Hematologic syndrome b. Gastrointestinal syndrome c. Central nervous system syndrome

2. Local tissue damage a. Skin b. Gonads c. Extremities

3. Hematologic depression 4. Cytogenetic damage

LATE EFFECTS OF RADIATION ON HUMANS 1. Leukemia 2. Other malignant disease

a. Bone cancer b. Lung cancer c. Thyroid cancer d. Breast cancer

3. Local tissue damage a. Skin b. Gonads c. Eyes

4. Shortening of life span 5. Genetic damage

a. Cytogenetic damage b. Doubling of dose c. Genetically signifi cant dose

EFFECTS OF FETAL IRRADIATION 1. Prenatal death 2. Neonatal death 3. Congenital malformation 4. Childhood malignancy 5. Diminished growth and development

Radiobiology is the study of the effects of ionizing radiation on biologic tissue.

TABLE 32-1 Human Populations in Whom Radiation Effects Have Been Observed

Population Effect

American radiologists Leukemia, reduced life span

Atomic bomb survivors Malignant disease Radiation accident victims

(e.g., Chernobyl) Acute lethality

Marshall Islanders Thyroid cancer Uranium miners Lung cancer Radium watch-dial

painters Bone cancer

Patients treated with 131 I Thyroid cancer Children treated for

enlarged thymus Thyroid cancer

Children of Belarus (downwind from Chernobyl)

Thyroid cancer

Patients with ankylosing spondylitis

Leukemia

Patients who underwent Thorotrast studies

Liver cancer

Irradiation in utero Childhood malignancy Volunteer convicts Fertility impairment Cyclotron workers Cataracts

C H A P T E R 32 Human Biology 503

thereafter, in 1673, Anton van Leeuwenhoek accurately described a living cell on the basis of his microscopic observations.

It was longer than 100 years later, however, in 1838, that Schneider and Schwann showed conclusively that in all plants and animals, cells are the basic functional units. This is the cell theory.

The 1953 Watson and Crick description of the molecular structure of deoxyribonucleic acid (DNA) as the genetic substance of the cell was a major accomplishment. Precise mapping of the 40,000 human genes, which was the result of the Human Genome Project completed in the year 2000, promises exceptional solutions to the detection and management of human disease.

Molecular imaging is already making signifi cant contributions to human health.

Molecular Composition Five principal types of molecules are found in the body ( Box 32-3 ). Four of these molecules — proteins, lipids (fats), carbohydrates (sugars and starches), and nucleic acids — are macromolecules.

Proteins, lipids, and carbohydrates are the principal classes of organic molecules. An organic molecule is life-supporting and contains carbon. One of the rarest molecules — a nucleic acid concentrated in the nucleus of a cell (DNA) — is considered to be the most critical and radiosensitive target molecule.

Water is the most abundant molecule in the body, and it is the simplest. Water, however, plays a particu- larly important role in delivering energy to the target molecule, thereby contributing to radiation effects. In addition to water and the macromolecules, some trace elements and inorganic salts are essential for proper metabolism.

Water. The most abundant molecular constituent of the body is water. It consists of two atoms of hydrogen and one atom of oxygen (H 2 O) and constitutes approxi- mately 80% of human substance. Humans are basically made of structured water.

The water molecules exist both in the free state and in the bound state, that is, bound to other molecules. They provide some form and shape, assist in maintain- ing body temperature, and enter into some biochemical reactions.

During vigorous exercise, body water is lost through perspiration to stabilize temperature and respiration. Water loss must be replaced to maintain homeostasis, which is the concept of the relative constancy of the internal environment of the human body.

Water and carbon dioxide are end products in the catabolism (breaking down into smaller units) of macro- molecules. Anabolism, the production of large molecules from small, and catabolism collectively are referred to as metabolism. Some athletes use anabolic steroids to build muscle mass, but harmful adverse effects may occur.

Proteins. Approximately 15% of the molecular composition of the body is protein. Proteins are long- chain macromolecules that consist of a linear sequence of amino acids connected by peptide bonds. Twenty- two amino acids are used in protein synthesis, the metabolic production of proteins. The linear sequence, or arrangement, of these amino acids determines the precise function of the protein molecule.

Figure 32-2 shows the general chemical form of a protein molecule. The generalized formula for a protein is C n H n O n N n T n , where the subscript “n” refers to the number of atoms of each element in the molecule; T represents trace elements. In general, 50% of the mass of a protein molecule is carbon, 20% oxygen, 17% nitrogen, 7% hydrogen, and 6% other elements.

BOX 32-2 Atomic Composition of the Body

• 60.0% hydrogen • 25.7% oxygen • 10.7% carbon • 2.4% nitrogen • 0.2% calcium • 0.1% phosphorus • 0.1% sulfur • 0.8% trace elements

BOX 32-3 Molecular Composition of the Body

• 80% water • 15% protein • 2% lipids • 1% carbohydrates • 1% nucleic acid • 1% other

Protein = AA — AA — AA — AA... where AA is the amino acid, and — is the peptide bond.

Macromolecules are very large molecules that sometimes consist of hundreds of thousands of atoms.

METABOLISM

Catabolism Anabolism

PART VI Radiobiology504

Proteins have a variety of uses in the body. They provide structure and support. Muscles are very high in protein content. Proteins also function as enzymes, hormones, and antibodies.

Enzymes are molecules that are necessary in small quantities to allow a biochemical reaction to continue, even though they do not directly enter into the reaction.

Hormones are molecules that exercise regulatory control over some body functions, such as growth and development. Hormones are produced and secreted by the endocrine glands — the pituitary, adrenal, thyroid, parathyroid, pancreas, and gonads.

Antibodies constitute a primary defense mechanism of the body against infection and disease. The molec- ular confi guration of an antibody may be precise and designed for attacking a particular type of invasive or infectious agent, the antigen.

Lipids. Lipids are organic macromolecules composed solely of carbon, hydrogen, and oxygen. They are rep- resented by the general formula, C n H n O n . Structurally, lipids are seen in the form shown in Figure 32-3 , and it is this structure that distinguishes them from carbohy- drates. In general, lipids are composed of two types of smaller molecules — glycerol and fatty acid. Each lipid molecule is composed of one molecule of glycerol and three molecules of fatty acid.

Lipids are present in all tissues of the body and are the structural components of cell membranes. Lipids often are concentrated just under the skin and

serve as a thermal insulator from the environment. Penguins, for instance, have a particularly thick layer of subcutaneous fat (blubber) that protects them from the cold.

Lipids also serve as fuel for the body by providing energy stores. It is more diffi cult, however, to extract energy from lipids than from the other major fuel source, carbohydrates; this relationship, of course, is associated with one of the major dilemmas in modern nutrition — obesity.

Carbohydrates. Carbohydrates, similar to lipids, are composed solely of carbon, hydrogen, and oxygen, but their structure is different ( Figure 32-4 ). This structural difference determines the contribution of the carbohy- drate molecule to body biochemistry. The ratio of the number of hydrogen atoms to oxygen atoms in a carbo- hydrate molecule is 2:1 (as in water), and a large fraction of this molecule consists of these atoms. Consequently, carbohydrates were fi rst considered to be watered, or hydrated, carbons, hence their name.

Carbohydrates also are called saccharides. Mono- saccharides and disaccharides are sugars. The chemical formula for glucose, a simple sugar, is C 6 H 12 O 6 . These molecules are relatively small. Polysaccharides are large and include plant starches and animal glycogen. The chemical formula for a polysaccharide is (C 6 H 10 O 5 ) n , where n is the number of simple sugar molecules in the macromolecule.

Amino acids Protein

Oxygen Carbon Nitrogen Various side chains Hydrogen

FIGURE 32-2 Proteins consist of amino acids linked by peptide bonds. The creation of the peptide bond requires the removal of a molecule of water.

Oxygen Carbon Hydrogen

FIGURE 32-3 The structural confi guration of a lipid is repre- sented by a molecule of oleic acid: CH 2 (CH 2 )7CH = CH(CH 2 ) 7 COOH.

Oxygen Carbon Hydrogen

FIGURE 32-4 Carbohydrates are structurally different from lipids, even though their composition is similar. This is a molecule of sucrose, or ordinary table sugar: (C 12 H 22 O 11 ).

C H A P T E R 32 Human Biology 505

Some carbohydrates are incorporated into the structure of cells and tissues to provide shape and stability. The human polysaccharide, glycogen, is stored in the tissues of the body and is used as fuel only when quantities of the simple sugar, glucose, are inadequate.

Glucose is the ultimate molecule that fuels the body. Lipids can be catabolized into glucose for energy, but only with great diffi culty. Polysaccharides are much more readily transformed into glucose. This explains why a chocolate bar, which is high in glucose, can provide a quick burst of energy for an athlete.

Nucleic Acids. Two principal nucleic acids are important to human metabolism: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Located principally in the nucleus of the cell, DNA serves as the command or control molecule for cell function. DNA contains all the hereditary information that represents a cell and, of course, if the cell is a germ cell, all the hereditary information of the whole individual.

Located principally in the cytoplasm, RNA also is found in the nucleus. Two types of RNA have been identifi ed: messenger RNA (mRNA) and transfer RNA (tRNA). These are distinguished according to their

biochemical functions. These molecules are involved in the growth and development of the cell through a number of biochemical pathways, most notably, protein synthesis.

The nucleic acids are very large and extremely complex macromolecules. Figure 32-5 shows the structural composition of DNA and reveals how the component molecules are joined. DNA consists of a backbone com- posed of alternating segments of deoxyribose (a sugar) and phosphate. For each deoxyribose – phosphate conju- gate formed, a molecule of water is removed.

Attached to each deoxyribose molecule is one of four dif- ferent nitrogen-containing or nitrogenous organic bases: adenine, guanine, thymine, or cytosine. Adenine and gua- nine are purines; thymine and cytosine are pyrimidines.

The base sugar – phosphate combination is called a nucleotide, and the nucleotides are strung together in one long-chain macromolecule. Human DNA exists as two of these long chains attached together in ladder fashion ( Figure 32-6 ). The side rails of the ladder are the alternat- ing sugar – phosphate molecules, and the rungs of the ladder consist of bases joined together by hydrogen bonds.

Adenine Guanine Purines

Deoxyribose

Phosphate

Deoxyribonucleic acid (DNA)

Oxygen Carbon Nitrogen Phosphorus Hydrogen

Thymine Cytosine

Pyrimidines

FIGURE 32-5 DNA is the control center for life. A single molecule consists of a backbone of alternating sugar (deoxyribose) and phosphate molecules. One of the four organic bases is attached to each sugar molecule.

DNA is the radiation-sensitive target molecule.

The chief function of carbohydrates in the body is to provide fuel for cell metabolism.

PART VI Radiobiology506

To complete the picture, the ladder is twisted about an imaginary axis such as a spring. This produces a molecule with the double-helix confi guration ( Figure 32-7 ). The sequence of base bonding is limited to adenines bonded to thymines and cytosines bonded to guanines.

Structurally, RNA resembles DNA. In RNA, the sugar component is ribose rather than deoxyribose, and uracil replaces thymine as a base component. In contrast, RNA forms a single spiral, not a double helix.

THE HUMAN CELL The principal molecular components of the human body are made of intricate cellular structures. The distribu- tion of structures throughout the cell is reminiscent of the way the parts of an automobile are assembled. This assembly ensures proper growth, development, and function of the cell. Figure 32-8 is a cutaway view of a human cell, with its principal structures labeled.

The two major structures of the cell are the nucleus and the cytoplasm. The principal molecular component of the nucleus is DNA, the genetic material of the cell. The nucleus also contains some RNA, protein, and water.

Most of the RNA is contained in a rounded struc- ture, the nucleolus. The nucleolus often is attached to the nuclear membrane, a double-walled structure that at some locations is connected to the endoplasmic reticulum. This

connection by its nature controls the passage of molecules, particularly RNA, from nucleus to cytoplasm.

The cytoplasm makes up the bulk of the cell and contains great quantities of all molecular components except DNA. A number of intracellular structures are found in the cytoplasm. The endoplasmic reticulum is a channel or a series of channels that allows the nucleus to communicate with the cytoplasm.

The large bean-shaped structures are mitochondria. Macromolecules are digested in the mitochondria to produce energy for the cell. The mitochondria are there- fore called the engine of the cell.

The small, dot-like structures are ribosomes. Ribosomes are the site of protein synthesis and therefore are essen- tial to normal cellular function. Ribosomes are scattered throughout the cytoplasm or the endoplasmic reticulum.

The small pea-like sacs are lysosomes. The lysosomes contain enzymes capable of digesting cellular fragments and sometimes the cell itself. Lysosomes help to control intracellular contaminants.

All these structures, including the cell itself, are surrounded by membranes. These membranes consist principally of lipid-protein complexes that selectively allow small molecules and water to diffuse from one side to the other. These cellular membranes, of course, also provide structure and form for the cell and its components.

When the critical macromolecular cellular components are irradiated by themselves, a dose of approximately 1 Mrad (10 kGy t ) is required to produce a measurable change in any physical characteristic of the molecule.

Only adenine – thymine and cytosine – guanine base bonding is possible in DNA.

FIGURE 32-6 DNA consists of two long chains of alternating sugar and phosphate molecules fashioned similarly to the side rails of a ladder, with pairs of bases as rungs.

FIGURE 32-7 The DNA ladder is twisted about an imaginary axis to form a double helix.

C H A P T E R 32 Human Biology 507

When a macromolecule is incorporated into the apparatus of a living cell, only a quantity of a few rads is necessary to produce a measurable biologic response. The lethal dose in some single-cell organisms, such as bacteria, is measured in kilorad, whereas human cells can be killed with a dose of less than 100 rad (1 Gy t ).

A number of experiments have shown that the nucleus is much more sensitive than the cytoplasm to the effects of radiation. Such experiments are conducted with the use of precise microbeams of electrons that can be focused and directed to a particular cell part, or through incorporation of the radioactive isotopes tritium ( 3 H) and carbon-14 ( 14 C) into cellular molecules that localize exclusively to the cytoplasm or the nucleus.

Cell Function Every human cell has a specifi c function in supporting the total body. Some differences are obvious, as in nerve cells, blood cells, and muscle cells. Similarities are also somewhat obvious.

In addition to its specialized function, each cell to some extent absorbs all molecular nutrients through the cell membrane and uses these nutrients in energy production and molecular synthesis. If this molecular synthesis is damaged by radiation exposure, the cell may malfunction and die.

Protein synthesis is a good example of a critical cellular function necessary for survival ( Figure 32-9 ). DNA, located in the nucleus, contains a molecular code that identifi es which proteins the cell will make.

This code is determined by the sequence of base pairs (adenine – thymine and cytosine – guanine). A series of three base pairs, called a codon, identifi es one of the 22 human amino acids available for protein synthesis.

This genetic message is transferred within the nucleus to a molecule of mRNA. mRNA leaves the nucleus by way of the endoplasmic reticulum and makes its way to a ribosome, where the genetic message is transferred to yet another RNA molecule (tRNA).

tRNA searches the cytoplasm for the amino acids for which it is coded. It attaches to the amino acid and carries it to the ribosome, where it is joined with other amino acids in sequence by peptide bonds to form the required protein molecule.

Interference with any phase of this procedure for protein synthesis could result in damage to the cell. Radiation interaction in which the molecule has primary control over protein synthesis (DNA) is more effective in producing a response than is radiation interaction with other molecules involved in protein synthesis.

Cell Proliferation Although many thousands of rad (many gray) are necessary to produce physically measurable disruption of macromolecules in vitro, single ionizing events at a particularly sensitive site of a critical target molecule are thought to be capable of disrupting cell proliferation.

The human body consists of two general types of cells: somatic cells and genetic cells. The genetic cells include the oogonium of the female and the spermatogonium of the male. All other cells of the body are somatic cells. When somatic cells proliferate or divide, they undergo mitosis. Genetic cells undergo meiosis.

Mitosis The cell biologist and the geneticist view the cell cycle differently ( Figure 32-10 ). Each cycle includes the various states of cell growth, development, and division. The geneticist considers only two phases of the cell cycle: mitosis (M) and interphase.

Mitochondria

Secretory channel

Cell membrane

Lysosomes

Ribosomes

Endoplasmic reticulum

Nucleus

Nuclear membrane

Nucleoli

FIGURE 32-8 Schematic view of a human cell shows the principal structural components.

Nucleus

Proteins

Ribosome

tRNA

mRNA

Amino acids

FIGURE 32-9 Protein synthesis is a complex process that involves many different molecules and cellular structures.

Cell proliferation is the act of a single cell or group of cells to reproduce and multiply in number.

PART VI Radiobiology508

Mitosis, the division phase, is characterized by four subphases: prophase, metaphase, anaphase, and telophase. The portion of the cell cycle between mitotic events is called interphase. Interphase is the period of growth of the cell between divisions.

The cell biologist usually identifi es four phases of the cell cycle: M, G 1 , S, and G 2 . These phases of the cell cycle are characterized by the structure of the chromo- somes, which contain the genetic material DNA. The gap in cell growth between M and S is G 1 . G 1 is the pre-DNA synthesis phase.

The DNA synthesis phase is S. During this period, each DNA molecule is replicated into two identical daughter DNA molecules.

During S phase, the chromosome is transformed from a structure with two chromatids attached to a centro- mere to a structure with four chromatids attached to a centromere ( Figure 32-11 ). The result is two pairs of homologous chromatids, that is, chromatids with precisely the same DNA content and structure.

The G 2 phase is the post-DNA synthesis gap of cell growth.

During interphase, the chromosomes are not visible; however, during mitosis, the DNA slowly takes the form of the chromosomes as seen microscopically. Figure 32-12 schematically depicts the process of mitosis.

During prophase, the nucleus swells and the DNA becomes more prominent and begins to take structural form. At metaphase, the chromosomes appear and are lined up along the equator of the nucleus. It is during metaphase that mitosis can be stopped and chromo- somes can be studied carefully under the microscope.

Anaphase is characterized by splitting of each chromo- some at the centromere, so that a centromere and two chromatids are connected by a fi ber to the poles of the nucleus. These poles are called spindles, and the fi bers are called spindle fi bers. The number of chromatids per centromere has been reduced by half, and these newly formed chromosomes migrate slowly toward the spindle.

The fi nal segment of mitosis, telophase, is characterized by the disappearance of structural chro- mosomes into a mass of DNA and the closing off of the nuclear membrane like a dumbbell into two nuclei. At the same time, the cytoplasm is divided into two equal parts, each of which accompanies one of the new nuclei.

Cell division is now complete. The two daughter cells look precisely the same as the parent and contain exactly the same genetic material.

Interphase

S

G1G2

M

Prophase

Metaphase Anaphase Telophase

FIGURE 32-10 Progress of the cell through one cycle involves several phases.

Chromatids

A B

FIGURE 32-11 During the synthesis portion of interphase, the chromosomes replicate from a two-chromatid structure (A) to a four-chromatid structure (B).

A B C

F E D

FIGURE 32-12 Mitosis is the phase of the cell cycle during which the chromosomes become visible, divide, and migrate to daughter cells. A, Interphase. B, Prophase. C, Metaphase. D, Anaphase. E, Telophase. F, Interphase.

Radiation-induced chromosome damage is analyzed during metaphase.

C H A P T E R 32 Human Biology 509

Meiosis Genetic material can change during the division process of genetic cells, which is called meiosis. Genetic cells begin with the same number of chromosomes as somatic cells — 23 pairs (46 chromosomes). However, for a genetic cell to be capable of marriage to another genetic cell, its complement of chromosomes must be reduced by half to 23, so that after conception and the union of two genetic cells, the daughter cells again will contain 46 chromosomes ( Figure 32-13 ).

The genetic cell begins meiosis with 46 chromosomes that appear the same as in a somatic cell that has com- pleted the G 2 phase. The cell then progresses through the phases of mitosis into two daughter cells, each containing 46 chromosomes of two chromatids each. The names of the subphases are the same for meiosis and mitosis.

Each of the daughter cells of this fi rst division now progresses through a second division in which all cellular material, including chromosomes, is divided. However, the second division is not accompanied by an S phase. Therefore, no replication of DNA occurs; consequently, no chromosomes are duplicated. Each of the resulting granddaughter cells contains only 23 chromosomes.

Each parent has undergone two division processes, which have resulted in four daughter cells. During the second division, some chromosomal material is exchanged among chromatids through a process called crossing ov er. Crossing over results in changes in genetic constitution and changes in inheritable traits.

TISSUES AND ORGANS During the development and maturation of a human from two united genetic cells, a number of different types of cells evolve. Collections of cells of similar structure

and function form tissues. Box 32-4 is a breakdown of the composition of the body according to its tissue constituents.

These tissues in turn are precisely bound together to form organs. The tissues and the organs of the body serve as discrete units with specifi c functional responsi- bilities. Some tissues and organs combine into an overall integrated organization known as an organ system.

The principal organ systems of the body are the nervous system, the digestive system, the endocrine system, the re- spiratory system, and the reproductive system. Effects of radiation that appear at the whole-body level result from damage to these organ systems that occurs as the result of radiation injury to the cells of that system.

The cells of a tissue system are identifi ed by their rate of proliferation and their stage of development. Immature cells are called undifferentiated cells, pre- cursor cells, or stem cells. As a cell matures through growth and proliferation, it can pass through various stages of differentiation into a fully functional and mature cell.

The sensitivity of the cell to radiation is determined somewhat by its state of maturity and its functional role. Table 32-2 lists a number of different types of cells in the body according to their degree of radiosensitivity.

2n 4n

2n

2n

n n

n n

Interphase DNA

replicates

Meitotic-like division

Meitotic-like division

No DNA replication

FIGURE 32-13 Meiosis is the process of reduction division, and it occurs only in reproduc- tive cells. n, Number of similar chromosomes.

Meiosis is the process whereby genetic cells undergo reduction division.

Stem cells are more sensitive to radiation than mature cells.

ORGAN SYSTEMS

• Nervous • Reproductive • Digestive • Respiratory • Endocrine

PART VI Radiobiology510

The tissues and organs of the body include both stem cells and mature cells. Several types of tissue can be classifi ed according to structural or functional features. These features infl uence the degree of radiosensitivity of the tissue.

Epithelium is the covering tissue, and it lines all exposed surfaces of the body, both exterior and

interior. Epithelium covers the skin, the blood vessels, the abdominal and chest cavities, and the gastrointes- tinal tract.

Connective and supporting tissues are high in protein and are composed principally of fi bers that are usually highly elastic. Connective tissue binds tissues and organs together. Bone ligaments and cartilage are examples of connective tissue.

Muscle is a special type of tissue that can contract. It is found throughout the body and is high in protein content.

Nervous tissue consists of specialized cells called neurons that have long, thin extensions from the cell to distant parts of the body. Nervous tissue is the avenue by which electrical impulses are transmitted throughout the body for control and response.

When these various types of tissue are combined to form an organ, they are identifi ed according to two parts of the organ. The parenchymal part contains tissues that represent that particular organ, whereas the stromal part is composed of connective tissue and vasculature that provide structure to the organ.

The early effects of high-dose radiation may include observable organ damage. The various organs of the body exhibit a wide range of sensitivity to radiation. This radiosensitivity is determined by the function of the organ in the body, the rate at which cells mature within the organ, and the inherent radiosensitivity of the cell type.

Precise knowledge of these various organ radio- sensitivities is unnecessary; however, knowledge of general levels of radiosensitivity is helpful toward understanding the effects of whole-body radiation exposure, particularly in the acute radiation syndrome ( Table 32-3 ).

TABLE 32-2 Response to Radiation Is Related to Cell Type

Radiosensitivity Cell Type

High Lymphocytes Spermatogonia Erythroblasts Intestinal crypt cells

Intermediate Endothelial cells Osteoblasts Spermatids Fibroblasts

Low Muscle cells Nerve cells

BOX 32-4 Tissue Composition of the Body

TISSUE ABUNDANCE • Muscle • 43% • Fat • 14% • Organs • 12% • Skeleton • 10% • Blood • 8% • Subcutaneous tissue • 6% • Bone marrow • 4% • Skin • 3%

* The minimum dose delivered at the rate of approximately 200 rad/day (2 Gy t /day), which will produce a response.

TABLE 32-3 Relative Radiosensitivity of Tissues and Organs Based on Clinical Radiation Oncology

Level of Radiosensitivity * Tissue or Organ Effects

High: 200 to 1000 rad (2 to 10 Gy t ) Lymphoid tissue Atrophy Bone marrow Hypoplasia Gonads Atrophy

Intermediate: 1000 to 5000 rad (10 to 50 Gy t ) Skin Erythema Gastrointestinal tract Ulcer Cornea Cataract Growing bone Growth arrest Kidney Nephrosclerosis Liver Ascites Thyroid Atrophy

Low: >5000 rad (>50 Gy t ) Muscle Fibrosis Brain Necrosis Spinal Transection

C H A P T E R 32 Human Biology 511

SUMMARY After radiation exposure, the human body responds in predictable ways. Radiobiology is the study of the effects of ionizing radiation on humans conducted to refi ne knowledge of the expected response.

If a response occurs within minutes or days of exposure, it is called an early effect of radiation. If an injury is not observable for months or years, it is called a late effect of radiation exposure.

The cell is the basic functional unit of all plants and animals. At the molecular level, the human body is composed primarily of water, protein, lipid, carbohy- drate, and nucleic acid. The two important nucleic acids in human metabolism are DNA and RNA.

DNA contains all the hereditary information in the cell. If the cell is a genetic cell, the DNA contains the hereditary information of the whole individual. DNA is a macromol- ecule that is made up of two long chains of base sugar – phosphate combinations twisted into a double helix.

Major cellular function consists of protein synthesis and cell division. Mitosis is the growth, development, and division of cells. Meiosis is the term applied to the division of genetic cells.

Cells of similar structure bind together to form tis- sue. Tissues bind together to form organs. An overall integrated organization of tissue and organs is called an organ system.

The principal organ systems of the body are the ner- vous, digestive, endocrine, and reproductive systems. The radiosensitivity of various tissue and organ systems varies widely. Reproductive cells are highly radiosensi- tive, whereas nerve cells are less radiosensitive.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. ALARA b. Cell theory c. Anabolism d. Carbohydrate e. M, G 1 , S, G 2 f. Epithelium g. Cytoplasm h. Enzyme i. Organic molecule j. Late effect of radiation

2. At what structural level do x-rays interact with humans to produce a radiation response?

3. How does ionizing radiation affect an atom within a large molecule?

4. List fi ve human groups in which radiation effects have been observed.

5. What are the effects of radiation on the populations mentioned in Question 4?

6. What is the most abundant atom and the most abundant molecule in the body?

7. What is a stem cell? 8. Why do we say that humans are basically a

structured aqueous suspension? 9. What is the meaning of epithelium? 10. How do proteins function in the human body? 11. What do carbohydrates do for us? 12. DNA is the abbreviation for what molecule? 13. Which molecule is considered the genetic material

of the cell? 14. What is the function of the endoplasmic reticulum? 15. What is the approximate dose of radiation

required to produce a measurable physical change in a macromolecule?

16. List the stages of cell division of a somatic cell. 17. List the stages of cell reduction division of a

genetic cell. 18. What cell type is the most radiosensitive? 19. What type of tissue is the least radiosensitive? 20. List three early radiation effects and three late

radiation effects in humans. The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

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C H A P T E R

33 Fundamental Principles of Radiobiology

OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. State the law of Bergonie and Tribondeau 2. Describe the physical factors that affect radiation response 3. Describe the biologic factors that affect radiation response 4. Explain radiation dose-response relationships 5. Describe fi ve types of radiation dose-response relationships OUTLINE Law of Bergonie and Tribondeau Physical Factors That Affect Radiosensitivity

Linear Energy Transfer Relative Biologic Effectiveness Protraction and Fractionation

Biologic Factors That Affect Radiosensitivity Oxygen Effect Age Recovery Chemical Agents Hormesis

Radiation Dose-Response Relationships Linear Dose-Response Relationships Nonlinear Dose-Response Relationships Constructing a Dose-Response Relationship

C H A P T E R 33 Fundamental Principles of Radiobiology 513

LAW OF BERGONIE AND TRIBONDEAU In 1906, two French scientists, Bergonie and Tribond- eau, theorized and observed that radiosensitivity was a function of the metabolic state of the tissue being irradi- ated. This has come to be known as the Law of Bergonie and Tribondeau and has been verifi ed many times. Basi- cally, the law states that the radiosensitivity of living tis- sue varies with maturation and metabolism ( Box 33-1 ).

This law is principally interesting as a historical note in the development of radiobiology. It has found some application in radiation oncology. In diagnostic imag- ing, the law serves to remind us that the fetus is con- siderably more sensitive to radiation exposure than the child or the mature adult.

PHYSICAL FACTORS THAT AFFECT RADIOSENSITIVITY When one irradiates tissue, the response of the tis- sue is determined principally by the amount of energy deposited per unit mass — the dose in rad (Gy t ). Even under controlled experimental conditions, however, when equal doses are delivered to equal specimens, the response may not be the same because of other modi- fying factors. A number of physical factors affect the degree of radiation response.

Linear Energy Transfer Linear energy transfer (LET) is a measure of the rate at which energy is transferred from ionizing radiation to soft tissue. It is another method of expressing radia- tion quality and determining the value of the radiation

weighting factor (W R ) used in radiation protection (see Chapter 37). LET is expressed in units of kiloelectron volt of energy transferred per micrometer of track length in soft tissue (keV/µm).

The ability of ionizing radiation to produce a biologic response increases as the LET of radiation increases. When LET is high, ionizations occur frequently, increasing the probability of interaction with the target molecule.

Relative Biologic Effectiveness As the LET of radiation increases, the ability to pro- duce biologic damage also increases. This relative effect is quantitatively described by the relative biologic effec- tiveness (RBE).

The standard radiation, by convention, is orthovoltage x-radiation in the range of 200 to 250 kVp. This type of x-ray beam was used for many years in radiation oncol- ogy and in essentially all early radiobiologic research.

Diagnostic x-rays have an RBE of 1. Radiations with lower LET than diagnostic x-rays have an RBE less than 1, whereas radiations with higher LET have a higher RBE.

Figure 33-1 shows the relationship between RBE and LET and identifies some of the more common types of radiation. Table 33-1 lists the approximate LET and RBE of various types of ionizing radiation.

BOX 33-1 Law of Bergonie and Tribondeau

• Stem cells are radiosensitive; mature cells are radioresistant.

• Younger tissues and organs are radiosensitive. • Tissues with high metabolic activity are

radiosensitive. • A high proliferation rate for cells and a high growth

rate for tissues result in increased radiosensitivity.

The LET of diagnostic x-rays is approximately 3 keV/µm.

The RBE of diagnostic x-rays is 1.

RELATIVE BIOLOGIC EFFECTIVENESS

RBE =

Dose of standard radiation necessary to produce a given effect

DDose of test radiation necessary to produce the same effect

OME TISSUES are more sensitive than others to radiation exposure. Such tissues usually respond more rapidly and to lower doses of radiation.

Reproductive cells are more sensitive than nerve cells. This and other radiobiologic concepts were detailed in 1906 by two French scientists.

Physical factors and biologic factors affect the radiobiologic response of tissue. Knowledge of these radiobiologic factors is essential for understanding the positive effects of radiation oncology and the potentially harmful effects of low-dose radiation exposure.

The principal aim of the study of radiobiology is to understand radiation dose – response relationships. A dose-response relationship is a mathematical and graphic function that relates radiation dose to observed response.

S

PART VI Radiobiology514

Question: When mice are irradiated with 250 kVp

x-rays, death occurs at 650 rad (6.5 Gy t ). If similar mice are irradiated with fast neutrons, death occurs at only 210 rad (2.1 Gy t ). What is the RBE for the fast neutrons?

Answer:

RBE = 650 rad 210 rad

= 3.1

Protraction and Fractionation If a dose of radiation is delivered over a long period of time rather than quickly, the effect of that dose is lessened. Stated differently, if the time of irradiation is lengthened, a higher dose is required to produce the same effect. This lengthening of time can be accomplished in two ways.

If the dose is delivered continuously but at a lower dose rate, it is said to be protracted. Six hundred rad (6 Gy t ) delivered in 3 min (200 rad/min [2 Gy t /min]) is lethal for a mouse. However, when 600 rad is delivered at the rate of 1 rad/hr (10 mGy t /hr) for a total time of 600 hr, the mouse will survive.

If the 600 rad dose is delivered at the same dose rate, 200 rad/min, but in 12 equal fractions of 50 rad (500 mGy t ), all separated by 24 hr, the mouse will survive. In this situation, the dose is said to be fractionated.

Dose fractionation reduces effect because cells undergo repair and recovery between doses. Dose frac- tionation is used routinely in radiation oncology.

BIOLOGIC FACTORS THAT AFFECT RADIOSENSITIVITY In addition to these physical factors, a number of bio- logic conditions alter the radiation response of tissue. Some of these factors, such as age and metabolic rate,

have to do with the inherent state of tissue. Other fac- tors are related to artifi cially introduced modifi ers of the biologic system.

Oxygen Effect Tissue is more sensitive to radiation when irradiated in the oxygenated, or aerobic, state than when irradiated under anoxic (without oxygen) or hypoxic (low- oxygen) conditions. This characteristic of tissue is called the oxygen effect and is described numerically by the oxygen enhancement ratio (OER).

Generally, tissue irradiation is conducted under condi- tions of full oxygenation. Hyperbaric (high- pressure) oxygen has been used in radiation oncology in an attempt to enhance the radiosensitivity of nodular, avascular tumors, which are less radiosensitive than tumors with an adequate blood supply.

Diagnostic x-ray imaging is performed under conditions of full oxygenation.

Dose protraction and fractionation cause less effect because time is allowed for intracellular repair and tissue recovery.

OXYGEN ENHANCEMENT RATIO

OER =

Dose necessary under anoxic conditions to produce a giveen effect

Dose necessary under aerobic conditions to producee the same effect

RBE

LET (keV/µm)

X-rays

Fast neurons

Alpha particles

Heavy nuclei

20

10

0 0.1 1.0 10 100 1000

60Co

FIGURE 33-1 As linear energy transfer (LET) increases, rela- tive biologic effectiveness (RBE) increases also, but a maximum value is reached followed by a lower RBE due to overkill.

Type of Radiation LET (keV/µm) RBE

25 MV x-rays 0.2 0.8 60 Co gamma rays 0.3 0.9 1 MeV electrons 0.3 0.9 Diagnostic x-rays 3.0 1.0 10 MeV protons 4.0 5.0 Fast neutrons 50.0 10 5 MeV alpha particles 100.0 20 Heavy nuclei 1000.0 30

LET, Linear energy transfer; RBE, relative biologic effectiveness.

TABLE 33-1 LET and RBE of Various Radiation Doses

C H A P T E R 33 Fundamental Principles of Radiobiology 515

Question: When experimental mouse mammary carcinomas are clamped and irradiated under hypoxic conditions, the tumor control dose is 10,600 rad (106 Gy t ). When these tumors are not clamped and are irradiated under aerobic conditions, the tumor control dose is 4050 rad (40.5 Gy t ). What is the OER for this system?

Answer: OER =

10,600 4050

= 2.6

The OER is LET-dependent ( Figure 33-2 ). The OER is highest for low-LET radiation, with a maximum value of approximately 3 that decreases to approximately 1 for high-LET radiation.

Age The age of a biologic structure affects its radiosensitiv- ity. The response of humans is characteristic of this age- related radiosensitivity ( Figure 33-3 ). Humans are most sensitive before birth.

After birth, sensitivity decreases until maturity, at which time humans are most resistant to radiation effects. In old age, humans again become somewhat more radiosensitive.

Recovery In vitro experiments show that human cells can recover from radiation damage. If the radiation dose is not suf- fi cient to kill the cell before its next division (interphase

death), then given suffi cient time, the cell will recover from the sublethal radiation damage it has sustained.

This intracellular recovery is due to a repair mechanism inherent in the biochemistry of the cell. Some types of cells have greater capacity than others for repair of sub- lethal damage. At the whole-body level, this recovery from radiation damage is assisted through repopulation by surviving cells.

If a tissue or organ receives a suffi cient radiation dose, it responds by shrinking. This is called atrophy, and it occurs because some cells die and disintegrate and are carried away as waste products.

If a suffi cient number of cells sustain only sublethal damage and survive, they may proliferate and repopu- late the irradiated tissue or organ.

X-rays

Fast neutrons

Alpha particles

LET (keV/µm)

OER

3

2

1

0 0.1 1 10 100 1000

60Co

FIGURE 33-2 The oxygen enhancement ratio (OER) is high for low linear energy transfer (LET) radiation and decreases in value as the LET increases.

Birth

HIGH

LOW

Sensitivity to

radiation

In utero Childhood Adult Aged

FIGURE 33-3 Radiosensitivity varies with age. Experiments with animals have shown that the very young and the very old are more sensitive to radiation.

The combined processes of intracellular repair and repopulation contribute to recovery from radiation damage.

RECOVERY

Recovery = Intracellular repair + Repopulation

Interphase death occurs when the cell dies before replicating.

PART VI Radiobiology516

Chemical Agents Some chemicals can modify the radiation response of cells, tissues, and organs. For chemical agents to be effective, they must be present at the time of irradiation. Post-irradiation application does not usually alter the degree of radiation response.

Radiosensitizers. Agents that enhance the effect of radiation are called sensitizing agents. Examples include halogenated pyrimidines, methotrexate, actinomycin D, hydroxyurea, and vitamin K.

The halogenated pyrimidines become incorporated into the DNA of the cell and amplify the effects of radiation on that molecule. All radiosensitizers have an effectiveness ratio of approximately 2, that is, if 90% of a cell culture is killed by 200 rad (2 Gy t ), then in the presence of a sensitizing agent, only 100 rad (1 Gy t ) is required for the same percentage of lethality.

Radioprotectors. Radioprotective compounds include molecules that contain a sulfhydryl group (sulfur and hydrogen bound together), such as cysteine and cyste- amine. Hundreds of others have been tested and found effective by a factor of approximately 2. For example, if 600 rad (6 Gy t ) is a lethal dose to a mouse, then in the presence of a radioprotective agent, 1200 rad (12 Gy t ) would be required to produce lethality.

Radioprotective agents have not found human appli- cation because, to be effective, they must be adminis- tered at toxic levels. The protective agent can be worse than the radiation!

Hormesis A growing body of radiobiologic evidence suggests that a little bit of radiation is good for you. Studies have shown that animals given low radiation doses live longer than controls. The prevailing explanation is that a little radiation stimulates hormonal and immune responses to other toxic environmental agents.

Many nonradiation examples of hormesis can be found. In large quantities, fl uoride is deadly. In small quantities, it is a known tooth preservative.

Regardless of radiation hormesis, we continue to practice ALARA (“as low as reasonably achievable”) vigorously as a known safe approach to radiation management.

RADIATION DOSE-RESPONSE RELATIONSHIPS Radiobiology is a relatively new science. Although some scientists were working with animals to observe the effects of radiation a few years after the discovery of x-rays, these studies were not experimentally sound, nor were their results applied. With the advent of the age of the atomic bomb in the 1940s, however, interest in radiobiology increased enormously.

The object of nearly all radiobiologic research is the establishment of radiation dose-response relationships.

A radiation dose-response relationship is a mathemati- cal relationship between various radiation dose levels and magnitude of the observed response.

Radiation dose-response relationships have two important applications in radiology. First, these experi- mentally determined relationships are used to design therapeutic treatment routines for patients with cancer.

Radiobiologic studies also have been designed to yield information on the effects of low-dose irradiation. These studies and the dose-response relationships revealed provide the basis for radiation control activities and are particularly signifi cant for diagnostic radiology.

Human responses to radiation exposure fall into two types: early or late, high dose or low dose, and deter- ministic or stochastic. Deterministic radiation responses usually follow high-dose exposure and an early response. Radiation-induced skin burns represent a deterministic response.

Stochastic responses are cancer, leukemia, or genetic effects. Such responses usually follow low radiation exposure and appear as a late radiation response.

Every radiation dose-response relationship has two characteristics. It is either linear or nonlinear, and it is either threshold or nonthreshold. These characteristics can be described mathematically or graphically. This discussion avoids the math.

Linear Dose-Response Relationships Figure 33-4 shows examples of the linear dose-response relationship, which is so named because the response is directly proportionate to the dose. When the radiation

Radiation dose

A

C DB

DT DT

RN

Response

FIGURE 33-4 Linear dose-response relationships A and B are nonthreshold types; C and D are threshold types. R N is the normal incidence or response with no radiation exposure.

C H A P T E R 33 Fundamental Principles of Radiobiology 517

dose is doubled, the response to radiation likewise is doubled.

Dose-response relationships A and B intersect the dose axis at zero or below (see Figure 33-4 ). These re- lationships are therefore the linear, nonthreshold type. In a nonthreshold dose-response relationship, any dose, regardless of its size, is expected to produce a response.

At zero dose, relationship A exhibits a measurable response, R N . The level R N , called the natural response level, indicates that even without radiation exposure, that type of response, such as cancer, occurs.

Dose-response relationships C and D are identified as linear, threshold because they intercept the dose axis at some value greater than zero. The threshold dose for C and D is D T .

At radiation doses below D T , no response is expected. Relationship D has a steeper slope than C; therefore, above the threshold dose, any increment of dose produces a larger response if that response follows relationship D rather than C.

Nonlinear Dose-Response Relationships All other radiation dose-response relationships are non- linear ( Figure 33-5 ). Curves A and B are nonlinear, non- threshold. Curve A shows that a large response results from a very small radiation dose. At high dose levels, radiation is not so effi cient because an incremental dose at high levels results in less relative damage than the same incremental dose at low levels.

The dose-response relationship represented by curve B is just the opposite. Incremental doses in the low dose range produce very little response. At high doses, however, the same increment of dose produces a much larger response.

Curve C is a nonlinear, threshold relationship. At doses below D T , no response is measured. As the dose is increased to above D T , it becomes increasingly effective per increment of dose until the dose that corresponds to the infl ection point of the curve is reached. This type of dose-response relationship is characteristic of a deter- ministic response.

The infl ection point occurs when the curve stops bending up and begins bending down. Above this level, incremental doses become less effective. Relationship C is sometimes called an S-type, or sigmoid-type, radiation dose-response relationship.

We shall refer to these general types of radiation dose- response relationships when discussing the type and degree of human radiation injury. Diagnostic radiol- ogy is concerned almost exclusively with the late effects of radiation exposure and therefore, with linear, non- threshold dose-response relationships. For completeness, however, Chapter 35 briefl y discusses early radiation damage.

Constructing a Dose-Response Relationship Determining the radiation dose-response relationship for a whole-body response is tricky. It is very diffi cult to determine the degree of response, even that of early effects, because the number of experimental animals that can be used is usually small. It is nearly impossible to measure low-dose, late effects — the area of greatest interest in diagnostic imaging.

Therefore, we resort to irradiating a limited number of animals to very large doses of radiation in the hope of observing a statistically signifi cant response. Figure 33-6 shows the results of such an experiment, in which four groups of animals were irradiated to a different dose. The observations on each group result in an ordered pair of data: a radiation dose and the associated biologic response.

The error bars in each ordered pair indicate the confi - dence associated with each data point. Error bars on the dose measurements are very narrow; thus, we can measure radiation dose very accurately. Error bars on the response, however, are very wide because of biologic variability and the limited number of observations at each dose.

Skin effects resulting from high-dose fl uoroscopy follow a sigmoid-type dose-response relationship.

DT

Radiation dose

Response

Inflection point

A C

B

FIGURE 33-5 Nonlinear dose-response relationships can as- sume several shapes. Curve A is nonthreshold. Curves B and C are threshold. D T , Threshold dose.

Radiation-induced cancer, leukemia, and genetic effects follow a linear-nonthreshold dose-response relationship.

PART VI Radiobiology518

The principal interest in diagnostic imaging is to es- timate response at very low radiation doses. Because this cannot be done directly, we extrapolate the dose- response relationship from the high-dose, known region into the low-dose, unknown region.

This extrapolation invariably results in a linear, non- threshold dose-response relationship. Such an extrapo- lation, however, may not be correct because of the many qualifying conditions on the experiment.

The radiation dose-response relationship that dem- onstrates radiation hormesis appears as in Figure 33-7 . At very low doses, irradiated subjects experience less response than controls. The existence of radiation hormesis is a highly controversial topic in radiologic science. Regardless of its existence, no human radiation responses have been observed after doses less than 10 rad (100 mGyt).

SUMMARY In 1906, two French scientists fi rst theorized that radio- sensitivity was a function of the metabolic state of tissue being irradiated. Their theories, known as the Law of Bergonie and Tribondeau, state the following: (1) Stem cells are radiosensitive, mature cells are less so, (2) young tissue is more radiosensitive than older tissue, (3) high metabolic activity is radiosensitive, low metabolic rate is radioresistant, and (4) increases in proliferation and growth rates of cells make them more radiosensitive.

Physical and biologic factors affect tissue radiosensi- tivity. Physical factors include LET, RBE, fractionation (dose delivered over a long time), and protraction. Biologic factors that affect radiosensitivity include the oxygen effect, the age-related effect, and the recovery effect.

Some chemicals can modify cell response. These are called radiosensitizers and radioprotectors.

Radiobiologic research concentrates on radiation dose-response relationships. In linear dose-response re- lationships, the response is directly proportional to the dose. In nonlinear dose-response relationships, varied doses produce varied responses.

The threshold dose is the level below which there is no response. The nonthreshold dose-response relationship means that any dose is expected to produce a response. For establishing radiation protection guidelines for diag- nostic imaging, the linear, nonthreshold dose-response model is used.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. Linear energy transfer b. Standard radiation c. Oxygen enhancement ratio d. Repopulation e. Extrapolation f. Threshold dose g. Interphase death h. Dose protraction i. Radiation weighting factor j. Tribondeau 2. Write the formula for relative biologic

effectiveness. 3. Give an example of fractionated radiation.

Radiation dose

Response Extrapolation

Natural incidence

FIGURE 33-6 A dose-response relationship is produced when high-dose experimental data are extrapolated to low doses.

Radiation hormesis

Radiation dose

RN RN = Natural response

Response

FIGURE 33-7 Dose-response relationship for radiation hormesis.

C H A P T E R 33 Fundamental Principles of Radiobiology 519

4. Why is high-pressure (hyperbaric) oxygen used in radiation oncology?

5. Write the formula for the oxygen enhancement ratio. 6. How does age affect the radiosensitivity of tissue? 7. When a radiobiologic experiment is conducted in

vitro, what does this mean? 8. Name three agents that enhance the effects of

radiation. 9. Name three radioprotective agents. 10. Are radioprotective agents used for human

application? 11. Explain the meaning of a radiation dose-response

relationship. 12. What occurs in a nonlinear radiation dose-

response relationship? 13. Explain why the linear, nonthreshold dose-

response relationship is used as a model for diagnostic imaging radiation management.

14. State two of the corollaries to the law of Bergonie and Tribondeau.

15. Approximately 800 rad of 220 kVp x-rays is necessary to produce death in the armadillo. Cobalt-60 gamma rays have a lower LET than 220 kVp x-rays; therefore, 940 rad is required for armadillo lethality. What is the RBE of 60 CO compared with 220 kVp?

16. Under fully oxygenated conditions, 90% of human cells in culture will be killed by 150 rad x-rays. If cells are made anoxic, the dose required for 90% lethality is 400 rad. What is the OER?

17. What are the units of LET? 18. Describe how RBE and LET are related. 19. Is occupational radiation exposure fractionated,

protracted, or continuous? 20. Describe how OER and LET are related. The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

520

C H A P T E R

34 Molecular and Cellular Radiobiology

OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Discuss three effects of in vitro irradiation of macromolecules 2. Explain the effects of radiation on DNA 3. Identify the chemical reactions involved in the radiolysis of water 4. Describe the effects of in vivo irradiation 5. Describe the principles of target theory 6. Discuss the kinetics of cell survival following irradiation

OUTLINE Irradiation of Macromolecules

Main-Chain Scission Cross-Linking Point Lesions Macromolecular Synthesis Radiation Effects on DNA

Radiolysis of Water Direct and Indirect Effects Target Theory Cell-Survival Kinetics

Single-Target, Single-Hit Model Multitarget, Single-Hit Model Recovery

Cell-Cycle Effects LET, RBE, and OER

C H A P T E R 34 Molecular and Cellular Radiobiology 521

The effect of irradiation of macromolecules is quite different from that of irradiation of water. When mac- romolecules are irradiated in vitro, that is, outside the body or outside the cell, a considerable radiation dose is required to produce a measurable effect. Irradiation in vivo, that is, within the living cell, demonstrates that macromolecules are considerably more radiosensitive in their natural state.

IRRADIATION OF MACROMOLECULES A solution is a liquid that contains dissolved substances. A mixture of fl uids such as water and alcohol is also a solution. When macromolecules are irradiated in solu- tion in vitro, three major effects occur: main-chain scis- sion, cross-linking, and point lesions ( Figure 34-1 ).

Main-Chain Scission Main-chain scission is the breakage of the backbone of the long-chain macromolecule. The result is the reduc- tion of a long, single molecule into many smaller mol- ecules, each of which may still be macromolecular.

Main-chain scission reduces not only the size of the macromolecule but also the viscosity of the solution. A viscous solution is one that is very thick and slow to fl ow, such as cold maple syrup. Tap water, on the other hand, has low viscosity. Measurements of viscos- ity determine the degree of main-chain scission.

Cross-Linking Some macromolecules have small, spur-like side struc- tures that extend off the main chain. Others produce these spurs as a consequence of irradiation.

These side structures can behave as though they had a sticky substance on the end, and they attach to a neighboring macromolecule or to another segment of the same molecule. This process is called cross-linking. Radiation-induced molecular cross-linking increases the viscosity of a macromolecular solution.

Point Lesions Radiation interaction with macromolecules also can result in disruption of single chemical bonds, producing point lesions. Point lesions are not detectable, but they can cause a minor modifi cation of the molecule, which in turn can cause it to malfunction within the cell.

Laboratory experiments have shown that all these types of radiation effects on macromolecules are reversible through intracellular repair and recovery.

Macromolecular Synthesis Modern molecular biology has developed a generalized scheme for the function of a normal human cell. Molecular nutrients are brought to the cell and are diffused through the cell membrane, where they are broken down (catabolism) into smaller molecules with an accompanying release of energy.

This energy is used in several ways, but one of the more important ways is that they are used in the con- struction or synthesis of macromolecules from smaller molecules (anabolism). The synthesis of proteins and nucleic acids is critical to the survival of the cell and to its reproduction.

A B C

X-rays

FIGURE 34-1 The results of irradiation of macromolecules. A , Main-chain scission. B , Cross-linking. C , Point lesions.

In vitro is irradiation outside of the cell or body. In vivo is irradiation within the body.

VEN THOUGH the initial interaction between radiation and tissue occurs at the electron level, observable human radiation injury results from

change at the molecular level. The occurrence of molecular lesions is categorized into effects on macromolecules and effects on water. This chapter discusses irradiation of macromolecules and radi- olysis of water.

Because the human body is an aqueous solution that contains 80% water molecules, radiation inter- action with water is the principal radiation interac- tion in the body. However, the ultimate damage occurs to the target molecule, DNA, which controls cellular metabolism and reproduction.

E

At low radiation doses, point lesions are considered to be the cellular radiation damage that results in the late radiation effects observed at the whole-body level.

PART VI Radiobiology522

Chapter 32 describes the scheme of protein synthesis and its dependence on nucleic acids. Proteins are manufactured by translation of the genetic code from transfer RNA (tRNA), which had been transferred from messenger RNA (mRNA). The information carried by the mRNA was in turn transcribed from DNA. This chain of events is shown schematically in Figure 34-2 .

Radiation damage to any of these macromolecules may result in cell death or late effects. Proteins are continuously synthesized throughout the cell cycle and occur in much more abundance than nucleic acids. Fur- thermore, multiple copies of specifi c protein molecules are always present in the cell. Consequently, proteins are less radiosensitive than nucleic acids.

Similarly, multiple copies of both types of RNA molecules are present in the cell, although they are less abundant than protein molecules. On the other hand, the DNA molecule, with its unique assembly of bases, is not so abundant.

DNA is synthesized somewhat differently from proteins. During the G1 portion of interphase, the deoxyri- bose, phosphate, and base molecules accumulate in the nucleus. These molecules combine to form a sin- gle large molecule that, during the S portion of inter- phase, is attached to an existing single chain of DNA ( Figure 34-3 ). During G 1 , molecular DNA is in the familiar double-helix form.

As the cell moves into S phase, the ladder begins to open up in the middle of each rung, much like a zipper. Now the DNA consists of only a single chain, and no pairing of bases occurs.

This state does not exist long, however, because the combined base sugar – phosphate molecule attaches to the single-strand DNA sequence, as determined by permitted base pairing. Consequently, where one double-helix DNA molecule was present, now two similar molecules exist, each a duplicate of the original. Parent DNA is said to be replicated into two duplicate DNA daughter molecules.

Radiation Effects on DNA DNA is the most important molecule in the human body because it contains the genetic information for each cell. Each cell has a nucleus that contains DNA complexed with other molecules in the form of chromosomes. Chro- mosomes therefore control the growth and development of the cell; these in turn determine the characteristics of the individual ( Figure 34-4 ).

If radiation damage to the DNA is severe enough, visible chromosome aberrations may be detected. Figure 34-5 is a representation of a normal chromosome and sev- eral distinct types of chromosome aberrations. Radiation- induced chromosome aberrations or cytogenetic damage is discussed more completely in Chapter 35.

The DNA molecule can be damaged without the pro- duction of a visible chromosome aberration. Although

Transcription Transfer Translation

DNA duplication during S

mRNA

tRNA

Protein

FIGURE 34-2 The genetic code of DNA is transcribed by messenger RNA (mRNA) and is transferred to transfer RNA (tRNA), which translates it into a protein.

DNA is the most radiosensitive molecule.

Parent molecule

Daughter molecules

FIGURE 34-3 During S phase, the DNA separates like a zip- per and two daughter DNA molecules are formed, each alike and each a replicate of the parent molecule.

Half as much DNA is present in G 1 as in G 2 .

Metabolism consists of catabolism (the reduction of nutrient molecules for energy) and anabolism (the pro duction of large molecules for form and function).

C H A P T E R 34 Molecular and Cellular Radiobiology 523

such damage is reversible, it can lead to cell death. If enough cells of the same type respond similarly, then a particular tissue or organ can be destroyed.

Damage to the DNA also can result in abnormal metabolic activity. Uncontrolled rapid proliferation of cells is the principal characteristic of radiation- induced malignant disease. If damage to the DNA occurs within a germ cell, then it is possible that the response to radiation exposure will not be observed until the following generation, or even later.

The chromosome contains miles of DNA; therefore, when a visible aberration does appear, it signifi es a considerable amount of radiation damage. Unobserved damage to the DNA also can produce responses at cel- lular and whole-body levels. The types of damage that can occur in the DNA molecule are as follows:

The gross structural radiation response of DNA is diagrammed schematically in Figure 34-6 . Although each of these effects results in a structural change in the DNA molecule, they are all reversible. In some of these types of damage, the sequence of bases can be altered; therefore, the triplet code of codons may not remain intact. This represents a genetic mutation at the molecular level.

The fi fth type of damage, the change or loss of a base, also destroys the triplet code and may not be reversible. This type of radiation damage is a molecular lesion of the DNA. These molecular lesions are called point mutations, and they can be of minor or major importance to the cell. One critical consequence of point mutations is the transfer

of the incorrect genetic code to one of the two daughter cells. This sequence of events is shown in Figure 34-7 .

The three principal observable effects that may result from irradiation of DNA are cell death, malignant disease, and genetic damage. The latter two effects at the molecular level conform to the linear, nonthreshold dose-response relationship.

RADIOLYSIS OF WATER Because the human body is an aqueous solution that contains approximately 80% water molecules, irradia- tion of water represents the principal radiation interac- tion in the body. When water is irradiated, it dissociates into other molecular products; this action is called radi- olysis of water ( Figure 34-8 ).

When an atom of water (H 2 O) is irradiated, it is ionized and dissociates into two ions — an ion pair, as shown by the following:

DNA Chromosome Cell Human

FIGURE 34-4 DNA is the target molecule for radiation dam- age. It forms chromosomes and controls cell and human growth and development.

A B C D

FIGURE 34-5 Normal and radiation-damaged human chro- mosomes. A, Normal. B, Terminal deletion. C, Dicentric for- mation. D, Ring formation.

A B C D

FIGURE 34-6 Types of damage that can occur in DNA. A, One side rail severed. B, Both side rails severed. C, Cross-linking. D, Rung breakage.

RADIATION RESPONSE OF DNA

• Main-chain scission with only one side rail severed

• Main-chain scission with both side rails severed • Main-chain scission and subsequent cross-linking • Rung breakage causing a separation of bases • Change in or loss of a base

IONIZATION

H O HOH e2 + + + −↑ →

PART VI Radiobiology524

After this initial ionization, a number of reactions can happen. First, the ion pair may rejoin into a stable water molecule. In this case, no damage occurs. Second, if these ions do not rejoin, it is possible for the negative ion (the electron) to attach to another water molecule through the following reaction to produce yet a third type of ion.

The HOH and HOH − ions are relatively unstable and can dissociate into still smaller molecules as follows:

The fi nal result of the radiolysis of water is the forma- tion of an ion pair, H + and OH − , and two free radicals, H * and OH * . The ions can recombine; therefore, no biologic damage would occur.

These types of ions are not unusual. Many molecules in aqueous solution exist in a loosely ionized state because of their structure. Salt (NaCl), for instance, easily dissociates into Na + and Cl − ions. Even in the absence of radiation, water can dissociate into H + and OH − ions.

Free radicals are another story. They are highly reactive. Free radicals are unstable and therefore exist with a life- time of less than 1 ms. During that time, however, they are capable of diffusion through the cell and interaction at a distant site. Free radicals contain excess energy that can be transferred to other molecules to disrupt bonds and produce point lesions at some distance from the initial ionizing event.

The H* and OH* molecules are not the only free rad- icals that are produced during the radiolysis of water. The OH* free radical can join with a similar molecule to form hydrogen peroxide.

Hydrogen peroxide is poisonous to the cell and there- fore acts as a toxic agent.

The H* free radical can interact with molecular oxygen to form the hydroperoxyl radical as follows:

The hydroperoxyl radical, along with hydrogen perox- ide, is considered to be the principal damaging product after the radiolysis of water. Hydrogen peroxide also can be formed by the interaction of two hydroperoxyl radicals as follows:

Codon

Codon

Abnormal codon

Normal codon

FIGURE 34-7 A point mutation results in the change or loss of a base, which creates an abnormal gene. This is therefore a genetic mutation that is passed to one of the daughter cells.

Water H2O

Ions HOH+, HOH–

Ions H+, OH–

Free radicals OH*, H*

e–

FIGURE 34-8 The radiolysis of water results in the formation of ions and free radicals.

A free radical is an uncharged molecule that contains a single unpaired electron in the outer shell.

HYDROGEN PEROXIDE

OH OH H O2 2 ∗ ∗+ →

HYDROPEROXYL FORMATION

H O HO2 2 ∗ ∗+ →

ADDITIONAL IONIZATION

H O e HOH2 + − −→

DISSOCIATION

HOH H OH

HOH OH H

+ + ∗

− − ∗

+

+

→

→

C H A P T E R 34 Molecular and Cellular Radiobiology 525

Some organic molecules, symbolized as RH, can become reactive free radicals as follows:

When oxygen is present, yet another species of free radi- cal is possible as follows:

Free radicals are energetic molecules because of their unique structure. This excess energy can be transferred to DNA, and this can result in bond breaks.

DIRECT AND INDIRECT EFFECTS When biologic material is irradiated in vivo, the harm- ful effects of irradiation occur because of damage to a particularly sensitive molecule, such as DNA. Evidence for the direct effect of radiation comes from in vitro experiments wherein various molecules can be irradi- ated in solution. The effect is produced by ionization of the target molecule.

On the other hand, if the initial ionizing event occurs on a distant, noncritical molecule, which then trans- fers the energy of ionization to the target molecule, indirect effect has occurred. Free radicals, with their excess energy of reaction, are the intermediate mol- ecules. They migrate to the target molecule and trans- fer their energy, which results in damage to that target molecule.

It is not possible to identify whether a given interaction with the target molecule resulted from direct or indirect effect. However, because the human body consists of approximately 80% water and less than 1% DNA, it is concluded that essentially all of the effects of irradiation in vivo result from indirect effect. When oxygen is present, as in living tissue, the indirect effects are amplifi ed because of the additional types of free radicals that are formed.

TARGET THEORY The cell contains many species of molecules, most of which exist in overabundance. Radiation damage to such molecules probably would not result in noticeable injury to the cell because similar molecules would be available to continue to support the cell.

On the other hand, some molecules in the cell are considered to be particularly necessary for normal cell function. These molecules are not abundant; in fact, there may be only one such molecule. Radiation damage to such a molecule could affect the cell severely because no similar molecules would be available as substitutes.

This concept of a sensitive key molecule serves as the basis for the target theory. According to the target the- ory, for a cell to die after radiation exposure, its target molecule must be inactivated ( Figure 34-9 ).

The key molecular target is the DNA. Originally, the target theory was used to represent cell lethality. It can be used equally well, however, to describe nonlethal radiation-induced cell abnormalities.

In the target theory, the target is considered to be an area of the cell occupied by the target molecule or by a sensitive site on the target molecule. This area changes position with time because of intracellular molecular movement.

The interaction between radiation and cellular com- ponents is random; therefore, when an interaction does occur with a target, it occurs randomly. No favoritism is seen in radiation to the target molecule. Its sensitivity to radiation occurs simply because of its vital function in the cell.

If the initial ionizing event occurs on the target molecule, the effect of radiation is direct.

The principal effect of radiation on humans is indirect.

Cell death

No effect

Target molecule

FIGURE 34-9 According to target theory, cell death will occur only if the target molecule is inactivated. DNA, the target molecule, is located within the cell nucleus.

DNA is the target molecule.

HYDROGEN PEROXIDE FORMATION

HO HO H O O2 2 2 2 2 ∗ ∗+ +→

ORGANIC FREE RADICAL FORMATION

RH RH H R+ ↑ +∗ ∗ ∗→ →

ORGANIC FREE RADICAL FORMATION

R O RO2 2 ∗ ∗+ →

PART VI Radiobiology526

When radiation does interact with the target, a hit is said to have occurred. Radiation interaction with mol- ecules other than the target molecule also can result in a hit. It is not possible to distinguish between a direct and an indirect hit.

When a hit occurs through indirect effect, the size of the target appears considerably larger because of the mobility of the free radicals. This increased target size contributes to the importance of the indirect effect of radiation.

Figure 34-10 illustrates some of the consequences of using target theory to explain the relationships among linear energy transfer (LET), the oxygen effect (oxygen enhancement ratio [OER]), and direct versus indirect effect. With low-LET radiation, in the absence of oxygen, the probability of a hit on the target molecule is low because of the relatively large distances between ionizing events.

If oxygen is present, free radicals are formed and the volume of effectiveness surrounding each ionization is enlarged. Consequently, the probability of a hit is increased.

When high-LET radiation is used, the distance between ionizations is so close that the probability of a hit by direct effect is high. When oxygen is added to the system and high-LET radiation is used, the added sphere of infl uence for each ionizing event, although

somewhat larger, does not result in additional hits. The maximum number of hits has already been produced by direct effect with high-LET radiation.

CELL-SURVIVAL KINETICS Early radiation experiments at the cell level were con- ducted with simple cells, such as bacteria. It was not until the middle 1950s that laboratory techniques were developed to allow the growth and manipulation of human cells in vitro.

One technique for measuring the lethal effects of radiation on cells is shown in Figure 34-11 . If normal cells are planted individually in a Petri dish and are incubated for 10 to 14 days, they divide many times and produce a visible colony that consists of many cells. This is cell cloning.

After irradiation of such single cells, some do not sur- vive and, therefore, fewer colonies are formed. A higher radiation dose leads to the formation of fewer colonies.

When a mathematical extension of target theory is used, two models of cell survival result. The single-target, single-hit model applies to biologic targets, such as enzymes, viruses, and simple cells like bacteria. The multitarget, single-hit model applies to more compli- cated biologic systems, such as human cells.

The following discussion concerns the equation of these models. The mathematics of these models is relatively unimportant but is given here for the interested student.

Single-Target, Single-Hit Model Consider for a moment the situation illustrated in Figure 34-12 . It is raining on a large concrete runway that contains 100 squares. A square is considered wet when one or more raindrops have fallen on it.

Hits occur through both direct and indirect effects.

Cytoplasm Nucleus Low LET Direct effect

Target molecule

Low LET Indirect effect

High LET Direct effect

High LET Indirect effect

FIGURE 34-10 In the presence of oxygen, the indirect effect is amplifi ed and the volume of action for low – linear energy transfer (LET) radiation is enlarged. The effective volume of ac- tion for high-LET radiation remains unchanged, in that maxi- mum injury will have been infl icted by direct effect.

No radiation Moderate dose High dose

Petri dish

Single cells Cell

colonies

FIGURE 34-11 When single cells are planted in a Petri dish, they grow into visible colonies. Fewer colonies develop if the cells are irradiated.

The lethal effects of radiation are determined by observing cell survival, not cell death.

C H A P T E R 34 Molecular and Cellular Radiobiology 527

When the fi rst drop falls on the pavement, 1 of the 100 squares becomes wet. When the second drop falls, it will probably fall on a dry square and not on the one already wet. Consequently, 2 out of 100 squares will be wet.

When the third raindrop falls, there will probably be 3 wet and 97 dry squares. As the number of raindrops increases, however, it becomes more probable that a given square will be hit by 2 or more drops.

Because the raindrops are falling randomly, the prob- ability that a square will become wet is governed by a statistical law called the Poisson distribution. According to this law, when the number of raindrops is equal to the number of squares (100 in this case), 63% of the squares will be wet and 37% of the squares will be dry. If the raindrops had fallen uniformly, all 100 squares would become wet with 100 raindrops.

Obviously, many of the 63 squares in this example have been hit twice or more. When the number of raindrops equals twice the number of squares, then 14 squares will be dry. After 300 raindrops, only 5 squares will remain dry.

Examine a graph of the number of dry squares as a function of the number of raindrops ( Figure 34-13 ). If the number of squares exposed to the rain was large or unknown, the scale on the right, expressed in percent, would be used.

The wet squares analogy can be extended to the irradiation of a large number of biologic specimens — for example, 1000 bacteria. Bacteria presumably contain a single sensitive site, or target, that must be inactivated for the cell to die. As 1000 bacteria are irradiated with increasing increments of dose, a greater number are killed ( Figure 34-14 ).

Just as with the wet squares, however, as the dose increases, some cells will suffer two or more hits. All hits per target in excess of one represent wasted radia- tion dose because the bacteria had been killed already by the fi rst hit.

When the radiation dose reaches a level suffi cient to kill 63% of the cells (37% survival), it is called D 37 . Follow- ing a dose equal to 2 × D 37 , 14% of the cells would sur- vive, and so forth. D 37 is a measure of the radiosensitivity of the cell. A low D 37 indicates a highly radiosensitive cell, and a high D 37 reveals radioresistance.

The equation that describes the dose-response rela- tionship represented by the graph in Figure 34-14 is

DryWet

FIGURE 34-12 When rain falls on a dry pavement that con- sists of a large number of squares, the number of squares that remains dry decreases exponentially as the number of rain- drops increases.

Radiation interacts randomly with matter.

100

37

10

1 0 100 200 300 400

Number of raindrops

Number of dry

squares

1

10

100

Percentage of dry

squares

FIGURE 34-13 When the number of dry squares is plotted on semilogarithmic paper as a function of the number of rain- drops, a straight line results, because when a few drops fall, some squares will be hit more than once.

A hit is not simply an ionizing event, but rather an ionization that inactivates the target molecule.

If there were no wasted hits (uniform interaction), D 37 is the dose that would be suffi cient to kill 100% of the cells.

PART VI Radiobiology528

the single-target, single-hit model of radiation-induced lethality as follows:

Multitarget, Single-Hit Model Returning to the wet squares analogy, suppose that each pavement square were divided into two equal parts, two targets ( Figure 34-15 ). By defi nition, each half now must be hit with a raindrop for the square to be consid- ered wet. The fi rst few raindrops probably will hit only one half of any given square; therefore, after a very light rain, no squares may be wet.

Many raindrops must fall before any single square suffers a hit in both halves so that it can be considered wet. This represents a threshold because, according to our defi nition, a number of raindrops can fall and all squares will remain dry. As the number of rain- drops increases, eventually some squares will have both halves hit and therefore will be considered wet.

This portion of the curve is represented by region A in Figure 34-16 .

When a large number of raindrops have fallen, region C will be reached, where every square will be at least half wet. When this occurs, each additional raindrop will produce a wet square. In region C, the

1000

100

10

0 Radiation dose

Number of

surviving cells

Percentage of

survival

D37 1

100

37

10

1

0.1

FIGURE 34-14 After irra diation of 1000 cells, the dose-response relationship is exponential. The D37 is that dose that results in 37% survival.

SINGLE-TARGET, SINGLE-HIT MODEL

S N N e D D= = −/ /0 37 where S is the surviving fraction, N is the number of cells surviving a dose D, N 0 is the initial number of cells, and D 37 is a constant dose related to cell radiosensitivity.

Dry squares

Wet square

FIGURE 34-15 If each pavement square has two equal parts, each part must be hit for the square to be considered wet.

C H A P T E R 34 Molecular and Cellular Radiobiology 529

relation between number of raindrops and wet squares is that described by the single-target, single-hit model. The intermediate region B is the region of accumula- tion of hits.

Complex biologic specimens such as human cells are thought to have more than a single critical target. Sup- pose that the human cell has two targets, each of which has to be inactivated for the cell to die. This would be analogous to the square having two halves, each of which had to be hit by rain for it to be considered wet. Figure 34-17 is a graph of single-cell survival for human cells that have two targets.

At very low radiation doses, cell survival is nearly 100%. As the radiation dose increases, fewer cells survive because more sustain a hit in both target molecules.

At a high radiation dose, all cells that survive have one target hit. Therefore, at still higher doses, the dose- response relationship would appear as the single-target, single-hit model.

The model of cell survival just described is the multi- target, single-hit model as follows:

The D 0 is called the mean lethal dose and is a constant related to the radiosensitivity of the cell. It is equal to D 37 in the linear portion of the graph and therefore represents the dose that would result in one hit per tar- get in the straight-line portion of the graph if no radia- tion were wasted.

The extrapolation number is also called the target number. When this type of experiment was fi rst con- ducted with human cells, the observed extrapolation number was 2. That result agreed with the hypothesis that similar regions on two homologous chromosomes (an identical pair) had to be inactivated to produce cell death. Because chromosomes come in pairs, the experi- mental results confi rmed the hypothesis.

Subsequent experiments, however, have resulted in extrapolation numbers ranging from 2 to 12, and there- fore the precise meaning of n is unknown.

The D Q is called the threshold dose. It is a mea- sure of the width of the shoulder of the multitarget, single-hit model and is related to the capacity of the cell to recover from sublethal damage. Table 34-1 lists reported values for D 0 and D Q for various experimen- tal cell lines.

A B

C

Number of raindrops

Percentage of dry

squares

0.1

1

10

100

FIGURE 34-16 When a square contains two equal parts, both of which have to be hit to be considered wet, three regions of the dry square versus raindrops relationship can be identifi ed.

Radiation dose

DQ

D0 Percentage

of surviving cells (N/N0)

n

0.1

0.037

0.001

0.01

0.1

1

2

FIGURE 34-17 The multitarget, single-hit model of cell sur- vival is characteristic of human cells that contain two targets.

MULTITARGET, SINGLE-HIT MODEL

S N N eD D n= = − −/ ( )/0 01 1 where S is the surviving fraction, N is the number of cells surviving a dose D, N 0 is the initial number of cells, D 0 is the dose necessary to reduce survival to 37% in the straight-line portion of the graph, and n is the extrapolation number.

A large D 0 indicates radioresistant cells, and a small D 0 is characteristic of radiosensitive cells.

A large D Q indicates that the cell can recover readily from sublethal radiation damage.

PART VI Radiobiology530

Recovery The shoulder of the graph of the multitarget, single- hit model shows that for mammalian cells, some damage must be accumulated before the cell dies. This accumulated damage is called sublethal damage. The wider the shoulder, the more sublethal damage that can be sustained and the higher the value of D Q .

Figure 34-18 demonstrates the results of a split-dose irradiation designed to describe the capacity of a cell to recover from sublethal damage. This illustration shows a rather typical human cell survival curve with D 0 = 160 rad (1.6 Gyt), D Q = 110 rad (1.1 Gy t ), and n = 2. If one takes those cells that survive any large dose (e.g., 470 rad [4.7 Gyt]) and reincubates them in a growth medium, they will grow into another large population.

This new population of cells then can be used to per- form a second cell survival experiment. When the cells that survived the fi rst dose are subsequently subjected to additional incremental radiation doses, a second dose- response curve is generated that has precisely the same shape as the fi rst.

After such a split occurs, the extrapolation number is the same and the mean lethal dose is the same, and the sec- ond dose-response curve is separated along the dose axis from the fi rst dose-response curve by D Q . For full recovery to occur, the time between such split doses must be at least as long as the cell generation time, usually 24 hours.

Such experiments show that cells that survive an initial radiation insult exhibit precisely the same char- acteristics as nonirradiated cells; therefore, the surviving cells have fully recovered from the sublethal damage produced by the initial irradiation.

Question: From Figure 34-18 , estimate the overall surviving fraction for a cell receiving a split dose of 400 rad followed by 400 rad (4 Gy t ).

Answer: At a dose of 400 rad, approximately 0.15 of the cells survive. Therefore, at a split dose of 400 rad and 400 rad, the surviving fraction should equal 0.15 × 0.15 = 0.023. The total dose is 800 rad (8 Gy t ), and the surviving fraction on the split-dose curve at 800 rad should equal 0.023, and it does. Had the 800 rad been delivered at one time, the surviving fraction would have been 0.012, as is shown by the single-dose curve of Figure 34-18 .

CELL-CYCLE EFFECTS When human cells replicate by mitosis, the average time from one mitosis to another is called the cell-cycle time or the cell generation time. Most human cells that are in a state of normal proliferation have generation times of approximately 24 hours.

Some specialized cells have generation times that extend to hundreds of hours, and other cells, such as neurons (nerve cells), do not normally replicate. Longer generation times primarily result from lengthening of the G 1 phase of the cell cycle.

n

DQ = 110 rad

DO = 630-470 = 160 rad

0 200 400 600 800 1000 1200

Radiation dose

Percentage of surviving cells

N/NO

0.001

0.01

0.1

1.0

DQ = 110 rad

FIGURE 34-18 Split-dose irradiation results in a second cell survival curve with the same characteristics as the fi rst and dis- placed along the dose axis by DQ.

Cell Type D 0 (rad) D Q (rad)

Mouse oocytes 91 62 Mouse skin 135 350 Human bone

marrow 137 100

Human fi broblasts

150 160

Mouse spermatogonia

180 270

Chinese hamster ovary

200 210

Human lymphocytes

400 100

TABLE 34-1 Doses for Various Experimental Mammalian Cell Lines

D 0 , Mean lethal dose; D Q , threshold dose.

D Q is a measure of the capacity to accumulate sublethal damage and the ability to recover from sublethal damage.

G 1 is the most time variable of cell phases.

C H A P T E R 34 Molecular and Cellular Radiobiology 531

A randomly growing population of cells that are uni- formly distributed in position throughout the cell cycle can be synchronized in various ways. A population of synchronized cells then can be subdivided into smaller populations and irradiated sequentially as they pass through the phases of the cell cycle.

Figure 34-19 represents results obtained from human fi broblasts. The fraction of cells that survive a given dose can vary by a factor of 10 from the most sensitive to the most resistant phase of the cell cycle.

This pattern of change in radiosensitivity as a func- tion of phase in the cell cycle is the age-response func- tion, and it varies among cells. Cells in mitosis are always most sensitive. The fraction of surviving cells is lowest in this phase. The next most sensitive phase of the cell cycle occurs at the G 1 -S transition. The most resistant portion of the cell cycle is the late S phase.

LET, RBE, AND OER Mammalian cell survival experiments have been used extensively to measure the effects of various types of radiation and to determine the magnitude of various dose-modifying factors, such as oxygen. Because the mean lethal dose, D 0 , is related to radiosensitivity, the ratio of D 0 for one condition of irradiation compared with another is a measure of the effectiveness of the dose modifi er, whether it is physical or biologic.

If the same cell type is irradiated by two different radiations under identical conditions, results may appear as in Figure 34-20 . At very high LET (as with alpha particles and neutrons), cell-survival kinetics follow the single-target, single-hit model. With low-LET radiation (x-rays), the multitarget, single-hit model applies.

The mean lethal dose after low-LET irradiation is always greater than that after high-LET irradiation. If the low-LET D 0 represents x-rays, then the ratio of one D 0 to another equals the relative biologic effectiveness (RBE) for the high-LET radiation as follows:

Question: Figure 34-20 shows the radiation dose- response relationship of human fi broblasts exposed to x-rays and those exposed to 14 MeV neutrons. The D 0 after x-radiation is 170 rad (1.7 Gyt); the D 0 for neutron irradiation is 100 rad (1 Gy t ). What is the RBE of 14 MeV neutrons relative to x-rays?

Answer:

RBE rad rad

= = 170 100

1 7.

The most completely studied dose modifier is oxy- gen. The presence of oxygen maximizes the effect of low-LET radiation. When anoxic cells are exposed, a considerably higher dose is required to produce a given effect.

M

S

G2G1

M S MG1 G2

Surviving fraction

0.001

0.01

0.1

500 rad

FIGURE 34-19 The age response of human fi broblasts after irradiation shows minimum survival during the M phase and maximum survival during the late S phase. Such cells are most radiosensitive during mitosis and most radioresistant during the late S phase.

D0 = 100 rad

Low LET (x-rays)

D0 = 170 rad

High LET (neutrons)

N/N0

Radiation dose 1000 12008006004002000

0.001

0.01

0.1

1.0

10

FIGURE 34-20 Representative cell-survival curves after expo- sure to 200 kVp x-rays and 14 MeV neutrons.

Human cells are most radiosensitive in M and most radioresistant in late S.

Irradiation of mammalian cells with high-LET radiation follows the single-target, single-hit model.

RELATIVE BIOLOGIC EFFECTIVENESS RBE

D x radiation to produce an effect D test radiatio

= −0

0

( ) ( nn to produce the same effect)

PART VI Radiobiology532

With high-LET radiation, little difference is noted between the response of oxygenated cells and that of anoxic cells. Figure 34-21 shows typical cell-survival curves for each of these combinations of LET and oxygen.

Such experiments are designed to measure the magnitude of the oxygen effect. The OER determined from single-cell survival experiments is defi ned as follows:

Question: With reference to Figure 34-21 , what is the estimated OER for human cells exposed to low-LET radiation and to high- LET radiation?

Answer:

Low LET no oxygen D rad

Low LET oxygen D rad

OER

0

0

,

,

= =

340

140

== =

=

340 140

2 4

90

rad rad

High LET no oxygen, D rad

High LET 0

.

,

,,

.

oxygen, D rad

OER rad rad

0 =

= =

70

90 70

1 3

The interrelationships among LET, RBE, and OER are complex. However, it is LET that determines the magnitude of RBE and OER.

SUMMARY When macromolecules are irradiated in vitro, three major effects occur: (1) main-chain scission, (2) cross-linking, and (3) disruption of single chemical bonds in a macro- molecule, causing point lesions. All three types of damage are reversible through intracellular repair and recovery.

DNA, with its unique assembly of bases, is not abun- dant in the cell. As a result, DNA is the most radiosensi- tive of all macromolecules. Chromosome aberrations or abnormal metabolic activity can result from DNA dam- age. DNA irradiation has three observable effects: cell death, malignant disease, and genetic damage.

Because the human body is 80% water, irradiation of water is the principal interaction that occurs in the body. Water dissociates into free radicals that are highly reactive and can diffuse through the cell to cause dam- age at some distance.

The initial ionizing event is said to be a direct effect if the interaction occurs with a DNA molecule. If the ion- izing event occurs with water and transfers that energy to DNA, the event is said to be an indirect effect.

The concept of a sensitive key molecule within a cell serves as the basis for the target theory. For a cell to die after radiation exposure, the target molecule, DNA, must be inactivated.

Radiation exposure results in two models of cell survival. The single-target, single-hit model applies to simple cells such as bacteria. The multitarget, single- hit model implies a dose threshold. However, at higher doses, the relationship becomes a single-hit, single- target model. Experiments in cell recovery show that cells can recover from sublethal radiation damage.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. In vitro b. Cytogenetic damage c. Point mutation d. Free radical e. Target theory f. D 37 g. Mean lethal dose h. Radiation hit i. Extrapolation number j. D Q 2. List the effects of irradiation of macromolecules in

solution in vitro. 3. How is solution viscosity used to determine the

degree of radiation macromolecular damage? 4. What is the difference between catabolism and

anabolism? 5. In what phase of the cell cycle does the DNA lad-

der open up in the middle of each rung and consist of only a single chain?

Low LET

No oxygen

Oxygen High LET

N/N0

Radiation dose

180016001400120010008006004002000 0.001

0.01

0.1

1.0

FIGURE 34-21 Cell-survival curves for human cells irradi- ated in the presence and the absence of oxygen with high- and low – linear energy transfer (LET) radiation.

OXYGEN ENHANCEMENT RATIO

OER

D anoxic to produce an effect D oxygenated to pro

= 0 0

( ) ( ) dduce the same effect

C H A P T E R 34 Molecular and Cellular Radiobiology 533

6. Name the three principal observable effects of DNA irradiation.

7. Differentiate between transcription, transfer, and translation when applied to molecular genetics.

8. Draw a diagram that illustrates the point mutations of DNA that transfer the incorrect genetic code to one of the two daughter cells.

9. Write the formula for radiolysis of water in which the atom of water is ionized and dissociates into two ions.

10. What happens to radiation-induced free radicals within the cell?

11. What is the target theory of radiobiology? 12. Does radiation interact with tissue uniformly or

randomly? 13. Draw cell-survival curves to show the difference

between irradiation with low LET and high-LET radiation.

14. What is the difference between in vitro and in vivo?

15 Complete the following chemical equations:

H O Radiation

HOH dissociation

HOH dissociation

2 + →

→ →

?

? ?

+

−

( )

( ) 16. The D 37 of a cellular species that follows the

single-target, single-hit model is 150 rad. What percentage of cells will survive 450 rad?

17. What is the RBE of alpha radiation if the D 0 is 40 rad, compared with 180 rad for x-rays?

18. What is the difference between direct effect and indirect effect?

19. How does the radiosensitivity of human cells vary with stages of the cell cycle?

20. Draw cell-survival curves to show the difference between low-LET irradiation of aerobic cells and anoxic cells.

The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com.

534

C H A P T E R

35 Early Effects of Radiation OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Describe the three acute radiation syndromes 2. Identify the two stages that lead to acute radiation lethality 3. Defi ne LD 50/60 4. Discuss local tissue damage after high-dose irradiation 5. Review the cytogenetic effects of radiation exposure

OUTLINE Acute Radiation Lethality

Prodromal Period Latent Period Manifest Illness LD 50/60 Mean Survival Time

Local Tissue Damage Effects on Skin Effects on Gonads

Hematologic Effects Hemopoietic System Hemopoietic Cell Survival

Cytogenetic Effects Normal Karyotype Single-Hit Chromosome Aberrations Multi-Hit Chromosome Aberrations Kinetics of Chromosome Aberration

C H A P T E R 35 Early Effects of Radiation 535

ACUTE RADIATION LETHALITY Death, of course, is the most devastating human response to radiation exposure. No cases of death after diagnos- tic x-ray exposure have ever been recorded, although some early x-ray pioneers died from the late effects of x-ray exposure. In each of these cases, however, the total radiation dose was extremely high by today's standards.

Acute radiation-induced human lethality is of only academic interest in diagnostic radiology. Diagnostic x-ray beams are neither intense enough nor large enough to cause death.

Some accidental exposures of persons in the nuclear weapons and nuclear energy fi elds have resulted in immediate death, but the number of such accidents has been small considering the length and activity of the atomic age. The unfortunate incident at Chernobyl in April 1986 is the one notable exception.

Thirty people at Chernobyl experienced the acute radiation syndrome and died. A number of minor late effects have been observed. No one died or was even seri- ously exposed in the March 1979 incident at the nuclear power reactor at Three Mile Island, Pennsylvania. Employment in the nuclear power industry is a safe occupation.

The sequence of events that follow high-level radia- tion exposure leading to death within days or weeks is called the acute radiation syndrome. There are, in fact, three separate syndromes that are dose related and that follow a rather distinct course of clinical responses.

These syndromes are hematologic death, gastroin- testinal (GI) death, and central nervous system (CNS) death. The clinical signs and symptoms of each are outlined in Table 35-2 . CNS death requires radiation doses in excess of 5000 rad (50 Gy t ) and results in death within hours. Hematologic death and GI death follow lower exposures and require a longer time for death to occur.

In addition to the three lethal syndromes, two periods are associated with acute radiation lethality. The pro- dromal period consists of acute clinical symptoms that occur within hours of exposure and continue for up to a day or two. After the prodromal period has ended, there may be a latent period, during which the subject is free of visible effects.

D URING THE 1920s and the 1930s, it would not have been unusual for a radiologic technolo- gist to visit the hematology laboratory once

a week for a routine blood examination. Before the introduction of personnel radiation monitors, periodic blood examination was the only way to monitor x-ray workers.

There was great concern over the danger of occupational radiation exposure. Today's occupa- tional radiation exposures are quite low. Still, the radiologic technologist must understand the early effects of high radiation doses.

This chapter explores such early effects from the most severe (death) to the most worrisome today (skin effects). The chapter also reviews hematologic and cytogenetic effects.

To produce a radiation response in humans within a few days to months, the dose must be substantial. Such a response is called an early effect of radiation exposure. A dose of this magnitude is rare in diag- nostic radiology.

These early effects have been studied exten- sively with laboratory animals, and some data have been obtained from observations of humans. This chapter considers only the more important effects as identifi ed in Table 35-1 , along with the minimum radiation dose necessary to produce each.

Early radiation responses are described as deterministic. Deterministic radiation responses are those that exhibit increasing severity with increas- ing radiation dose. Furthermore, there is usually a dose threshold.

D TABLE 35-1 Principal Early Effects of Radiation

Exposure on Humans and the Approximate Threshold Dose

Effect Anatomic Site Threshold Dose

Death Whole body 200 rad/2 Gy t Hematologic

depression Whole body 25 rad/250 mGy t

Skin erythema Small fi eld 200 rad/2 Gy t Epilation Small fi eld 300 rad/3 Gy t Chromosome

aberration Whole body 5 rad/50 mGy t

Gonadal dysfunction

Local tissue 10 rad/100 mGy t

Diagnostic x-ray beams always result in partial-body exposure, which is less harmful than whole-body exposure.

PART VI Radiobiology536

Prodromal Period At radiation doses above approximately 100 rad (1Gy t ) delivered to the total body, signs and symptoms of radiation sickness may appear within minutes to hours. The symptoms of early radiation sickness most often take the form of nausea, vomiting, diarrhea, and a reduction in the white cells of the peripheral blood (leukopenia).

The prodromal period may last from a few hours to a couple of days. The severity of the symptoms is dose related; at doses in excess of 1000 rad (10 Gy t ), symp- toms can be violent. At still higher doses, the duration of the prodromal syndrome becomes shorter, until it is difficult to separate the prodromal syndrome from the period of manifest illness.

Latent Period After the period of initial radiation sickness, a period of apparent well-being occurs, which is called the la- tent period. The latent period extends from hours or less (at doses in excess of 5000 rad) to weeks (at doses from 100 to 500 rad).

The latent period is sometimes mistakenly thought to indicate an early recovery from a moderate radiation dose. It may be misleading, however, because it gives no indication of the extensive radiation response yet to follow.

Manifest Illness The dose necessary to produce a given syndrome and the mean survival time are the principal quantita- tive measures of human radiation lethality (see Table 35-2 ). Although ranges of dose and resultant mean

survival times are given, there is rarely a precise dif- ference in the dose and time-related sequence of events associated with each syndrome. At very high radia- tion doses, the latent period disappears altogether. At very low radiation doses, there may be no prodromal period at all.

Hematologic Syndrome. Radiation doses in the range of approximately 200 to 1000 rad (2 to 10 Gy t ) produce the hematologic syndrome. The patient initially experiences mild symptoms of the prodromal syndrome, which appear in a matter of a few hours and may persist for several days.

The latent period that follows can extend as long as 4 weeks and is characterized by a general feeling of well- ness. There are no obvious signs of illness, although the number of cells in the peripheral blood declines during this time.

The period of manifest illness is characterized by pos- sible vomiting, mild diarrhea, malaise, lethargy, and fever. Each of the types of blood cells follows a rather characteristic pattern of cell depletion. If the dose is not lethal, recovery begins in 2 to 4 weeks, but as long as 6 months may be required for full recovery.

If the radiation injury is severe enough, the reduc- tion in blood cells continues unchecked until the body's defense against infection is nil. Just before death, hem- orrhage and dehydration may be pronounced. Death occurs because of generalized infection, electrolyte imbalance, and dehydration.

Gastrointestinal (GI) Syndrome. Radiation doses of approximately 1000 to 5000 rad (10 to 50 Gy t ) result in the GI syndrome . The prodromal symptoms of vomiting and diarrhea occur within hours of exposure and persist for hours to as long as a day. A latent period of 3 to 5 days follows, during which no symptoms are present.

The manifest illness period begins with a second wave of nausea and vomiting, followed by diarrhea.

TABLE 35-2 Summary of Acute Radiation Lethality

Period Approximate Dose (rad ) Mean Survival Time (days) Clinical Signs and Symptoms

Prodromal >100 — Nausea, vomiting, diarrhea Latent 100 to 10,000 — None Hematologic 200 to 1000 10 to 60 Nausea, vomiting, diarrhea, anemia,

leukopenia, hemorrhage, fever, infection Gastrointestinal 1000 to 5000 4 to 10 Same as hematologic plus electrolyte

imbalance, lethargy, fatigue, shock Central nervous

system >5000 0 to 3 Same as gastrointestinal plus ataxia,

edema, system vasculitis, meningitis

The latent period is the time after exposure during which there is no sign of radiation sickness.

This immediate response of radiation sickness is the prodromal period.

The hematologic syndrome is characterized by a reduction in white cells, red cells, and platelets.

C H A P T E R 35 Early Effects of Radiation 537

The victim experiences a loss of appetite (anorexia) and may become lethargic. The diarrhea persists and becomes more severe, leading to loose and then watery and bloody stools. Supportive therapy cannot prevent the rapid progression of symptoms that ultimately leads to death within 4 to 10 days of exposure.

Intestinal cells are normally in a rapid state of prolifera- tion and are continuously being replaced by new cells. The turnover time for this cell renewal system in a nor- mal person is 3 to 5 days.

Radiation exposure kills the most sensitive cells — stem cells; this controls the length of time until death. When the intestinal lining is completely denuded of functional cells, fl uids pass uncontrollably across the intestinal membrane, electrolyte balance is destroyed, and conditions promote infection.

At doses consistent with the GI syndrome, measur- able and even severe hematologic changes occur. It takes a longer time for the cell renewal system of the blood to develop mature cells from the stem cell population; therefore, there is not enough time for maximum hema- tologic effects to occur.

Central Nervous System (CNS) Syndrome. After a radiation dose in excess of approximately 5000 rad (50 Gy t ) is received, a series of signs and symptoms occur that lead to death within a matter of hours to days. First, severe nausea and vomiting begins, usually within a few minutes of exposure.

During this initial onset, the patient may become extremely nervous and confused, may describe a burning sensation in the skin, may lose vision, and can even lose consciousness within the fi rst hour. This may be followed by a latent period that lasts up to 12 hours, during which earlier symptoms subside or disappear.

The latent period is followed by the period of mani- fest illness, during which symptoms of the prodromal stage return but are more severe. The person becomes disoriented; loses muscle coordination; has diffi culty breathing; may go into convulsive seizures; experiences loss of equilibrium, ataxia, and lethargy; lapses into a coma; and dies.

Regardless of the medical attention given the patient, the symptoms of manifest illness appear rather suddenly and always with extreme severity. At radiation doses high enough to produce CNS effects, the outcome is always death within a few days of exposure.

The CNS syndrome is characterized by increased intra- cranial pressure, infl ammatory changes in the blood ves- sels of the brain (vasculitis), and infl ammation of the meninges (meningitis). At doses suffi cient to produce CNS damage, damage to all other organs of the body is equally severe. The classic radiation-induced changes in the GI tract and the hematologic system cannot occur because there is insuffi cient time between exposure and death for them to appear.

LD 50/60 If experimental animals are irradiated with varying doses of radiation, for example, 100 to 1000 rad (1 to 10 Gy t ), the plot of the percentage that die as a function of radiation dose would appear as in Figure 35-1 . This fi gure illustrates the radiation dose-response relationship for acute human lethality.

At the lower dose of approximately 100 rad (1 Gy t ), no one is expected to die. Above approximately 600 rad (6 Gy t ), all those irradiated die unless vigorous medical support is available. Above 1000 rad (10 Gy t ), even vigorous medical support does not prevent death.

0 100 200 300 400 500 600

Radiation dose (rad)

Lethality (%)

100

75

50

25

0

LD50/60

FIGURE 35-1 Radiation-induced death in humans follows a nonlinear, threshold dose-response relationship.

GI death occurs principally because of severe damage to the cells lining the intestines.

The ultimate cause of death in CNS syndrome is elevated fl uid content of the brain.

The LD 50/60 is the dose of radiation to the whole body that causes 50% of irradiated subjects to die within 60 days.

Acute radiation lethality follows a nonlinear, threshold dose-response relationship.

PART VI Radiobiology538

If death is to occur, it usually happens within 60 days of exposure. Acute radiation lethality is measured quantitatively by the LD 50/60 , which is approximately 350 rad (3.5 Gy t ) for humans. With clinical support, humans can tolerate much higher doses; the maximum is reported to be 850 rad (8.5 Gy t ). Table 35-3 lists values of LD 50/60 for various species.

Question: From Figure 35-1 , estimate the radiation

dose that will produce 25% lethality in humans within 60 days.

Answer: First, draw a horizontal line from the 25% level on the y-axis until it intersects the S curve. Now, drop a vertical line from this point to the x-axis. This intersection with the x-axis occurs at the LD 25/60 , which is approximately 250 rad (2.5 Gy t ).

Mean Survival Time As the whole-body radiation dose increases, the average time between exposure and death decreases. This time is known as the mean survival time. A graph of radiation dose versus mean survival time is shown in Figure 35-2 . This graph depicts three distinct regions associated with the three radiation syndromes.

As the radiation dose increases from 200 to 1000 rad (2 to 10 Gy t ), the mean survival time decreases from approximately 60 to 4 days; this region is consistent with death resulting from the hematologic syndrome. Mean survival time is dose dependent with the hemato- logic syndrome.

In the dose range associated with the GI syndrome, however, the mean survival time remains relatively constant, at 4 days. With larger doses, those associated with the CNS syndrome, the mean survival time is again dose dependent, varying from approximately 3 days to a matter of hours.

LOCAL TISSUE DAMAGE When only part of the body is irradiated, in contrast with whole-body irradiation, a higher dose is required to produce a response. Every organ and tissue of the body can be affected by partial-body irradiation. The effect is cell death, which results in shrinkage of the organ or tissue. This effect can lead to total lack of function for that organ or tissue, or it can be followed by recovery.

There are many examples of local tissue damage imme- diately after radiation exposure. In fact, if the dose is high enough, any local tissue will respond. The manner in which local tissues respond depends on their intrinsic radiosensitivity and the kinetics of cell proliferation and maturation. Examples of local tissues that can be affected immediately are skin, gonads, and bone marrow.

All early radiation responses — local tissue damage is a good example — follow a threshold-type dose-response relationship. This is characteristic of a deterministic radiation response. A minimum dose is necessary to produce a deterministic response. Once that threshold

Atrophy is the shrinkage of an organ or tissue due to cell death.

LD 50/60 , Dose of radiation to the whole body that causes 50% of irradiated subjects to die within 60 days.

TABLE 35-3 Approximate LD 50/60 for Various Species After Whole-Body Radiation Exposure

Species LD 50/60 (rad)

Pig 250 Dog 275 Human 350 Guinea pig 425 Monkey 475 Opossum 510 Mouse 620 Goldfi sh 700 Hamster 700 Rat 710 Rabbit 725 Gerbil 1050 Turtle 1500 Armadillo 2000 Newt 3000 Cockroach 10,000

Not lethal

Hematologic death

Gastro- intestinal

death Central nervous system death

Radiation dose (rad)

50

10

1

0.01

Mean survival

time (days)

102 103 104 10 5

FIGURE 35-2 Mean survival time after radiation exposure shows three distinct regions. If death is due to hematologic or CNS effects, the mean survival time will vary with dose. If gastrointestinal (GI) effects cause death, it occurs in approxi- mately 4 days.

C H A P T E R 35 Early Effects of Radiation 539

dose has been exceeded, the severity of the response increases with increasing dose.

Effects on Skin The tissue with which we have had the most experience is the skin. Normal skin consists of three layers: an outer layer (the epidermis), an intermediate layer of connec- tive tissue (the dermis), and a subcutaneous layer of fat and connective tissue.

The skin has additional accessory structures, such as hair follicles, sweat glands, and sensory receptors ( Figure 35-3 ). All cell layers and accessory structures participate in the response to radiation exposure.

The skin, similar to the lining of the intestine, repre- sents a continuing cell renewal system, only with a much slower rate than that experienced by intestinal cells. Almost 50% of the cells lining the intestine are replaced every day, whereas skin cells are replaced at the rate of only approximately 2% per day.

The outer skin layer, the epidermis, consists of several layers of cells; the lowest layer consists of basal cells. Basal cells are the stem cells that mature as they migrate to the surface of the epidermis. Once these cells arrive at the surface as mature cells, they are slowly lost and have to be replaced by new cells from the basal layer.

In earlier times, the tolerance of the patient's skin determined the limitations of radiation oncology with orthovoltage x-rays (200 to 300 kVp x-rays). The object

of x-ray therapy was to deposit energy in the tumor while sparing the surrounding normal tissue. Because the x-rays had to pass through the skin to reach the tumor, the skin was necessarily subjected to higher radiation doses than the tumor. The resultant skin damage was seen as erythema (a sunburn-like reddening of the skin), followed by desquamation (ulceration and denudation of the skin), which often required interruption of treatment.

After a single dose of 300 to 1000 rad (3 to 10 Gy t ), an initial mild erythema may occur within the fi rst or second day. This fi rst wave of erythema then subsides, only to be followed by a second wave that reaches maxi- mum intensity in about 2 weeks.

At higher doses, this second wave of erythema is fol- lowed by a moist desquamation, which in turn may lead to a dry desquamation. Moist desquamation is known as clinical tolerance for radiation therapy.

During radiation therapy, the skin is exposed accord- ing to a fractionated scheme, usually approximately 200 rad/day (2 Gy t /d), 5 days a week. To assist the radiation oncologist in planning patient treatment, isoeffect curves have been generated that accurately project the dose necessary to produce skin erythema or clinical tolerance after a prescribed treatment routine ( Figure 35-4 ). Contemporary radiation oncology uses high- energy x- radiation from linear accelerators; this protects the skin from radiation damage.

Erythema was perhaps the fi rst observed biologic response to radiation exposure. Many of the early x-ray pioneers, including Roentgen, suffered skin burns induced by x-rays.

One of the hazards to the patient during the early years of radiology was x-ray – induced erythema. During

Epidermis Basal cells

Dermis

Subcutaneous tissue

Fascia covering muscle Muscle

FIGURE 35-3 A sectional view of the anatomic structures of the skin. The basal cell layer is most radiosensitive.

Damage to basal cells results in the earliest manifestation of radiation injury to the skin.

PART VI Radiobiology540

those years, x-ray tube potentials were so low that it was usually necessary to position the tube very close to the patient's skin; exposures of 10 to 30 minutes were required. Often, the patient would return several days later with an x-ray burn.

These skin effects follow a nonlinear, threshold dose-response relationship similar to that described for radiation-induced lethality. Small doses of x-radiation do not cause erythema. Extremely high doses of x-radia- tion cause erythema in all persons so irradiated.

Whether intermediate radiation doses produce ery- thema depends on the individual's radiosensitivity, the dose rate, and the size of the irradiated skin fi eld. Analy- sis of persons irradiated therapeutically with superfi cial x-rays has shown that the skin erythema dose required to affect 50% of those irradiated (SED 50 ), is about 500 rad (5 Gy t ).

Before the roentgen was defi ned and accurate radiation-measuring apparatus was developed, the skin was observed, and its response to radiation was used in formulating radiation protection practices. The unit used was the SED 50 , and permissible radiation exposures were specifi ed in fractions of SED 50 .

Another response of the skin to radiation exposure is epilation, or loss of hair. For many years, soft x-rays (10 to 20 kVp), called grenz rays, were used as the treat- ment of choice for persons with skin diseases, such as tinea capitis (ringworm).

Tinea capitis of the scalp, not uncommon in children, was successfully treated by grenz radiation; unfortu- nately, the patient's hair would fall out for weeks or even months. Sometimes, an unnecessarily high dose of grenz rays resulted in permanent epilation.

High-dose fl uoroscopy has focused more attention on the response of the skin to x-rays. The longer fl uo- roscopy times required for cardiovascular and inter- ventional procedures, coupled with allowed exposure

rates exceeding 20 R/min, are of great concern. Inju- ries to patients have been reported, and steps are being taken to establish better control over such exposures. Table 35-4 summarizes the potential effects of high- dose fl uoroscopy.

Effects on Gonads Human gonads are critically important target organs. As an example of local tissue effects, they are particularly sensitive to radiation. Responses to doses as low as 10 rad have been observed. Because these organs produce the germ cells that control fertility and heredity, their response to radiation has been studied extensively.

Much of what is known about the types of radia- tion response and about dose-response relationships has been derived from numerous animal experiments. Signifi cant data are also available from human popula- tions. Radiotherapy patients, radiation accident victims, and volunteer convicts all have provided data; this has resulted in a rather complete description of the gonadal response to radiation.

The cells of the testes (the male gonads) and the ova- ries (the female gonads) respond differently to radiation because of differences in progression from the stem cell to the mature cell. Figure 35-5 illustrates this progres- sion, indicating the most radiosensitive phase of cell maturation.

Germ cells are produced by both ovaries and testes, but they develop from the stem cell phase to the mature cell phase at different rates and at different times. This process of development is called gametogenesis.

The stem cells of the ovaries are the oogonia, and they multiply in number only before birth, during fetal life. The oogonia reach a maximum number of several

Clinical tolerance

Erythema

Time (days) 1 2 4 6 810 20 40 6080

8000

4000

2000

1000

600

Radiation dose (rad)

FIGURE 35-4 These isoeffect curves show the relationship between the number of daily fractions and the total radiation dose that will produce erythema or moist desquamation. As the fractionation of the dose increases, so does the total dose required.

TABLE 35-4 Potential Radiation Responses of Skin From High-Dose Fluoroscopy

Potential Radiation Response

Threshold Dose

Approximate Time of Onset

Early transient erythema

200 rad/2 Gy t Hours

Main erythema 600 rad/6 Gy t 10 days Temporary

epilation 300 rad/3 Gy t 3 weeks

Permanent epilation

700 rad/7 Gy t 3 weeks

Moist desquamation

1500 rad/15 Gy t 4 weeks

Ovaries and testes produce oogonia and spermatogonia, which mature into ovum and sperm, respectively.

C H A P T E R 35 Early Effects of Radiation 541

million and then begin to decline because of spontane- ous degeneration.

During late fetal life, many primordial follicles grow to encapsulate the oogonia, which become oocytes. These follicle-containing oocytes remain in a suspended state of growth until puberty. By the time of prepuberty, the number of oocytes has been reduced to only several hundred thousand.

Commencing at puberty, the follicles rupture with regularity, ejecting a mature germ cell, the ovum. Only 400 to 500 such ova are available for fertilization (num- ber of years of menstruation times 13 per year).

The germ cells of the testes are continually being pro- duced from stem cells progressively through a number of stages to maturity, and similar to the ovaries, the tes- tes provide a sustaining cell renewal system.

The male stem cell is the spermatogonia, which matures into the spermatocyte. The spermatocyte in turn multiplies and develops into a spermatid, which fi nally differentiates into the functionally mature germ cell, the spermatozoa or sperm. The maturation process from stem cell to spermatozoa requires 3 to 5 weeks.

Ovaries. Irradiation of the ovaries early in life reduces their size (atrophy) through germ cell death. After puberty, such irradiation also causes suppression and delay of menstruation.

Radiation effects on the ovaries depend somewhat on age. At fetal life and in early childhood, the ovaries are especially radiosensitive. They decline in radiosensitiv- ity, reaching a minimum in the age range of 20 to 30 years, and then increase continually with age.

Doses as low as 10 rad (100 mGy t ) may delay or suppress menstruation in the mature female. A dose of approximately 200 rad (2 Gy t ) produces temporary infertility; approximately 500 rad (5 Gy t ) to the ovaries results in permanent sterility.

In addition to the destruction of fertility, irradiation of the ovaries of experimental animals has been shown to produce genetic mutations. Even moderate doses, such as 25 to 50 rad (250 to 500 mGy t ), have been associated with measurable increases in genetic muta- tions. Evidence also indicates that oocytes that survive such a modest dose can repair some genetic damage as they mature into ova.

Testes. The testes, similar to the ovaries, atrophy after high doses of radiation. A large volume of data on testicular damage has been gathered from observa- tions of volunteer convicts and patients treated for carcinoma in one testis while the other was shield- ed. Many investigators have recorded normal births in such patients, whose remaining functioning testis received a radiation dose between 50 and 300 rad (0.5 and 3 Gy t ).

The spermatogonial stem cells signify the most sensitive phase in the gametogenesis of the spermato- zoa. After irradiation of the testes, maturing cells, sper- matocytes, and spermatids are relatively radioresistant and continue to mature. Consequently, no signifi cant reduction in spermatozoa occurs until several weeks after exposure; therefore, fertility continues through- out this time, during which irradiated spermatogonia would have developed into mature spermatozoa had they survived.

Radiation doses as low as 10 rad (100 mGy t ) can reduce the number of spermatozoa ( Table 35-5 ) in a manner reminiscent of the radiation response of the ovaries. With increasing dose, the depletion of sperma- tozoa increases and extends over a longer period.

Two hundred rad (2 Gy t ) produces temporary in- fertility, which commences approximately 2 months after irradiation and persists for up to 12 months. Five hundred rad (5 Gy t ) to the testes produces per- manent sterility. Even after doses suffi cient to produce

Male:

Female:

Spermatagonia Spermatocyte Spermatid Sperm

Primordial follicle

Mature follicle

Corpus Luteum Ovum

FIGURE 35-5 Progression of germ cells from the stem cell phase to the mature cell. Asterisk indicates the most radiosen- sitive cell.

The most radiosensitive cell during female germ cell development is the oocyte in the mature follicle.

TABLE 35-5 Response of Ovaries and Testes to Radiation

Approximate Dose Response

10 rad/100 Gy t Minimal detectable response 200 rad/2 Gy t Temporary infertility 500 rad/5 Gy t Sterility

PART VI Radiobiology542

permanent sterility, the male normally retains his abil- ity to engage in sexual intercourse.

Male gametogenesis is a self-renewing system; some evidence suggests that the most hazardous mutations are the genetic ones induced in surviving postspermatogo- nial cells. Consequently, after testicular irradiation of doses exceeding approximately 10 rad (100 mGy t ), the male should refrain from procreation for 2 to 4 months until all cells that were in the spermatogonial and post- spermatogonial stages at the time of irradiation have matured and disappeared.

This reduces but probably does not eliminate any increase in genetic mutations caused by the persistence of the stem cell. Evidence from animal experiments sug- gests that genetic mutations undergo some repair even when the stem cell is irradiated.

HEMATOLOGIC EFFECTS If you were a radiologic technologist in practice during the 1920s and the 1930s, you might have visited the hematology laboratory once a week for a routine blood examination. Before the introduction of personnel radiation monitors, periodic blood examination was the only monitoring performed on x-ray and radium workers. This examination included total cell counts and a white cell (leukocyte) differential count.

Most institutions had a radiation safety regulation such that, if the leukocytes were depressed by greater than 25% of normal level, the employee was given time off or was assigned to nonradiation activities until the count returned to normal.

What was not entirely understood at that time was that the minimum whole-body dose necessary to produce a measurable hematologic depression was approximately 25 rad (250 mGy t ). These workers were being heavily irradiated by today's standards.

Hemopoietic System The hemopoietic system consists of bone marrow, circulat- ing blood, and lymphoid tissue. Lymphoid tissues are the lymph nodes, spleen, and thymus. With this system, the prin- cipal effect of radiation is a depressed number of blood cells in the peripheral circulation. Time- and dose-related effects on the various types of circulating blood cells are determined by the normal growth and maturation of these cells.

All cells of the hemopoietic system apparently develop from a single type of stem cell ( Figure 35-6 ). This stem cell is called a pluripotential stem cell because it can develop into several different types of mature cells.

Although the spleen and the thymus manufacture one type of leukocyte (the lymphocyte), most circulating blood cells, including lymphocytes, are manufactured in the bone marrow. In a child, the bone marrow is rather uniformly distributed throughout the skeleton. In an adult, the active bone marrow responsible for producing circulating cells is restricted to fl at bones, such as the ribs, sternum, and skull, and the ends of long bones.

Erythrocytes

Reticulocyte

Pronormoblast

Stem cell

Megakaryoblast

Platelet-producing megakaryocyte

Thrombocytes (platelets)Lymphocyte

Lymphoblast

EosinophilicNeutrophilic

Granulocytes

Myeloblast

FIGURE 35-6 Four principal types of blood cells — lymphocytes, granulocytes, erythrocytes, and thrombocytes — develop and mature from a single pluripotential stem cell.

Under no circumstances is a periodic blood examination recommended as a feature of any current radiation protection program.

C H A P T E R 35 Early Effects of Radiation 543

From the single pluripotential stem cell, a number of cell types are produced. Principally, these are lympho- cytes (those involved in the immune response), granu- locytes (scavenger type of cells used to fi ght bacteria), thrombocytes (also called platelets and involved in the clotting of blood to prevent hemorrhage), and erythro- cytes (red blood cells that are the transportation agents for oxygen). These cell lines develop at different rates in the bone marrow and are released to the peripheral blood as mature cells.

While in the bone marrow, the cells proliferate in number, differentiate in function, and mature. Develop- ing granulocytes and erythrocytes spend about 8 to 10 days in the bone marrow. Thrombocytes have a lifetime of approximately 5 days in the bone marrow.

Lymphocytes are produced over varying times and have varying lifetimes in the peripheral blood. Some are thought to have lives measured in terms of hours and others in terms of years. In the peripheral blood, granu- locytes have a lifetime of only a couple of days. Throm- bocytes have a lifetime of approximately 1 week, and erythrocytes a lifetime of nearly 4 months.

The hemopoietic system, therefore, is another example of a cell renewal system. Normal cell growth and devel- opment determine the effects of radiation on this system.

Hemopoietic Cell Survival The principal response of the hemopoietic system to radiation exposure is a decrease in the numbers of all types of blood cells in the circulating peripheral blood. Lethal injury to the stem cells causes depletion of these mature circulating cells.

Figure 35-7 shows the radiation response of three cir- culating cell types. Examples are given for low, moder- ate, and high radiation doses, showing that the degree of cell depletion increases with increasing dose. These fi gures are the results of observations on experimental animals, radiotherapy patients, and the few radiation accident victims.

After exposure, the fi rst cells to become affected are the lymphocytes. These cells are reduced in number (lymphopenia) within minutes or hours after exposure, and they are very slow to recover. Because the response is so immediate, the radiation effect is apparently a direct one on the lymphocytes themselves rather than on the stem cells.

Granulocytes experience a rapid rise in number (granu- locytosis), followed fi rst by a rapid decrease and then a slower decrease in number (granulocytopenia). If the radiation dose is moderate, then an abortive rise in gran- ulocyte count may occur 15 to 20 days after irradiation. Minimum granulocyte levels are reached approximately 30 days after irradiation. Recovery, if it is to occur, takes approximately 2 months.

The depletion of platelets (thrombocytopenia) after irradiation develops more slowly, again because of the longer time required for the more sensitive precursor cells to reach maturity. Thrombocytes reach a minimum

Number of cells

per cubic centimeter (x1000)

10

8

6

4

2

0 40 80 120

25 rad

Platelets

Granulocytes

Lymphocytes

40 80 120

200 rad

Time after irradiation (days)

10 20 30

0

4

2

Number of cells

per cubic centimeter

(x 105)

600 rad

A B C

FIGURE 35-7 These graphs show the radiation response of the major circulating blood cells. A , 25 rad. B , 200 rad. C , 600 rad.

The lymphocytes and the spermatogonia are the most radiosensitive cells in the body.

PART VI Radiobiology544

in about 30 days and recover in approximately 2 months, similar to the response kinetics of granulocytes.

Erythrocytes are less sensitive than the other blood cells, apparently because of their very long lifetime in the peripheral blood. Injury to these cells is not appar- ent for a matter of weeks. Total recovery may take 6 months to a year.

CYTOGENETIC EFFECTS A technique developed in the early 1950s contributed enormously to human genetic analysis and radiation genetics. The technique calls for a culture of human cells to be prepared and treated so that the chromosomes of each cell can be easily observed and studied. This has resulted in many observations on radiation-induced chromosome damage.

The photomicrograph shown in Figure 35-8 shows the chromosomes of a human cancer cell following radiation therapy. The many chromosome aberrations represent a high degree of damage.

Radiation cytogenetic studies have shown that nearly every type of chromosome aberration can be radiation induced, and that some aberrations may be specifi c to radiation. The rate of induction of chromo- some aberrations is related in a complex way to the radiation dose and differs among the various types of aberrations.

Attempts to measure chromosome aberrations in pa- tients after diagnostic x-ray examination have been largely unsuccessful. However, some studies involving high-dose fl uoroscopy have shown radiation-induced chromosome aberrations soon after the examination was performed.

Without question, high doses of radiation cause chromosome aberrations. Low doses no doubt also do so, but it is technically diffi cult to observe aberrations at doses that are less than approximately 10 rad (100 mGy t ). An even more diffi cult task is to identify the link between radiation-induced chromosome aber- rations and latent illness or disease.

Dicentric

Isochromatids

Dicentric

Ring

FIGURE 35-8 Chromosome damage in an irradiated human cancer cell. (Courtesy Neil Wald, University of Pittsburgh.)

Cytogenetics is the study of the genetics of cells, particularly cell chromosomes.

Radiation-induced chromosome aberrations follow a nonthreshold dose-response relationship.

C H A P T E R 35 Early Effects of Radiation 545

When the body is irradiated, all cells can suffer cytogenetic damage. Such damage is classifi ed here as an early response to radiation because, if the cell survives, the damage manifests during the next mitosis after the radiation exposure.

Human peripheral lymphocytes are most often used for cytogenetic analysis, and these lymphocytes do not move into mitosis until stimulated in vitro by an appro- priate laboratory technique.

Cytogenetic damage to the stem cells is sustained immediately but may not be manifested for the consid- erable time required for that stem cell to reach maturity as a circulating lymphocyte.

Although chromosome damage occurs at the time of irradiation, it can be months and even years before the damage is measured. For this reason, chromosome abnor- malities in circulating lymphocytes persist in some workers who were irradiated in industrial accidents 20 years ago.

Normal Karyotype The human chromosome consists of many long strings of DNA mixed with a protein and folded back on itself many times. Refer back to Figure 32-11 , which shows a normal chromosome as it would appear in the G 1 phase of the cell cycle, when only two chromatids are present, and in the G 2 phase of the cell cycle after DNA repli- cation. The chromosome structure of four chromatids represented for the G 2 phase is that which is visualized in the metaphase portion of mitosis.

For certain types of cytogenetic analysis of chromo- somes, photographs are taken and enlarged so that each chromosome can be cut out like a paper doll and paired

with its sister into a chromosome map, which is called a karyotype ( Figure 35-9 ).

Structural radiation damage to individual chromosomes can be visualized without constructing a karyotype. These are the single- and double-hit chromosome aber- rations. Reciprocal translocations require a karyotype for detection. Point genetic mutations are undetectable even with karyotype construction.

Single-Hit Chromosome Aberrations When radiation interacts with chromosomes, the interaction can occur through direct or indirect effect. In either mode, these interactions result in a hit. The hit, however, is somewhat different from the hit described previously in radiation interaction with DNA.

The DNA hit results in an invisible disruption of the molecular structure of the DNA. A chromosome hit, on the other hand, produces a visible derangement of the chromosome. Because the chromosomes contain DNA, this indicates that such a hit has disrupted many molecular bonds and has severed many chains of DNA.

A B

C

D E

F

X Y Sex

Chromosomes

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22

A B

C

D E

F

X Y Sex

Chromosomes

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22

GG

FIGURE 35-9 A photomicrograph of the human cell nucleus at metaphase shows each chro- mosome distinctly. The karyotype is made by cutting and pasting each chromosome similar to paper dolls and aligning them largest to smallest. The left karyotype is male, the right female. (Courtesy Carolyn Caskey Goodner, Identigene, Inc.)

Each cell consists of 22 pairs of autosomes and a pair of sex chromosomes — the X chromosome from the female and the Y chromosome from the male.

A chromosome hit represents severe damage to the DNA.

PART VI Radiobiology546

Single-hit effects produced by radiation during the G 1 phase of the cell cycle are shown in Figure 35-10 . The breakage of a chromatid is called chromatid deletion. During S phase, both the remaining chromosome and the deletion are replicated.

The chromosome aberration visualized at metaphase consists of a chromosome with material missing from the ends of two sister chromatids and two acentric (without a centromere) fragments. These fragments are called isochromatids.

Chromosome aberrations also can be produced by single-hit events during the G 2 phase of the cell cycle (see Figure 35-10 ). The probability that ionizing radiation will pass through sister chromatids to produce isochro- matids is low. Usually, radiation produces a chromatid deletion in only one arm of the chromosome. The result is a chromosome with an arm that is obviously missing genetic material and a chromatid fragment.

Multi-Hit Chromosome Aberrations A single chromosome can sustain more than one hit. Multi-hit aberrations are not uncommon ( Figure 35-11 ).

In the G 1 phase of the cell cycle, ring chromosomes are produced if the two hits occur on the same chromosome. Dicentrics are produced when adjacent chromosomes each suffer one hit and recombine. The mechanism for the joining of chromatids depends on a condition called stickiness that is radiation-induced and appears at the site of the severed chromosome.

Similar aberrations can be produced in the G 2 phase of the cell cycle; however, such aberrations again require that (1) either the same chromosome be hit two or more times, or (2) adjacent chromosomes be hit and joined together. However, these events are rare.

Reciprocal Translocations. The multi-hit chromo- some aberrations previously described represent rath- er severe damage to the cell. At mitosis, the acentric fragments are lost or are attracted to only one of the daughter cells because they are unattached to a spindle fi ber. Consequently, one or both of the daughter cells can be missing considerable genetic material.

Reciprocal translocations are multi-hit chromosome aberrations that require karyotypic analysis for detec- tion ( Figure 35-12 ). Radiation-induced reciprocal trans- locations result in no loss of genetic material, simply a rearrangement of the genes. Consequently, all or nearly all genetic codes are available; they simply may be orga- nized in an incorrect sequence.

Kinetics of Chromosome Aberration At very low doses of radiation, only single-hit aberrations occur. When the radiation dose exceeds approximately 100 rad (1 Gy t ), the frequency of multi-hit aberrations increases more rapidly.

Irradiation in G1

Irradiation in G2

causes chromatid

break

that is replicated in S and passed

through G2

to be seen at M

to be seen at M

can cause a single or double

chromatid break

that is replicated in S and passed

through G2

FIGURE 35-10 Single-hit chromosome aberrations after irradiation in G 1 and G 2 . The aberrations are visualized and recorded during the M phase.

Irradiation in G1

causes chromatid breaks,

which rejoin

during S

to be seen at M

Ring

Dicentric+

FIGURE 35-11 Multi-hit chromosome aberrations after irradiation in G 1 result in ring and dicentric chromosomes, in addition to chromatid fragments. Similar aberrations can be produced by irradiation during G 2 , but they are rarer.

+

FIGURE 35-12 Radiation-induced reciprocal translocations are multi-hit chromosome aberrations that require karyotypic analysis for detection.

C H A P T E R 35 Early Effects of Radiation 547

The general dose-response relationship for produc- tion of single- and multi-hit aberrations is shown in Figure 35-13 . Single-hit aberrations are produced with a linear, nonthreshold dose-response relationship. Multi- hit aberrations are produced following a nonlinear, non- threshold relationship. A number of investigators have experimentally characterized these relationships.

where Y is the number of single- or multi-hit chromo- some aberrations, a is the naturally occurring frequency of chromosome aberrations, and b and c are radiation dose (D) coeffi cients of damage for single- and multi-hit aberrations, respectively.

Some laboratories use cytogenetic analysis as a bio- logic radiation dosimeter. Multi-hit aberrations are considered to be the most signifi cant in terms of latent human damage. If the radiation dose is unknown yet is not life threatening, the approximate chromosome aberration frequency is two single-hit aberrations per rad per 1000 cells and one multi-hit aberration per 10 rad per 1000 cells.

SUMMARY After exposure to a high radiation dose, humans can experience a response within a few days to a few weeks. This immediate response is called an early effect of

radiation exposure. Such early effects are usually deter- ministic, that is, the severity of response is dose related and there is a dose threshold.

The sequence of events that follows high-dose radia- tion exposure leading to death within days or weeks is called the acute radiation syndrome, which includes the hematologic syndrome, the GI syndrome, and the CNS syndrome. These syndromes are dose related.

LD 50/60 is the dose of radiation to the whole body in which 50% of subjects will die within 60 days. For humans, this dose is estimated at 350 rad (3.5 Gy t ). As radiation dose increases, the time between exposure and death decreases.

When only part of the body is irradiated, higher doses are tolerated. Examples of local tissue damage include effects on the skin, gonads, and bone marrow. The fi rst manifestation of radiation injury to the skin is damage to the basal cells. Resultant skin damage occurs as ery- thema, desquamation, or epilation.

Radiation of the male testes can result in a reduc- tion of spermatozoa. A dose of 200 rad (2 Gy t ) produces temporary infertility. A dose of 500 rad (5 Gy t ) to the testes produces permanent sterility. In the male as in the female, the stem cell is the most radiosensitive phase.

The hemopoietic system consists of bone marrow, cir- culating blood, and lymphoid tissue. The principal effect of radiation on this system is fewer blood cells in the periph- eral circulation. Radiation exposure decreases the numbers of all precursor cells; this reduces the number of mature cells in the circulating blood. Lymphocytes and spermatogonia are considered the most radiosensitive cells in the body.

The study of chromosome damage from radiation exposure is called cytogenetics. Chromosome damage takes on the following different forms: (1) chromatid deletion, (2) dicentric chromosome aberration, and (3) reciprocal translocations.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. GI death b. Latent period c. LD 50/60 d. Erythema e. Clinical tolerance f. Primordial follicle g. Erythrocyte h. Karyotype i. Epilation j. Multi-hit aberration 2. What is the minimum dose that results in

reddening of the skin? 3. Explain the prodromal syndrome. 4. Clinical signs and symptoms of the manifest illness

stage of acute radiation lethality are classifi ed into what three groups?

Radiation dose

Chromosome aberrations

Multi-hit aberrations

Single-hit aberrations

FIGURE 35-13 Dose-response relationships for single-hit aberrations are linear, nonthreshold, whereas those for multi- hit aberrations are nonlinear, nonthreshold.

RADIATION DOSE-RESPONSE RELATIONSHIPS FOR CYTOGENETIC DAMAGE Single -hit: Y = a + bD Multi-hit: Y = a + bD + cD2

PART VI Radiobiology548

5. During which stage of the acute radiation syndrome is recovery stimulated?

6. What dose of radiation results in the gastrointestinal syndrome?

7. Why does death occur with the GI syndrome? 8. Identify the cause of death from the CNS syndrome. 9. Describe the stages of gametogenesis in the female.

Identify the most radiosensitive phases. 10. What cells of the hemopoietic system arise from

pluripotential stem cells? 11. Discuss the maturation of basal cells in the epidermis. 12. What two cells are the most radiosensitive cells in

the human body? 13. Describe the changes in mean survival time

associated with increasing dose. 14. What are the approximate values of LD 50/60 and

SED 50 in humans? 15. What are the four principal blood cell lines and

what is the function of each?

16. Diagram the mechanism for the production of a reciprocal translocation.

17. List the clinical signs and symptoms of the hematologic syndrome.

18. What mature cells form from the omnipotential stem cell?

19. If the normal incidence of single hit – type chromosome aberrations is 0.15 per 100 cells and the dose coeffi cient is 0.0094, how many such aberrations would be expected after a dose of 38 rad?

20. If the normal incidence of multi-hit chromosome aberrations is 0.082 and the dose coeffi cient is 0.0047, how many dicentrics per 100 cells would be expected after a whole-body dose of 16 rad?

The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

549

C H A P T E R

36 Late Effects of Radiation OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Defi ne late effects of radiation exposure 2. Identify the radiation dose needed to produce late effects 3. Discuss the results of epidemiologic studies of persons exposed to

radiation 4. List the local tissue effects of low-dose radiation to various types of

organs 5. Explain the estimates of radiation risk 6. Analyze radiation-induced leukemia and cancer 7. Review the risks of low-dose radiation on fertility and pregnancy

OUTLINE Local Tissue Effects

Skin Chromosomes Cataracts

Life-Span Shortening Risk Estimates

Relative Risk Excess Risk Absolute Risk

Radiation-Induced Malignancy Leukemia Cancer

Total Risk of Malignancy Three Mile Island BEIR Committee

Radiation and Pregnancy Effects on Fertility Irradiation In Utero Genetic Effects

PART VI Radiobiology550

The radiation exposures that we experience in diagnostic radiology are low and of low LET; they are chronic in nature because they are delivered intermittently over long periods. Therefore, late radiation effects are of particular importance.

The principal late effects are radiation-induced malignancy and genetic effects. Most late effects are also known as stochastic effects. Stochastic effects of radiation exposure exhibit an increasing incidence of response — not severity — with increasing dose. No dose threshold has been established for a stochastic response.

Studies of large numbers of people exposed to a toxic substance require considerable statistical analyses. Such studies, called epidemiologic studies, are required when the number of persons affected is small.

Epidemiologic studies of people exposed to radiation are diffi cult because (1) the dose usually is not known but is presumed to be low, and (2) the frequency of response

is very low. Consequently, the results of radiation epi- demiologic studies do not convey the statistical accuracy associated with observations of early radiation effects.

Table 36-1 illustrates the diffi culty of the problem. It shows the minimum number of persons that must be observed as a function of radiation dose if a defi nite link is to be established between an increase of leukemia and the radiation dose in question.

LOCAL TISSUE EFFECTS Skin In addition to the early effects of erythema and des- quamation and late-developing carcinoma, chronic irradiation of the skin can result in severe nonmalignant changes. Early radiologists who performed fl uoroscopic examinations without protective gloves developed a very callused, discolored, and weathered appearance to the skin of their hands and forearms. In addition, the skin would be very tight and brittle and sometimes would severely crack or fl ake.

This late effect was observed many years ago in radiologists and is called radiodermatitis. The dose nec- essary to produce such an effect is very high. No such effects occur in the current practice of radiology.

Chromosomes Irradiation of blood-forming organs can produce hema- tologic depression as an early response or leukemia as a late response. Chromosome damage in the circulating lymphocytes can be produced as both an early and a late response.

The types and frequency of chromosome aberra- tions have been described previously; however, even a low dose of radiation can produce chromosome aber- rations that may not be apparent until many years after radiation exposure. For example, individuals irradiated accidentally with rather high radiation doses continue to show chromosome abnormalities in their peripheral lymphocytes for as long as 20 years.

This late effect presumably occurs because of radia- tion damage to the lymphocytic stem cells. These cells may not be stimulated into replication and maturation for many years.

ARLY EFFECTS of radiation exposure are pro- duced by high radiation doses. Late effects of radiation exposure are the result of low doses

delivered over a long period. Radiation exposures experienced by personnel

in diagnostic imaging are low dose and low linear energy transfer (LET). In addition, the exposures in diagnostic imaging are delivered intermittently over long periods.

The principal late effects of low-dose radiation over long periods consist of radiation-induced malignancy and genetic effects. Life span short- ening and effects on local tissues also have been reported as late effects, but these are not consid- ered signifi cant. Radiation protection guides are based on suspected or observed late effects of radiation and on an assumed linear, nonthreshold dose-response relationship.

This chapter reviews these late effects and introduces the subject of risk estimation. Radia- tion effects during pregnancy are of considerable importance in diagnostic x-ray imaging, and such effects are discussed here as well.

E

TABLE 36-1 Minimum Population Sample Required to Show That the Given Radiation Dose Signifi cantly Elevated the Incidence of Leukemia

Dose Required Sample Size (No. of People)

5 rad (0.05 Gy t ) 6,000,000 10 rad (0.1 Gy t ) 1,600,000 15 rad (0.15 Gy t ) 750,000 20 rad (0.2 Gy t ) 500,000 50 rad (0.5 Gy t ) 100,000

Our radiation protection guides are based on the late effects of radiation and on linear, nonthreshold dose-response relationships.

C H A P T E R 36 Late Effects of Radiation 551

Cataracts In 1932, E.O. Lawrence of the University of California developed the fi rst cyclotron, a 5-inch-diameter device capable of accelerating charged particles to very high energies. These charged particles are used as “bullets” that are shot at the nuclei of target atoms in the study of nuclear structure. By 1940, every university physics department of any worth had built its own cyclotron and was engaged in what has become high-energy physics.

The modern cyclotron is used principally to produce radionuclides for use in nuclear medicine ( Figure 36-1 ), especially fl uorine-18 for positron emission tomography (PET). The largest particle accelerators in the world are located at Argonne National Laboratory in the United States and at CERN in Switzerland. These accelerators are used to discover the ultimate fi ne structure of matter and to describe exactly what happened at the moment of creation of the universe.

Early cyclotrons were located in one room and a beam of high-energy particles was extracted through a tube and steered and focused by electromagnets onto the target material in the adjacent room. At that time, sophisticated electronic equipment was not available for controlling this high-energy beam.

Cyclotron physicists used a tool of the radiologic technologist, the radiographic intensifying screen, to aid them in locating the high-energy beam. Unfortunately,

in so doing, these physicists received high radiation doses to the lens of the eye because they had to look directly into the beam.

In 1949, the fi rst paper reporting cataracts in cyclo- tron physicists appeared. By 1960, several hundred such cases of radiation-induced cataracts had been reported. This was particularly tragic because there were few high-energy physicists.

On the basis of these observations and animal experi- mentation, several conclusions can be drawn regarding radiation-induced cataracts. The radiosensitivity of the lens of the eye is age dependent. As the age of the indi- vidual increases, the radiation effect becomes greater and the latent period becomes shorter.

Latent periods ranging from 5 to 30 years have been observed in humans, and the average latent period is approximately 15 years. High-LET radiation, such as neutron and proton radiation, has a high relative biologic effectiveness (RBE) for the production of cataracts.

FIGURE 36-1 Cyclotron used to produce radionuclides for nuclear medicine. (Courtesy CTI, Molecular Imaging, Inc.)

Radiation-induced cataracts occur on the posterior pole of the lens.

The dose-response relationship for radiation- induced cataracts is nonlinear, threshold.

PART VI Radiobiology552

If the lens dose is high enough, in excess of approxi- mately 1000 rad (10 Gy t ), cataracts develop in nearly 100% of those who are irradiated. The precise level of the threshold dose is diffi cult to assess.

Most investigators would suggest that the threshold after an acute x-ray exposure is approximately 200 rad (2 Gy t ). The threshold after fractionated exposure, such as that received in radiology, is probably in excess of 1000 rad (10 Gy t ). Occupational exposures to the lens of the eye are too low to require protective lens shields for radiologic technologists. It is nearly impossible for a medical radiation worker to reach the threshold dose.

Radiation administered to patients who are under- going head and neck examination by fl uoroscopy or computed tomography can be signifi cant. In computed tomography, the lens dose can be 5 rad (50 mGy t ) per slice. In this situation, however, usually no more than one or two slices intersect the lens. In either case, pro- tective lens shields are not normally required. However, in computed tomography, it is common to modify the examination to reduce the dose to the eyes.

LIFE-SPAN SHORTENING Many experiments have been conducted with animals after both acute and chronic exposures that show that ir- radiated animals die young. Figure 36-2 , which has been redrawn from several such representative experiments, shows that the relationship between life span shortening and dose is apparently linear, nonthreshold. When all animal data are considered collectively, it is diffi cult to attempt a meaningful extrapolation to humans.

The data presented in Table 36-2 were compiled by Cohen of the University of Pittsburgh and were extrap- olated from various statistical sources of mortality. The expected loss of life in days is given as a function of occupation, disease, or other condition.

As one can see, the most grievous risk is being male rather than female. Whereas the average life shorten- ing caused by occupational accidents amounts to 74 days, for radiation workers, life is shortened by only 12 days.

Radiation-induced life span shortening is nonspecific, that is, no characteristic diseases are associated with it, and it does not include late malignant effects. It occurs simply as accelerated premature aging and death.

One investigator has evaluated the death records of radiologic technologists who operated fi eld x-ray equip- ment during World War II. These imaging systems were poorly designed and inadequately shielded, so that tech- nologists received higher-than-normal exposures. Seven thousand such technologists have been studied, and no radiation effects have been observed.

An investigation of health effects from radiation exposure of American radiologic technologists is cur- rently under way. This is being conducted as a mail survey that is covering many work-related conditions of

40

20

0 200 400 600 800

Radiation dose

Life shortening

(wk)

FIGURE 36-2 In chronically irradiated animals, the relation- ship between extent of life shortening and dose appears linear, nonthreshold. This graph shows the representative results of several such experiments with mice.

TABLE 36-2 Risk of Life Span Shortening as a Consequence of Occupation, Disease, or Various Other Conditions

Risky Condition Expected Days of Life Lost

Being male rather than female 2800 Heart disease 2100 Being unmarried 2000 One pack of cigarettes a day 1600 Working as a coal miner 1100 Cancer 980 30 pounds overweight 900 Stroke 20 All accidents 435 Service in Vietnam 400 Motor vehicle accidents 200 Average occupational accidents 74 Speed limit increase from

55 to 65 mph 40

Radiation worker 12 Airplane crashes 1

At worst, humans can expect a reduced life span of approximately 10 days for every rad.

Radiologic technology is a safe occupation.

C H A P T E R 36 Late Effects of Radiation 553

approximately 150,000 subjects; it will take many years to complete. Early reports show no effects.

Observations on human populations have not been totally convincing. No life span shortening has been observed among atomic bomb survivors, although some received rather substantial radiation doses. Life span shortening in radium watch-dial painters, x-ray patients, and other human radiation-exposed populations has not been reported.

American radiologists have been fairly extensively studied, and early radiologists appeared to have a reduced life span. Such research has many shortcom- ings, not the least of which is its retrospective nature. Figure 36-3 shows the results obtained when the age at death for radiologists was compared with the age at death for the general population. Radiologists dying in the early 1930s were approximately 5 years younger than members of the general population who died at an average age. However, this difference in age at death had shrunk to zero by 1965.

A more thorough study used two other physician groups as controls rather than the general population. Table 36-3 summarizes the results of this investigation. Physicians in the high-risk group observed in this study were members of the Radiological Society of North America (RSNA); the low-risk groups consisted of mem- bers of the American Academy of Ophthalmology and Otolaryngology (AAOO). Members of the American College of Physicians (ACP) represented an intermedi- ate-risk group.

A comparison of median age at death and age- adjusted death rates for these physician specialties demonstrates a signifi cant difference in age at death during the early years of radiology.

RISK ESTIMATES The early effects of high-dose radiation exposure are usually easy to observe and measure. The late effects are also easy to observe, but it is nearly impossible to associate a particular late response with a previous radiation exposure.

Consequently, precise dose-response relationships are often not possible to formulate, and we therefore resort to risk estimates. There are three types of risk estimates — relative, excess, and absolute risk; all of these represent dif- ferent statements of risk and have different dimensions.

Relative Risk If one observes a large population for late radiation effects without having any precise knowledge of the radiation dose to which they were exposed, then the concept of relative risk is used. The relative risk is computed by comparing the number of persons in the exposed popu- lation showing a given late effect with the number in an unexposed population who show the same late effect.

A relative risk of 1.0 indicates no risk at all. A relative risk of 1.5 indicates that the frequency of a late response is 50% higher in the irradiated population than in the nonirradiated population. The relative risk for radiation- induced late effects of particular importance observed in human populations is in the range of 1 to 2.

General population

Radiologists

75

70

65

60

55

50

Year

1930 1940 1950 1960

Average age

at death

FIGURE 36-3 Radiation-induced life span shortening is shown for American radiologists. The age at death among ra- diologists was lower than that of the general population, but this difference has disappeared.

TABLE 36-3 Death Statistics for Three Groups of Physicians

Died During Median Age at Death

Age-Adjusted Deaths per 1000

1935 TO 1944 RSNA 71.4 18.4 ACP 73.4 15.4 AAOO 76.2 13.0

1945 TO 1954 RSNA 72.0 16.4 ACP 74.8 13.7 AAOO 76.0 11.9

1955 TO 1958 RSNA 73.5 13.6 ACP 76.0 11.4 AAOO 76.4 10.6

ACP , American College of Physicians; AOO , American Academy of Ophthal- mology and Otolaryngology; RSNA , Radiological Society of North America.

RELATIVE RISK

Relative risk = Observed cases Expected cases

PART VI Radiobiology554

Occasionally, an investigation results in the identifi - cation of a relative risk of less than 1. This indicates that the exposed population receives some protective benefi t, which is consistent with the theory of radiation horme- sis. However, the usual interpretation of such studies is that the results are not statistically signifi cant because of the small number of observations conducted or because irradiated and control populations were not adequately identifi ed.

Some evidence supports the principle of radiation hormesis. Radiation hormesis suggests that low levels of radiation — less than approximately 10 rad (100 mGy t ) — are good for you! Such low doses may provide a protective effect by stimulating molecu- lar repair and immunologic response mechanisms. Nevertheless, radiation hormesis remains a theory at this time, and until it has been proved, we will continue to practice ALARA — as low as reasonably achievable.

An example of a reported dose-response relationship indicating radiation hormesis was shown in Figure 33-7 . The low-dose region where the relative risk is less than 1 is the hormetic region. The crossover at a relative risk of 1 usually occurs in the 5- to 20-rad (50- to 200-mGy t ) range.

Question: In a study of radiation-induced leukemia

after diagnostic levels of radiation, 227 cases were observed in 100,000 persons so irradiated. The normal incidence of leukemia in the United States is 150 cases per 100,000. On the basis of these data, what is the relative risk of radiation-induced leukemia?

Answer:

Relative risk Observed cases Expected cases

=

÷ 227

100 000 150

100, ,, . .

. 000

0 00227 0 00150

1 51= =

Excess Risk Often, when an investigation of human radiation response reveals the induction of some late effect, the magnitude of the effect is refl ected by the excess num- ber of cases induced. Leukemia, for instance, is known to occur spontaneously in nonirradiated populations. If the leukemia incidence in an irradiated population exceeds that which is expected, then the difference between the observed number of cases and the expected number would be excess risk.

The excess cases in this instance are assumed to be radiation induced. To determine the number of excess cases, one must be able to measure the observed number of cases in the irradiated popula- tion and compare this with the number that would have been expected on the basis of known popula- tion levels.

Question: Twenty-three cases of skin cancer

were observed in a population of 1000 radiologists. The incidence in the general population is 0.5/100,000. How many excess skin cancers were produced in the population of radiologists?

Answer: Excess cases = Observed cases − Expected cases

= − = − ≅ 23

1000 0 5

100 000 23

1000 0 005 1000

23 . ,

.

Because none would be expected, all 23

cases represent radiation risk.

Absolute Risk If at least two different dose levels are known, then it may be possible to determine an absolute risk factor. In contrast to the relative risk, which is a dimensionless ratio, the absolute risk consists of units of cases/popula- tion/dose.

The absolute risk of radiation-induced malignant disease has been determined by the National Academy of Science (NAS) Committee on the Biologic Effects of Ionizing Radiation (BEIR). This value (5 × 10 − 4 rem − 1 / 5 × 10 − 2 Sv − 1 ) is a considerable simplifi cation of the results of many studies.

To determine the absolute radiation risk, one must assume a linear dose-response relationship. If the dose- response relationship is assumed to be nonthreshold, then only one dose level is required. The value of the absolute radiation risk is equal to the slope of the dose- response relationship ( Figure 36-4 ). The error bars on each data point indicate the precision of the observation of response.

Question: The absolute risk for radiation-induced breast

cancer is 5 × 10 − 4 rem − 1 (5 × 10 − 2 Sr − 1 ) for a 20-year at-risk period (actually it's much less than this). If 100,000 women receive 100 mrem (1 mSv) during mammography, how many fatal cancers would be expected to be induced?

The theory of radiation hormesis suggests that very low radiation doses are benefi cial.

EXCESS RISK

Excess risk = Observed cases − Expected cases

C H A P T E R 36 Late Effects of Radiation 555

Answer:

5 10 5

10 000 5

100 000 100 5

4 1× = ×

= ×

=

− −rem ,

,

rem

mrem fatalcancers

Question: There are approximately 300,000 American

radiologic technologists, and they receive an annual effective dose of 50 mrem (0.5 mSv). What is the expected number of annual deaths because of this occupational exposure?

Answer:

5 10 5

10 000 25

1 000 000 50 300 0

4 1× = ×

= ×

− −rem

Therefore in

,

, , ,

rem

mrem 000

7 5 RTs

= . deaths frommalignantdisease

The reader should realize that death from malig- nant disease occurs in approximately 20% of the population.

RADIATION-INDUCED MALIGNANCY All the late effects, including radiation-induced malignancy, have been observed in experimental animals, and on the basis of these animal experiments, dose-response relation- ships have been developed. At the human level, these late effects have been observed, but often, data are insuffi cient to allow precise identifi cation of the dose-response relation- ship. Consequently, some of the conclusions drawn regard- ing human responses are based in part on animal data.

Most of these late effects are stochastic effects. A stochas- tic effect is one that has no dose threshold. Even the smallest radiation dose can produce an effect. With increasing dose, the incidence, not the severity, of the response increases. All radiation-induced malignancies are stochastic.

Leukemia When one considers radiation-induced leukemia in lab- oratory animals, there is no question that this response is real and that the incidence increases with increasing radiation dose. The form of the dose-response relation- ship is linear and nonthreshold. A number of human population groups have exhibited an elevated incidence of leukemia after radiation exposure — atomic bomb survivors, American radiologists, radiotherapy patients, and children irradiated in utero, to name a few.

Atomic Bomb Survivors. Probably the greatest wealth of information that we have accumulated regar- ding radiation-induced leukemia in humans has been drawn from observations of survivors of the atomic bombings of Hiroshima and Nagasaki. At the time of the bombings, approximately 300,000 people lived in those two cities. Nearly 100,000 were killed from the blast and from early effects of radiation. Another 100,000 people received signifi cant doses of radiation and survived. The remainder were unaffected because their radiation dose was less than 10 rad (100 mGy t ).

After World War II, scientists of the Atomic Bomb Casualty Commission (ABCC), now known as the Radiation Effects Research Foundation (RERF), attempted to determine the radiation dose received by each of the atomic bomb survivors in both cities. They estimated the dose to each survivor by considering not only distance from the explosion but also terrain, type of bomb, type of building construction if the survivor was inside, and other factors that might infl uence dose.

A summary of the data obtained through these inves- tigations is given in Table 36-4 , and the data analysis is shown graphically in Figure 36-5 . After high doses were delivered by these bombs, the leukemia incidence was as much as 100 times that in the nonirradiated population. Even though large error bars are seen at each dose incre- ment, the response appears linear, nonthreshold.

B

A

Radiation dose (rad) 0 50 100 150 200 250 300

1200

1000

800

600

400

200

0

Response (number of

cases/106/yr)

FIGURE 36-4 Slope of the linear, nonthreshold dose- response relationship is equal to the absolute risk. A and B show absolute risks of 3.4 and 6.2 cases per 10 6 persons/rad/year, respectively.

TABLE 36-4 Summary of the Incidence of Leukemia in Atomic Bomb Survivors

Hiroshima Nagasaki Total

Total number of survivors in study

74,356 25,037 99,393

Observed cases

102 42 144 of leukemia

Expected cases

39 13 52 of leukemia

PART VI Radiobiology556

If, however, one expands the data in the low-dose region (e.g., below 200 rad), one could conclude that a threshold exists in the neighborhood of 50 rad (500m Gy t ). Nevertheless, neither this information nor other available information is interpreted to support a threshold response.

Figure 36-6 demonstrates the temporal distribution of the onset of leukemia among atomic bomb survivors for the 40 years after the bombings. The data are pre- sented as cases per 100,000 and include for comparison the leukemia rate in the population at large and in the nonexposed populations of the bombed cities. A rather rapid rise in leukemia incidence reached a plateau after approximately 5 years. The incidence declined slowly for approximately 20 years, when it reached the natural level experienced by the nonexposed.

The at-risk period is that time after irradiation during which one might expect the radiation effect to occur. The at-risk period for radiation-induced cancer is lifetime.

Data from atomic bomb survivors show without a doubt that radiation exposure to those survivors caused the later development of leukemia. It is interesting,

3000

2000

1000

0

H

N Incidence in unexposed

Leukemia incidence

(cases/106

survivors/yr)

Radiation dose (rad) 0 200 400 600 800 1000

FIGURE 36-5 Data from the atomic bomb survivors of Hiroshima (H) and Nagasaki (N) suggest a linear, nonthreshold dose-response relationship.

Heavily exposed cases (>100 rad)

All Japan

Nonexposed cases (<10 rad) Leukemia incidence

(cases/105 survivors)

15

10

5

0 1950 1955 1960 1965 1970 1975 1980 1985

FIGURE 36-6 The incidence of leukemia among atomic bomb survivors increased rapidly for the fi rst few years, then declined to natural incidence by approximately 1975.

Radiation-induced leukemia follows a linear, nonthreshold dose-response relationship.

Radiation-induced leukemia is considered to have a latent period of 4 to 7 years and an at-risk period of approximately 20 years.

C H A P T E R 36 Late Effects of Radiation 557

however, to refl ect on some additional aspects of these events.

Of the 300,000 total residents, 335 persons are estimated to have survived doses in excess of 600 rad (6 Gy). The leukemia risk estimates are based on only 144 cases in the total exposed population. Acute leukemia and chronic myelocytic leukemia were observed most often among atomic bomb survivors.

Taken to the final analysis, data from the atomic bomb survivors pointed to an absolute risk of 5 × 10 − 4 rem − 1 (5 × 10 − 2 Sv − 1 ). The overall relative risk based on the total number of observed leukemia deaths (144) ver- sus the number of expected leukemia deaths (52) is approximately 3:1.

Radiologists. By the second decade of radiology, reports of pernicious anemia and leukemia in radiolo- gists began to appear. In the early 1940s, several inves- tigators reviewed the incidence of leukemia in American radiologists and found it alarmingly high. These early radiologists functioned without the benefi t of modern radiation protection devices and procedures, and many served as both radiation oncologists and diagnostic radiologists.

It has been estimated that some of these early radio- logists received doses exceeding 100 rad/yr (1 Gy t /yr). Currently, American radiologists do not exhibit an elevated incidence of leukemia compared with other physician specialists.

A rather exhaustive study of mortality among radiolo- gists in Great Britain during the period from the turn of the century to 1960 did not show an elevated risk of leuke- mia. The reasons for such a different experience between American and British radiologists are unknown.

Studies of radiation-induced leukemia among American radiologic technologists consistently show no evidence of any radiation effect.

Patients With Ankylosing Spondylitis. In the 1940s and 1950s, particularly in Great Britain, it was common practice to treat patients with ankylosing spondylitis with radiation. Ankylosing spondylitis is an arthritis- like condition of the vertebral column.

Patients cannot walk upright or move except with great diffi culty. For relief, they would be given fairly high doses of radiation to the spinal column, and the treatment was quite successful. Patients who previously had been hunched over were able to stand erect.

Radiation therapy was a permanent cure and re- mained the treatment of choice for approximately 20 years, until it was discovered that some who had been cured by radiation were dying from leukemia. Graphic

results on the observations of these patients are shown in Figure 36-7 .

During the period from 1935 to 1955, 14,554 male patients were treated at 81 different radiation therapy centers in Great Britain. Review of treatment records showed that the dose to the bone marrow of the spinal column ranged from 100 to 4000 rad (1 to 40 Gy).

Fifty-two cases of leukemia occurred in this popula- tion. When this incidence of leukemia is compared with that of the general population, the relative risk is 10:1.

Absolute risk can be obtained from these data by determining the slope of the best-fi t line through the data points ( Figure 36-7 ). Such an analysis yields a result of approximately 8 × 10 − 4 rem − 1 (8 × 10 − 2 Sv − 1 ). If 95% confi dence limits are placed on the data, one cannot rule out the possibility of a threshold dose at approximately 300 rad (3 Gy t ).

Leukemia in Other Populations. Several studies have been designed to link leukemia incidence with environmental radiation. Natural background radiation levels increase in general with altitude and with latitude, but the range of levels observed is not suffi cient to dem- onstrate a causal relationship with leukemia.

Other population groups that have provided evidence, both positive and negative, regarding the leukemia-in- ducing action of radiation include radium watch-dial painters, children receiving superfi cial x-ray treatment, and some additional adult radiation therapy groups.

Cancer What has been discussed regarding radiation-induced leu- kemia also can be reported for radiation-induced cancer. We do not have similar quantities of human data regard- ing cancer as we do for leukemia. Nevertheless, it can be said without question that radiation can cause cancer.

0 500 1000 1500 2000 2500 Bone marrow dose (rad)

95% confidence level

Leukemia incidence

(cases/104 persons/yr)

30

25

20

15

10

5

0

FIGURE 36-7 Results of observations of leukemia in patients with ankylosing spondylitis treated with x-ray therapy suggest a linear, nonthreshold dose-response relationship.

Chronic lymphocytic leukemia is rare and therefore is not considered to be a form of radiation- induced leukemia.

PART VI Radiobiology558

The relative risks and absolute risks have been shown to be similar to those reported for leukemia. Many types of cancer have been implicated as radiation induced, and a discussion of the more important ones is in order.

It is not possible to link any case of cancer to a pre- vious radiation exposure, regardless of its magnitude, because cancer is so common. Approximately 20% of all deaths are caused by cancer; therefore, any radiation- induced cancers are obscured. Leukemia, on the other hand, is a relatively rare disease; this makes analysis of radiation-induced leukemia easier.

Thyroid Cancer. Thyroid cancer has been shown to develop in three groups of patients whose thyroid glands were irradiated in childhood. The fi rst two groups, called the Ann Arbor series and the Rochester series, consisted of individuals who, in the 1940s and early 1950s, were treated shortly after birth for thymic enlargement. The thymus is a gland lying just below the thyroid gland that can enlarge shortly after birth in response to infection.

At these facilities, radiation was often the treatment of choice. After a dose of up to 500 rad (5 Gy t ), the thy- mus gland would shrink so that all enlargement disap- peared. No additional problems were evident until up to 20 years later, when thyroid nodules and thyroid cancer began to develop in some of these patients.

Another group included 21 children who were natives of the Rongelap Atoll in 1954; they were subjected to high levels of fallout during a hydrogen bomb test. The winds shifted during the test, carrying the fallout over an adjacent inhabited island rather than one that had been evacuated. These children received radiation doses to the thyroid gland from both external exposure and internal ingestion of approximately 1200 rad (12 Gy t ).

If one computes the incidence of thyroid nodularity, considered preneoplastic, in these three groups and plots this incidence as a function of estimated dose, the result is that shown in Figure 36-8 . Admittedly, the error bars on the dose data and on the incidence levels are large. Still, the implication of a linear, nonthreshold dose- response relationship is clear.

Data are just now becoming available on the nearly 100,000 persons exposed to radiation from the 1989 Chernobyl incident. No excess leukemia or cancer has been observed in this population, although a small increase in thyroid nodularity has been noted.

Bone Cancer. Two population groups have contrib- uted an enormous quantity of data showing that radia- tion can cause bone cancer. The fi rst group consists of radium watch-dial painters.

In the 1920s and 1930s, various small laborato- ries hired employees, most often female, who worked at benches painting watch dials with paint laden with radium sulfate. To prepare a fi ne point on the paint- brushes, the employees would touch the tip of the brush to the tongue. In this manner, substantial quantities of radium were ingested.

Radium salts were used because the emitted radia- tion, principally alpha and beta particles, would con- tinuously excite the luminous compounds so the watch dial would glow in the dark. Current technology uses harmlessly low levels of tritium ( 3 H) and promethium ( 147 Pm) for this purpose.

When ingested, the radium would behave metaboli- cally similar to calcium and deposit in bone. Because of radium's long half-life (1620 years) and alpha emission, these employees received radiation doses to bone of up to 50,000 rad (500 Gy t ).

Seventy-two bone cancers in approximately 800 per- sons have been observed during a follow-up period in excess of 50 years. Analysis of these data has disclosed an overall relative risk of 122:1. The absolute risk is equal to 1 × 10 − 4 rem − 1 (1 × 10 − 2 Sv − 1 ).

Another population in whom excess bone cancer de- veloped consisted of patients treated with radium salts for a variety of diseases, from arthritis to tuberculosis. Such treatments were common practice in many parts of the world until about 1950.

Skin Cancer. Skin cancer usually begins with the de- velopment of a radiodermatitis. Signifi cant data have been developed from several reports of skin cancer induced in radiation therapy recipients treated with orthovoltage (200 to 300 kVp) or superfi cial x-rays (50 to 150 kVp).

From these data, we conclude that the latent period is approximately 5 to 10 years, but we do not have enough data to assign absolute risk values. When the dose delivered to the skin was in the range of

0 200 400 600 800 1000 1200 1400

Thyroid dose (rad)

Incidence of thyroid nodules

(%)

100

80

60

40

20

0

FIGURE 36-8 Radiation-induced preneoplastic thyroid nodu- larity in three groups of persons whose thyroid glands were irradiated in childhood follows a linear, nonthreshold dose- response relationship.

Radiation-induced skin cancer follows a threshold dose-response relationship.

C H A P T E R 36 Late Effects of Radiation 559

500 to 2000 rad (5 to 20 Gy t ), the relative risk of devel- oping skin cancer was 4:1. If the dose was 4000 to 6000 rad (40 to 60 Gy t ) or 6000 to 10,000 rad (60 to 100 Gy t ), the relative risks were 14:1 and 27:1, respectively.

Breast Cancer. In Chapter 19, some of the radio- graphic techniques used in mammography were dis- cussed. The radiation dose to mammography patients is considered in a later chapter. Here, we discuss the risk of radiation-induced breast cancer.

Controversy is ongoing regarding the risk of radia- tion-induced breast cancer, with implications for breast cancer detection by x-ray mammography. Concern over such risk fi rst surfaced in the mid-1960s, after reports were published of breast cancer developing in patients with tuberculosis.

Tuberculosis was for many years treated by isolation in a sanitarium. During the patient's stay, one mode of therapy was to induce a pneumothorax in the affected lung; this was done under non – image-intensifi ed fl uo- roscopy. Many patients received multiple treatments and up to several hundred fl uoroscopic examinations.

Precise dose determinations are not possible, but lev- els of several hundred rad would have been common. In some of these patient populations, the relative risk for radiation-induced breast cancer was shown to be as high as 10:1.

One such population exhibited no excess risk. This fi nding, however, was explained as a consequence of the fl uoroscopic technique. In the positive studies, the patient faced away from the radiologist, toward the fl u- oroscopic x-ray tube, during exposure. In the study that reported negative fi ndings, patients were imaged while facing the radiologist so that the radiation beam entered posteriorly. The breast tissue was exposed only to the low-intensity beam that exited the patient.

Additional studies have produced results suggest- ing that radiation-induced breast cancer developed in patients treated with x-rays for acute postpartum mas- titis. The dose to these patients ranged from 75 to 1000 rad (0.75 to 10 Gy t ). The relative risk factor in this pop- ulation was approximately 3:1.

Radiation-induced breast cancer has also been observed among atomic bomb survivors. Through 1980, observations on nearly 12,000 women who received radiation doses to the breasts of 10 rad or more showed a relative risk of 4:1.

In some of these studies, only one breast was irradi- ated. In nearly every such case, breast cancer developed only in the irradiated breast. These patients have now been followed for up to 35 years. On the basis of all available data regarding radiation-induced breast can- cer, the best estimate for absolute risk is 6 cases/10 6 persons/rad/yr.

Lung Cancer. Early in the 20th century, it was observed that approximately 50% of workers in the Bohemian pitchblende mines of Germany died of lung

cancer. Lung cancer incidence in the general population was negligible by comparison. The dusty mine environ- ment was considered to be the cause of this lung cancer. Now it is known that radiation exposure from radon in the mines contributed to the incidence of lung cancer in these miners.

Observations of American uranium miners active in the Colorado plateau in the 1950s and 1960s have also shown elevated levels of lung cancer. The peak of this activity occurred in the early 1960s, when approximately 5000 miners were active in nearly 500 underground mines and 150 open-pit mines. Most of the mines were worked by fewer than 10 men; therefore, for such a small operation, one could expect a lack of proper ventilation.

The radiation exposure in these mines occurred because of the high concentration of uranium ore. Ura- nium, which is radioactive with a very long half-life of 10 9 years, decays through a series of radioactive nuclides by successive alpha and beta emissions, each accompa- nied by gamma radiation.

One of the decay products of uranium is radon ( 222 Rn). This radionuclide is a gas that emanates through the rock to produce a high concentration in air. When breathed, radon can be deposited in the lung, where it undergoes an additional successive series of decay to a stable isotope of lead. During these subsequent decay actions, several alpha particles are released, resulting in a rather high local dose. Also, alpha particles emit high-LET radiation and therefore have a high RBE.

To date, more than 4000 uranium miners have been observed, and they have received estimated doses to lung tissue as high as 3000 rad (30 Gy t ); on this basis, the relative risk was approximately 8:1. It is interest- ing to note that smoking uranium miners have a relative risk of approximately 20:1.

Liver Cancer. Thorium dioxide (ThO 2 ) in a colloidal suspension known as Thorotrast was widely used in diagnostic radiology between 1925 and 1945 as a contrast agent for angiography. Thorotrast was approx- imately 25% ThO 2 by weight, and it contained several radioactive isotopes of thorium and its decay products. Radiation that was emitted produced a dose in the ratio of approximately 100:10:1 of alpha, beta, and gamma radiation, respectively.

The use of Thorotrast has been shown to be responsible for several types of carcinoma after a latent period of approximately 15 to 20 years. After extravascular injection, it is carcinogenic at the site of the injection. After intravascular injection, ThO 2 particles are deposited in phagocytic cells of the reticuloendothelial system and are concentrated in the liver and spleen. Its half-life and high alpha radiation dose have resulted in many cases of cancer in these organs.

PART VI Radiobiology560

TOTAL RISK OF MALIGNANCY On the basis of many of these observations on human population groups after exposure to low-level radiation, and considering all the risk estimates taken collectively for leukemia and cancer, a number of simplifi ed conclu- sions can be made. The overall absolute risk for induc- tion of malignancy is approximately 8 cases/10,000/rad (8 × 10 − 2 Sv − 1 ), with the at-risk period extending for 20 to 25 years after exposure.

Lethality from radiation-induced malignant disease is projected at approximately 50%. Five deaths from radiation-induced malignancy can be expected after an exposure of 1 rad to 10,000 persons. The risk of death from radiation- induced malignant disease is 5/10,000/ rad (5 × 10 − 2 Sv − 1 ).

Three Mile Island To make these values somewhat more meaningful, we can consider the celebrated Three Mile Island incident in 1979. Approximately 2,000,000 people resided within an 80-km (50-mile) radius of Three Mile Island, on the Susquehanna River, in Pennsylvania.

On the basis of population statistics, one would expect to observe approximately 330,000 cancer deaths in these persons. During the total period of the radiation inci- dent, the average dose to persons living within a 160-km (100-mile) radius was 1.5 mrad (15 µGy t ); to those within the 80-km (50-mile) radius, it was 8 mrad (80 µGy t ).

By applying 1.5 mrad as the population dose, one can predict that the Three Mile Island incident will result in no more than two additional malignant deaths as a result of this population radiation exposure. Clearly, this response is not detectable in the face of approximately 400,000 natural cancer deaths in this population.

BEIR Committee The Committee on the Biologic Effects of Ionizing Radia- tion (BEIR), an arm of the National Academy of Sciences, has reviewed the data on late effects of low-dose, low- LET radiation. This report showed the results summa- rized in Table 36-5 , which are considered authoritative.

BEIR committee members examined three situations. First, they estimated the excess mortality from malignant disease after a one-time accidental exposure to 10 rad; such a situation is highly unlikely in radiology. Second, they considered the response to a dose of 1 rad/yr for life; this situation is possible in diagnostic radiology but rare.

Finally, they considered excess radiation-induced can- cer mortality after a continuous dose of 100 mrad/yr.

This is still considerably higher than the experience of most radiologic technologists but can serve as a good upper limit of occupational radiation risk.

When a linear, nonthreshold dose-response relation- ship was assumed, these analyses showed an additional 800 cases of malignant disease death in a population of 100,000 after 10 rad and an additional 550 deaths after 100 mrad/yr. These cases represent an addition to the normal incidence of cancer death, which is approxi- mately 20,000 per 100,000 persons.

The BEIR Committee also has analyzed available human data with regard to age at exposure, a limited time of expression of effects, and whether the response was absolute or relative. This requires additional defini- tions of these terms.

If one is irradiated at an early age and the response is limited in time, radiation-induced excess malignant dis- ease appears as a bulge on the age-response relationship ( Figure 36-9 ). Childhood leukemia is a good example.

Age

Incidence

Exposure

Latent period

Spontaneous incidence

Excess incidence

FIGURE 36-9 Exposure at an early age can result in an excess bulge of cancer after a latent period.

PREDICTED RADIATION-INDUCED DEATHS AT THREE MILE ISLAND

2 × 10 6 people × 5 deaths/10 4 people/rad × 0.0015 rad = 1.5 deaths

TABLE 36-5 BEIR Committee Estimated Excess Mortality From Malignant Disease in 100,000 People

Male Female

Normal expectation 20,560 16,680 Excess cases

Single exposure to 10 rad (100 mGy t )

770 810

Continuous exposure to 1 rad/yr (10 mGy t /yr)

2880 3070

Continuous exposure to 100 mrad/yr (1 mGy t /yr)

520 600

The BEIR Committee has further stated that because of the uncertainty in its analysis, less than 1 rad/yr may not be harmful.

C H A P T E R 36 Late Effects of Radiation 561

An absolute age-response relationship is shown in Figure 36-10 . Here, the increased incidence of cancer is seen as a constant number of cases after a minimal latent period. Most subscribe to a relative age- response relationship, in which the increased inci- dence of cancer is proportional to the natural incidence ( Figure 36-11 ).

Perhaps the best way to present these radiation risk data is to compare them with other known causes of death. As one might imagine, volumes of tables are available that analyze risk. This information is presented in simplifi ed form in Table 36-6 .

Note that in these common situations, risk from radi- ation exposure is near the bottom of the list. Our actual occupational risk is even less because we use protective apparel during fl uoroscopy and the radiation risk esti- mate assumes whole-body exposure.

RADIATION AND PREGNANCY Since the fi rst medical applications of x-rays, concern and apprehension have arisen regarding the effects of radiation before, during, and after pregnancy. Before

pregnancy, the concern is interrupted fertility. During pregnancy, concern is directed to possible congenital effects in newborns. Postpregnancy concerns are related to suspected genetic effects. All these effects have been demonstrated in animals, and some have been observed in humans.

Effects on Fertility The early effect of high-level radiation on the interrup- tion of fertility in both men and women is discussed in Chapter 35. Ample evidence shows that such an effect does occur and is dose related. The effects of low-dose, long-term irradiation on fertility, however, are less well defi ned.

Animal data in this area are lacking. Those that are available indicate that, even when radiation is delivered at the rate of 100 rad per year, no noticeable depression in fertility is noted.

The health effects analysis of 150,000 American radio- logic technologists mentioned earlier has revealed no effect on fertility. The number of births that occurred during a 12-year sampling period equaled the number expected.

Irradiation In Utero Irradiation in utero concerns the following two types of exposures: that of the radiation worker and that of the patient. Recommended techniques and radiation con- trol procedures associated with these exposed persons are considered fully in Chapter 40. Here, we consider the biologic effects of such irradiation.

Age

Incidence

Exposure

Latent period

Excess incidence

Spontaneous incidence

FIGURE 36-10 The absolute risk model predicts that the excess radiation-induced cancer risk is constant for life.

Age

Incidence

Exposure

Latent period

Excess incidence

Spontaneous incidence

FIGURE 36-11 The relative risk model predicts that the excess radiation-induced cancer risk is proportional to the natural incidence.

TABLE 36-6 Average Annual Risk of Death From Various Causes

Cause Your Chance of Dying This Year

All causes (all ages) 1 in 100 20 cigarettes per day 1 in 280 Heart disease 1 in 300 Cancer 1 in 520 All causes (25-year-old) 1 in 700 Stroke 1 in 1200 Motor vehicle accident 1 in 4000 Drowning 1 in 30,000 Alcohol (light drinker) 1 in 50,000 Air travel 1 in 100,000 Radiation, 100 mrad 1 in 100,000 Texas Gulf Coast hurricane 1 in 4,500,000 Being a rodeo cowboy 1 in 6,200,000

Low-dose, chronic irradiation does not impair fertility.

PART VI Radiobiology562

Substantial animal data are available to describe fairly completely the effects of relatively high doses of radiation delivered during various periods of gestation. Because the embryo is a rapidly developing cell system, it is particularly sensitive to radiation. With age, the embryo (and then the fetus) becomes less sensitive to the effects of radiation, and this pattern continues into adulthood.

After maturity has been reached, radiosensitivity increases with age. Figure 36-12 summarizes the observed

LD 50/60 in mice exposed at various times, showing this aggregated radiosensitivity. Such fi ndings are of par- ticular concern because diagnostic x-ray exposure often occurs when pregnancy is unknown.

The effects of radiation in utero are time related and dose related. They include prenatal death, neonatal death, congenital abnormalities, malignancy induction, general impairment of growth, genetic effects, and men- tal retardation. Figure 36-13 has been redrawn from studies designed to observe the effects of a 200-rad (2-Gy t ) dose delivered at various stages in utero in mice. The scale along the x-axis indicates the approximate comparable time in humans.

Within 2 weeks of fertilization, the most pronounced effect of a high radiation dose is prenatal death, which manifests as a spontaneous abortion. Observations in radiation therapy patients have confi rmed this effect, but only after very high doses.

On the basis of animal experimentation, it would appear that this response is very rare. Our best estimate is that a 10-rad (100-mGy t ) dose during the fi rst 2 weeks will induce perhaps a 0.1% rate of spontaneous abor- tion. This occurs in addition to the 25% to 50% normal incidence of spontaneous abortions.

Mouse age (weeks) 0 20 40 60 80 100

In utero

Mouse LD 50/60 (rad)

700

600

500

400

300

200

100

0

0 20 40 60 7010 30 50 Human age (years)

FIGURE 36-12 LD 50/60 of mice in relation to age at time of irradiation.

Prenatal death

Congenital abnormalities

Neonatal death

LeukemiaRelative incidence

Gestational age (weeks)

0 4 8 12 16 20 24 28 32 36

1st trimester 2nd trimester 3rd trimester

FIGURE 36-13 After 200 rad are delivered at various times in utero, a number of effects can be observed.

All observations point to the fi rst trimester during pregnancy as the most radiosensitive period.

C H A P T E R 36 Late Effects of Radiation 563

Fortunately, this response is of the all-or-none vari- ety: Either a radiation-induced abortion occurs, or the pregnancy is carried to term with no ill effect.

During the period of major organogenesis, from the 2nd through the 10th week, two effects may occur. Early in this period, skeletal and organ abnormalities can be induced. As major organogenesis continues, congenital abnormalities of the central nervous system may be observed if the pregnancy is carried to term.

If radiation-induced congenital abnormalities are severe enough, the result will be neonatal death. After a dose of 200 rad (2 Gy t ) to the mouse, nearly 100% of fetuses suffered signifi cant abnormalities. In 80%, this was suffi cient to cause neonatal death.

Such effects are rare after diagnostic levels of exposure and are essentially undetectable after radia- tion doses of less than 10 rad (100 mGy t ). A dose of 10 rad (100 mGy t ) during organogenesis is expected to increase the incidence of congenital abnormalities by 1% above the natural incidence. To complicate mat- ters, an approximate 5% incidence of naturally occur- ring congenital abnormalities occurs in the unexposed population.

Irradiation in utero at the human level has been as- sociated with childhood malignancy by a number of investigators. Perhaps the most complete study of this effect was conducted by Alice Stewart and coworkers in a project known as the Oxford Survey, a study of child- hood malignancy in England, Scotland, and Wales.

Nearly every such case of childhood malignancy in these countries since 1946 has been investigated. Each case was fi rst identifi ed and then investigated by inter- view with the mother, review of the hospital charts, and review of the physician records.

Each “case” of childhood malignancy was matched with a “control” for age, sex, place of birth, socioeco- nomic status, and other demographic factors. The con- trol subject was a child who matched with the “case” in all respects, except that the control did not have can- cer or leukemia. The Oxford Survey is being continued at this time and has now considered more than 10,000 cases and a like number of matched control subjects.

Although the Oxford Survey has reviewed all malig- nancies, it is the fi ndings of radiation-induced leukemia that have been of particular importance. Table 36-7 shows the results of this survey in terms of relative risk.

A relative risk of 1.5 for the development of child- hood leukemia after irradiation in utero is significant. This indicates an increase of 50% over the nonirradi- ated rate. The number of cases involved, however, is small.

The incidence of childhood leukemia in the popula- tion at large is approximately 9 cases per 100,000 live births. According to the Oxford Survey, if all 100,000 had been irradiated in utero, perhaps 14 cases of leuke- mia would have resulted. Although these fi ndings have been substantiated in several American populations, no consensus has been reached among radiobiologists that this effect after such low doses is indeed real.

Other effects after irradiation in utero have been stud- ied rather fully in animals and have been observed in some human populations. An unexpected fi nding in the offspring of atomic bomb survivors is mental retarda- tion. Children of exposed mothers performed poorly on IQ tests and demonstrated poor scholastic performance compared with unexposed Japanese children.

These differences are marginal, yet signifi cant. When assessment is based on test scores, measurable mental retardation is apparent in approximately 6% of all chil- dren. A 10-rad dose in utero is expected to increase this incidence by an additional 0.5%.

Radiation exposure in utero does retard the growth and development of the newborn. Irradiation in utero, principally during the period of major organogenesis, has been associated with microcephaly (small head) and, as discussed, mental retardation.

Human data bearing on these effects have been obtained from patients irradiated medically, atomic bomb survivors, and residents of the Marshall Islands who were exposed to radioactive fallout in 1954 dur- ing weapons testing. For instance, heavily irradiated children at Hiroshima are, on average, 2.25 cm (0.9 in) shorter, 3 kg (6.6 lb) lighter, and 1.1 cm (0.4 in) smaller in head circumference than members of nonirradiated control groups.

These effects, as well as mental retardation, have been observed principally in those receiving doses in excess of 100 rad (1 Gy t ) in utero. The lack of appropriate and sensitive tests of mental function makes it impossible to draw similar conclusions at doses below 100 rad (1 Gy t ).

The fi rst 2 weeks of pregnancy may be of least concern because the response is all-or-nothing.

The relative risk of childhood leukemia after irradiation in utero is 1.5.

TABLE 36-7 Relative Risk of Childhood Leukemia After Irradiation In Utero by Trimester

Time of X-Ray Examination Relative Risk

First trimester 8.3 Second trimester 1.5 Third trimester 1.4 Total 1.5

PART VI Radiobiology564

A summary of the effects of irradiation in utero is given in Table 36-8 . Four responses of concern to ra- diology have been identifi ed: spontaneous abortion, congenital abnormalities, mental retardation, and child- hood malignancy.

Spontaneous abortion causes the least concern of the four because it is an all-or-none effect. Congenital ab- normalities, mental retardation, and childhood malig- nancy are of real concern, but it should be recognized that the probability of such a response after a fetal dose of 10 rad (100 mGy t ) is nil. Furthermore, 10 rad (100 mGy t ) to the fetus very rarely occurs in radiology.

The form of the dose-response relationship for each of these effects is unknown. However, several appear to be linear and nonthreshold when based on doses greater than 100 rad (1 Gy t ). When large experimental animal populations were acutely exposed, the minimum reported dose at which such effects were observed as statistically signifi cant was approximately 10 rad (100 mGy t ).

No evidence in humans or animals indicates that the levels of radiation exposure currently experienced oc- cupationally and medically are responsible for any such effects on growth and development.

Although our efforts in protecting the unborn from the harmful effects of radiation are principally directed at diagnostic x-ray exposures, we also must be aware of similar hazards resulting from radioisotope examina- tions. For example, radioiodine is known to concentrate principally in the thyroid gland. After administration of radioactive iodine, the dose to thyroid tissue will be sev- eral orders of magnitude higher than the whole-body dose because of this organ concentration effect.

The thyroid gland begins to function at approximately 10 weeks of gestation, and because radioiodine readily crosses the placental barrier from the mother's blood to the fetal circulation, radioiodine should be administered during pregnancy only in trace doses and before the 10-week gestation period begins. At any time thereafter, the hazard of such administration increases.

Genetic Effects Unfortunately, our weakest area of knowledge in radiation biology is the area of radiation genetics. Essentially all the data indicating that radiation causes genetic effects have come from large-scale experiments with fl ies or mice.

Observations of the atomic bomb survivors have shown no radiation-induced genetic effects, and descendants of survivors are now into the third generation. Other human populations have likewise provided only negative results. Consequently, in the absence of accurate human data, there is no choice but to rely on information from experimental labora- tory studies.

In 1927, the Nobel prize – winning geneticist H.J. Muller from the University of Texas reported the results of his irradiation of Drosophila , the fruit fl y. He irradiated mature fl ies before procreation and then measured the frequency of lethal mutations among the offspring. The radiation doses used were thousands of rad, but as the data in Figure 36-14 show, the dose- response relationship for radiation-induced genetic damage is unmistakably linear, nonthreshold.

Radiation dose (rad)

0 1000 2000 3000 4000

Lethal mutations

(%)

10

8

6

4

2

0

FIGURE 36-14 Irradiation of fl ies by H.J. Muller showed the genetic effects to be linear, nonthreshold. Note that the doses were exceedingly high.

We do not have any data that suggest that radiation-induced genetic effects occur in humans.

TABLE 36-8 Summary of Effects After 10 Rad In Utero

Time of Exposure Type of Response Natural Occurrence Radiation Response

0-2 wk Spontaneous abortion 25% 0.1% 2-10 wk Congenital abnormalities 5% 1% 2-15 wk Mental retardation 6% 0.5% 0-9 mo Malignant disease 8/10,000 12/10,000 0-9 mo Impaired growth and development 1% Nil 0-9 mo Genetic mutation 10% Nil

C H A P T E R 36 Late Effects of Radiation 565

On the basis of Muller's studies, other conclusions were drawn. Radiation does not alter the quality of mutations but rather increases the frequency of those mutations that are observed spontaneously. Muller's data showed no dose rate or dose fractionation effects. Hence, he concluded that such mutations were single-hit phenomena.

It was principally on the basis of Muller's work that the National Council on Radiation Protection in 1932 lowered the recommended dose limit and acknowl- edged offi cially for the fi rst time the existence of non- threshold radiation effects. Since then, all radiation protection guides have assumed a linear, nonthreshold dose- response relationship and have been based on the sus- pected genetic, as well as somatic, effects of radiation.

The only other experimental work of any signifi cance is that of Russell. Beginning in 1946, he began to irradi- ate a large mouse colony with radiation dose rates that varied from 0.001 to 90 rad/min (0.01 to 900 mGy t /min) and total doses up to 1000 rad (10 Gy t ). These studies are ongoing, and observations now have been reported on more than 8 million mice! The experiment requires the observation of seven specifi c genes that control read- ily recognizable characteristics, such as ear shape, coat color, and eye color.

Russell's data show that a dose rate effect does exist; this would indicate that the mouse has the capacity to repair genetic damage. He has confi rmed the linear, nonthreshold form of the dose-response relationship and has not detected any types of mutations that did not occur naturally.

The average mutation rate per unit dose in the mouse is approximately 15 times that observed in the fruit fl y. Whether an increased sensitivity exists in humans rela- tive to the mouse is unknown.

From these experimental studies, the concept of the doubling dose has been developed. The doubling dose in humans is estimated to lie in the range between 50 and 250 rad (0.5 and 2.5 Gy t ).

So, what is the signifi cance of all this in our daily practice? What is the signifi cance for patients or for radiologic technologists? First, it can be said with cer- tainty that the incidence of radiation-induced genetic mutations after the levels of exposure experienced in diagnostic radiology is essentially zero ( Box 36-1 ).

Under nearly all such diagnostic exposures, no action is required; however, should a high radiation dose be experienced (e.g., in excess of 10 rad), some protective action may be required. The prefertilized egg, in its vari- ous stages, exhibits a constant sensitivity to radiation;

however, it also demonstrates some capacity for repair of genetic damage. If repair occurs, it is rapid; therefore, a delay in procreation of only a few days may be ap- propriate. In the male, on the other hand, it might be prudent to refrain from procreation for a period of 60 days to allow cells that were in a resistant stage of devel- opment at the time of exposure to mature to functioning spermatids.

SUMMARY The late effects of radiation exposure occur a long time after exposure. Late effects can result from high-dose, short-term exposure, but the concern in diagnostic imaging involves low-dose exposures over time.

Many epidemiologic studies have reported positive results; however, problems include the following: (1) The exact dose usually is not known, and (2) the fre- quency of observable response is low. Most late effects are stochastic — the incidence of response is dose related and no dose threshold is evident.

Local tissues can be affected by low-dose radiation. Late effects appear as nonmalignant changes in the skin. The skin shows a weathered, callused, and discolored appearance. Chromosome damage in circulating lym- phocytes and cataracts in the lens of the eye have been observed as late effects of radiation exposure.

Because dose-response relationships are not precise when late effects of radiation exposure are observed, risk estimates are used to estimate radiation response in a population. Relative risk is calculated when the

BOX 36-1 Additional Conclusions Regarding Radiation Genetics

• Radiation-induced mutations are usually harmful. • Any dose of radiation, however small, to a germ cell

results in some genetic risk. • The frequency of radiation-induced mutations

is directly proportional to dose, so that a linear extrapolation of data obtained at high doses provides a valid estimate of low-dose effects.

• The effect depends on radiation protraction and fractionation.

• For most pre-reproductive life, the woman is less sensitive than the man to the genetic effects of radiation.

• Most radiation-induced mutations are recessive . These require that the mutant genes must be present in both the male and the female to produce the trait. Consequently, such mutations may not be expressed for many generations.

• The frequency of radiation-induced genetic mutations is extremely low. It is approximately 10 − 7 mutations/rad/gene.

The doubling dose is that dose of radiation that produces twice the frequency of genetic mutations as would have been observed without the radiation.

PART VI Radiobiology566

population's radiation dose is not known. Relative risk is computed by comparing the number of persons in the exposed population with late effects versus the number in an unexposed population in whom the same condi- tion developed. Excess risk determines the magnitude of the late effect as the difference between cases and con- trol subjects.

The effects of low-dose, long-term irradiation in utero can include the following: prenatal death, neonatal

death, congenital abnormalities, malignancy, impaired growth, genetic effects, and mental retardation. How- ever, these abnormalities are based on doses greater than 100 rad, with minimum reported doses in animal experiments at approximately 10 rad. No evidence at the human or animal level indicates that the levels of radiation exposure currently experienced occupation- ally or medically are responsible for any such effects on fetal growth or development.

C H A P T E R 36 Late Effects of Radiation 567

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. Epidemiology b. In utero c. ABCC-RERF d. Thorotrast e. Major organogenesis f. The Oxford Survey g. H.J. Muller h. Doubling dose i. Radon ( 222 Rn) j. Radium watch-dial painters 2. What population experienced radiation-induced

cataracts? 3. What is the risk of life span shortening for

radiation workers? 4. What is the signifi cance of the change in death sta-

tistics of American radiologists from the 1935 to 1944 time period to the 1955 to 1958 time period?

5. Approximately 300,000 radiologic technologists are working in the United States, and their annual exposure is 50 mrem (0.5 mSv). If a 40-year working period is assumed, how many are likely to die from occupational radiation exposure?

6. What is the absolute risk when three cases of radiation-induced leukemia develop per year in 100,000 persons after an average dose of 2 rad?

7. When should excess risk be used as the preferable risk index?

8. Twenty million people in Scandinavia were exposed to an average 0.7 mrad as a result of Chernobyl. If an absolute risk of 10 cases/10 6 /rad/yr over a 30-year period is assumed, how many malignancies will be induced?

9. What is the suspected reason why American radiologists have an elevated risk for leukemia?

10. Discuss the experience of radiation-induced leukemia in patients with ankylosing spondylitis.

11. Why was the thymus gland irradiated in the Ann Arbor and Rochester series? What were the late effects of the thymus irradiation?

12. Discuss the way that bone cancer developed in watch-dial painters in the 1920s and 1930s.

13. Explain the risk of radon gas to uranium miners. 14. During the period of the Three Mile Island

incident, what was the average dose to persons living within a 100-mile radius of the nuclear plant?

15. What are the effects on fertility caused by low-dose, long-term irradiation?

16. Is it true that most radiation-induced mutations are recessive?

17. In a population of 30,367 irradiated persons, 13 cases of leukemia developed; in a control population of 86,672 persons, 31 cases of leukemia developed. What was the relative risk?

18. What is the absolute risk if 32 cases of leukemia develop per year in 100,000 persons after an average dose of 2 rad?

19. How many cases of radiation-induced leukemia are suspected to have occurred among atomic bomb survivors?

20. What is the difference between relative risk and excess risk?

The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

570

37 Health Physics OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Defi ne health physics 2. List the cardinal principles of radiation protection and discuss

the ALARA concept 3. Explain the meaning of NCRP and the concept of dose limits 4. Name the recommended dose limits for radiation workers and

the public 5. Discuss the radiosensitivity of the stages of pregnancy 6. Describe the recommended management procedures for pregnant

radiation workers and for the pregnant patient OUTLINE Radiation and Health Cardinal Principles of Radiation Protection

Minimize Time Maximize Distance Use Shielding

Effective Dose Patient Effective Dose Radiologic Technologist Effective Dose

Radiologic Terrorism Radiologic Device Radiation Protection Guidance Radiation Detection and Measurement Equipment

C H A P T E R

C H A P T E R 37 Health Physics 571

The term health physics was coined during the early days of the Manhattan Project, the secret wartime effort un- dertaken to develop the atomic bomb. The group of phys- icists and physicians responsible for the radiation safety of persons involved in the production of atomic bombs were the fi rst health physicists. Thus, the health physicist is a radiation scientist who is concerned with the research, teaching, or operational aspects of radiation safety.

RADIATION AND HEALTH At the turn of the Millennium, the year 2000, the National Academy of Sciences identifi ed the 20 great- est scientifi c and technical accomplishments of the 20th century. Medical imaging was number 14 on this list.

This is important to point out to our patients, many of whom remain wary of radiation. One never reads the word “radiation” in a newspaper or a magazine without the modifi er “dangerous,” “deadly,” or “harmful.”

We practice ALARA because of the linear nonthresh- old radiation dose-response relationship (LNT) for sto- chastic effects — cancer, leukemia, and genetic effects. Yet we should also recognize that we actually employ low levels of radiation in diagnostic imaging.

Unquestionably, the application of this radiation has had a major impact on our health and increasing longevity. If you had been born in the United States in 1900, your life expectancy was 47 years. During the fi rst century of diagnostic x-ray imaging, life expectancy has soared. Life expectancy is now 78 years ( Figure 37-1 ).

Nevertheless, because of LNT, we must continue to be aware of patient and occupational radiation dose and must take those steps necessary to implement ALARA.

CARDINAL PRINCIPLES OF RADIATION PROTECTION All health physics activity in radiology is designed to minimize the radiation exposure of patients and person- nel. Three cardinal principles of radiation protection developed for nuclear activities — time, distance, and shielding — fi nd equally useful application in diagnostic radiology. When these cardinal principles are observed, radiation exposure can be minimized ( Box 37-1 ).

Minimize Time The dose to an individual is directly related to the dura- tion of exposure. If the time during which one is exposed to radiation is doubled, the exposure will be doubled, as follows:

Question: A radiation worker is exposed to 230 mR/hr (2.3 mGy a /hr) from a radiation source. If the worker remains in that position for 36 minutes, what will be the total occupational exposure?

Answer:

Occupational exposure = 230 mR/hr 36 min

60 min/hr

= 138mR

Question: The parent of a patient is asked to remain next to the patient during fl uoroscopy, where the radiation exposure level is 600 mR/hr (6 mGy a /hr). If the allowable daily exposure is 50 mR, how long may the parent remain? ( Figure 37-2 )

Answer: Time = Exposure ÷ Exposure rate = 50 mR ÷ 600 mR/hr = 1/12 hour = 5 minutes

TIME

Exposure = Exposure rate × Exposure time

Health physics is concerned with providing occupational radiation protection and minimizing radiation dose to the public.

MMEDIATELY AFTER their discovery, x-rays were applied to the healing arts. It was recognized with- in months, however, that radiation could cause

harmful effects. The fi rst American fatality that resulted from

radiation exposure was Thomas Edison's assistant, Clarence Dally. Since that event, a great deal of effort has been devoted to developing equipment, techniques, and procedures to control radiation levels and reduce unnecessary radiation exposure to radiation workers and the public.

The cardinal principles for radiation protection are simplifi ed rules designed to ensure safety in radiation areas for occupational workers. In 1931, the fi rst dose-limiting recommendations were made. Today, the National Council on Radiation Protection and Measurements (NCRP) continuously reviews the recommended dose limits.

Providing radiation protection for workers and the public is the practice of health physics. Health physi- cists design equipment, calculate and construct barri- ers, and develop administrative protocols to maintain radiation exposures as low as reasonably achievable (ALARA). That is the substance of this chapter.

I

PART VII Radiation Protection572

During radiography, the time of exposure is kept to a minimum to reduce motion blur. During fl uoroscopy, the time of exposure also should be kept to a minimum to reduce patient and personnel exposure. This is an area of radiation protection that is not directly controlled by the radiologic technologist.

Radiologists are trained to depress the fl uoroscopic foot switch in an alternating fashion, sequencing on-off rather than continuous on during the course of the ex- amination. A repeated up-and-down motion on the fl uo- roscopic foot switch permits a high-quality examination to be performed with considerably reduced exposure to

the patient. The use of pulse-progressive fl uoroscopy can reduce patient dose considerably.

The 5-minute reset timer on all fl uoroscopes reminds the radiologist that a considerable amount of fl uoro- scopic time has elapsed. The timer records the amount of x-ray beam on time. Most fl uoroscopic examinations take less than 5 minutes.

Only during diffi cult interventional radiology proce- dures should it be necessary to exceed 5 minutes of ex- posure time. A particular hazard lies in the use of mobile image intensifi ers in surgical suites where some physi- cians are less radiation conscious.

Question: A fl uoroscope emits 4.2 R/min (42 mGy a /min)

at the tabletop for every milliampere of operation (4.2 R/mAmin). What is the patient exposure in a barium enema examination that is conducted at 1.8 mA and requires 2.5 minutes of fl uoroscopic x-ray exposure time?

Answer:

Patient exposure = 4.2 R

mAmin (1.8 mA)(2.5 min)

= 1

 

 

88.9 R

Life expectancy at birth

Additional years of life expectancy if you are alive at age 65

1900 1950 1960 1970 1980 1990 1995 1996 1997 1998 1999 2000 2001 2002 2003

40

50

60

70

80

20

15

10

5

0

Years

Years

FIGURE 37-1 Life expectancy as a function of year of birth.

BOX 37-1 Cardinal Principles of Radiation Protection

• Keep the time of exposure to radiation as short as possible.

• Maintain as large a distance as possible between the source of radiation and the exposed person.

• Insert shielding material between the radiation source and the exposed person.

C H A P T E R 37 Health Physics 573

Maximize Distance As the distance between the source of radiation and the person increases, radiation exposure decreases rapidly. This decrease in exposure is calculated using the inverse square law, which was discussed in Chapter 4.

Most radiation sources are point sources. The x-ray tube target, for example, is a point source of radiation. The scat- tered radiation generated in a patient appears, however, to come not from a point source but rather from an extended area source. As a rule of thumb, even an extended source can be considered a point source at suffi cient distance.

Earlier, when the square law was used to calculate exposure in radiographic technique, the following for- mula may have been used:

New Exposure Old Exposure

= New Distance Squared Old Distance Squared

In this case, the exposure from the source (the x-ray

tube) was varied so that the optical density of the fi lm (OD) would remain constant.

When the inverse square law is used in calculations for radiation protection, it is usual to calculate the dose received at a point with the radiation from the tube as the constant.

Thus the above formula becomes

New Exposure Old Exposure

= Old Distance Squared New Distance SSquared

Note that the “distance” part of the equation is reversed.

Question: An x-ray tube has an output intensity of

2.6 mR/mAs at 100-cm source-to-image receptor distance (SID) when operated at 70 kVp. What would be the radiation exposure 350 cm from the target?

Answer: I I

= d d

I = I d d

= (2.6 mR/mAs) 100 cm 350 cm

= (2

1

2

2

1

1 2 2

1

2

2

2

2

2  

 

..6 mR/mAs)(0.082)

= 0.21 mR/mAs

In radiography, the distance from radiation source to patient usually is fi xed by the type of examination, and the radiologic technologist is positioned behind a pro- tective barrier.

During fl uoroscopy, the radiologic technologist can exercise good radiation protection procedures. Figure 37-2 shows approximate radiation exposure levels at waist height during a fl uoroscopic examination. The lines on the plot plan, called isoexposure lines, represent positions of equal radiation exposure in the fl uoroscopy room. At the normal position for a radiologist or a ra- diologic technologist, the exposure rate is approximately 300 mR/hr (3 mGy a /hr).

During portions of the fl uoroscopic examination, when it is not necessary for the radiologic technologist to re- main close to the patient, the technologist should step back. Two steps back, the exposure rate is only ap- proximately 5 mR/hr (50 µGy a /hr). This reduction in exposure does not follow the inverse square law because during fl uoroscopy, the patient is an extended source of radiation because of scattered x-rays generated within the body.

Protective curtain

Viewing window

Console

5 500

50

FIGURE 37-2 Typical isoexposure contours during fl uoro- scopic examination (mR/hr).

DISTANCE

Assume a point source and apply the inverse square law.

If the distance from the source exceeds fi ve times the source diameter, it can be treated as a point source.

During fl uoroscopy, the radiologic technologist should remain as far from the patient as practicable.

PART VII Radiation Protection574

Question: What is the approximate occupational expo- sure of a radiologic technologist at a position where the exposure rate is 300 mR/hr, and farther back where the exposure rate is 20 mR/hr, during a fl uoroscopic examination that lasts 4 minutes, 15 seconds?

Answer: Occupational exposure equals First position: (300 mR/hr) (4.25 min) (1 hr/60 min) = 21.25 mR Second position: (20 mR/hr) (4.25 min) (1 hr/60 min) = 1.4 mR

Better yet, after two steps back to take advantage of “Maximize distance,” take one step to the side and get behind the radiologist! This move results in additional shielding.

Use Shielding Positioning shielding between the radiation source and exposed persons greatly reduces the level of radiation exposure. Shielding used in diagnostic radiology usually consists of lead, although conventional building materi- als also are used.

The amount that a protective barrier reduces radiation intensity can be estimated if the half-value layer (HVL) or the tenth-value layer (TVL) of the barrier material is known. The HVL is defi ned and discussed in Chapter 12. The TVL is similarly defi ned as follows:

Table 37-1 shows approximate HVLs and TVLs for lead and concrete for diagnostic x-ray beams between 40 and 150 kVp.

Question: When operated at 80 kVp, an x-ray imaging system emits 3.6 mR/mAs at an SID of 100 cm. How much shielding (concrete or lead) would be required to reduce the intensity to less than 0.25 mR/mAs?

Answer: The amount of shielding in the fi rst or second column of the following data will reduce the beam intensity to the value in the third column. The last row is the answer. Pb Concrete Beam intensity (mm) (in) (mR/mAs) 0 0 3.60 0.19 0.42 1.80 0.38 0.84 0.90 0.57 1.26 0.45 0.76 1.68 0.23

Question: An x-ray imaging system is used strictly for chest radiography at 125 kVp. The useful beam is always directed to a wall that contains 0.8 mm Pb shielding. How much additional shielding will be required if the workload doubles?

Answer: When the workload doubles, so does the exposure on the other side of the wall. From Table 37-1 , it can be seen that one HVL, or 0.27 mm Pb, is necessary to reduce exposure to its original level.

Another example of the application of shielding in radiology is the use of protective apparel. Protective aprons usually contain 0.5 mm Pb. This is approxi- mately equivalent to 2 HVLs, which should reduce occupational exposure to 25%. Actual measurements show that such protective aprons reduce exposure to approximately 10% because scattered x-rays are inci- dent on the apron at an oblique angle.

Usually, application of the cardinal principles of radiation protection involves consideration of all three. The typical problem involves a known radia- tion level at a given distance from the source. The

HVL TVL

Tube Potential Lead (mm) Concrete (inches) Lead (mm) Concrete (inches)

40 kVp 0.03 0.13 0.06 0.40 60 kVp 0.11 0.25 0.34 0.87 80 kVp 0.19 0.42 0.64 1.4 100 kVp 0.24 0.60 0.80 2.0 125 kVp 0.27 0.76 0.90 2.5 150 kVp 0.28 0.86 0.95 2.8

TABLE 37-1 Approximate Half-Value and Tenth-Value Layers of Lead and Concrete at Various Tube Potentials

HVL , Half-value layer; TVL , tenth-value layer.

SHIELDING

1 TVL = 3.3 HVL

One TVL is the thickness of absorber that reduces the radiation intensity to one-tenth its original value.

C H A P T E R 37 Health Physics 575

level of exposure at any other distance, behind any shielding, for any length of time can be calculated. The order in which these calculations are made makes no difference.

Question: The kVp of a radiographic imaging

system rarely exceeds 100 kVp. The output intensity is 4.6 mR/mAs at 100-cm SID. The distance to a desk on the other side of the wall to which the x-ray beam is directed is 200 cm. The wall contains 0.96 mm Pb, and 300 mAs is anticipated daily. If the exposure is to be restricted to 2 mR/wk, how long each day may the desk be occupied?

Answer: Daily x-ray output at 100 cm

(4.6 mR/mAs)(300 mAs) 1380 mR = =

DDaily output at 200 cm

(1380)(100/200) 345 mR

Daily outpu

2

= =

tt behind 0.96 mm Pb or 4

HVLs 22 mR

110 mR/wk

Time allowed

= =

= 22mR

110 mR/wk 0.018 week

43 minutes

=

=

However, this analysis does not take into account the x-ray beam attenuation by the patient, which is approximately 2 TVLs or 0.01. Therefore,

Question: Suppose an analysis shows that if an administrator remains at her desk for longer than 24 minutes each week, the occupational dose limit will be exceeded. How much additional protective lead would be required?

Answer: Full occupancy is 40 hr 60 min/hr = 2400 min

2400 min 24 min

= 1

×

000

That is, 2 TVLs or an additional 1.6 mm Pb.

Figure 37-3 illustrates the use of these cardinal prin- ciples of radiation protection during a typical clinical situation.

EFFECTIVE DOSE It is relatively easy to measure patient radiation exposure and dose during medical x-ray imaging. However, x-ray imaging involves partial-body exposure. Radiographic images are collimated to the tissue of importance; there- fore, the total body is not exposed.

Radiation risk coeffi cients are based on total body radiation exposure, as for the atomic bomb survivors of Hiroshima and Nagasaki. When only part of the body is exposed, as in medical x-ray imaging, the risk of a stochastic radiation response is not proportional to the tissue dose but rather to the effective dose (E).

The equivalent whole-body dose is the weighted average of the radiation dose to various organs and tissues. The National Committee on Radiation Protection (NCRP) has identifi ed various tissues and organs and the relative radiosensitivity of each ( Table 37-2 ).

Effective dose is the weighted average dose to each of the tissues in Table 37-1 .

Patient Effective Dose Consider, for example, the relationship between patient dose and effective dose in computed tomography (CT) ( Figure 37-4 ). CT examination of the pelvis results in a rather uniform dose of 2000 mrad (20 mGy) to the tis- sues of the pelvis. Other tissues are not irradiated.

The exercise shown in Box 37-2 illustrates the man- ner in which one arrives at patient effective dose. This exercise shows that the effective dose for pelvic CT is 740 mrad.

Another example, as shown in Figure 37-5 , poste- rior-anterior chest radiography, should help explain this concept. Entrance skin dose for this examination is approximately 10 mrad. If one assumes an aver- age tissue dose of half the entrance skin dose, 5 mrad, the effective dose is 1.35 mrem, as computed in Box 37-3 .

Radiologic Technologist Effective Dose We receive essentially all of our occupational radiation exposure during fl uoroscopy. During radiography and mammography, the radiographer is positioned behind a protective barrier, resulting in zero occupational radia- tion exposure.

During fl uoroscopy, we position our occupational ra- diation monitor at the collar, as shown in Figure 37-6 , to

Effective dose is the equivalent whole-body dose.

E = Σ D i W t

Daily output behind 0.96 mm Pb and the

patient (110 mR)(0.= 001) 1.1 mR

Time allowed 2 mR

1.1 mR/wk 1.8 wk (unlimited)

=

= =

PART VII Radiation Protection576

estimate dose to the tissues of the head and neck. The tis- sues of the trunk of the body receive essentially zero dose; the protective apron does what it is designed to do.

So the estimation of effective occupational dose is shown in Box 37-4 for an occupation monitor response of 100 mrem (1 mSv). The result of this exercise is an occupational effective dose of 5 mrem.

Assuming an effective dose of 10% of the occupational monitor dose is conservative. In actual fact, it is some- thing less than 10%.

We will return to the concept of effective dose in Chapters 39 and 40. Be reminded that it is effective dose that should be used for radiation risk estimation.

RADIOLOGIC TERRORISM Emergency response to a radiologic incident conducted by terrorists, an exceptionally rare event, must be dealt with quickly and competently in order to save life and limit property and environmental damage. Emergency respond- ers are those individuals who must make the fi rst decisions and take the fi rst steps in the early stages of such an event.

The fi rst emergency responders are likely to be police, fi re, or emergency medical personnel. In the setting of a health care facility, radiologic technologists may likely be the fi rst emergency responders.

FIGURE 37-3 Application of the cardinal principles of radiation protection in radiology.

E = 740 mrem (7.4 mSv)

D = 2000 mrad (20 mGy)

FIGURE 37-4 The relationship between tissue dose and effec- tive dose during computed tomography.

TABLE 37-2 Weighting Factors for Various Tissues

Tissue Tissue Weighting Factor (W t )

Gonad 0.20 Active bone marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Esophagus 0.05 Liver 0.05 Thyroid 0.05 Bone surface 0.01 Skin 0.01

We assume the occupational effective dose to be 10% of the monitor dose.

C H A P T E R 37 Health Physics 577

The fi rst task of emergency responders is to prevent in- jury and death and to attend to the medical needs of victims. Such immediate responses include limiting acute, high-intensity radiation exposure and limiting low-intensity radiation exposure that could result in late stochastic effects. This is an ALARA exercise and will involve the application of the cardinal principles of radiation protection: Reduce time of exposure, increase distance from the source, and impose shielding between the source and the victim.

Radiologic Device The malevolent use of radiologic material by terrorists can be described as one of three devices: a radiation expo- sure device (RED), a radiologic dispersal device (RDD), and an improvised nuclear device (IND). Dealing with the effect of such devices requires specifi c response tech- niques for each.

An RED is a sealed source of radioactive material that directly exposes people. An RED will not disperse radio- active material; therefore, decontamination of an RED is not required.

An RDD is a bomb that when exploded disperses ra- dioactive contamination over a wide area. Although the contamination can be particularly troublesome, it is not usually life threatening. The RDD may not be explosive, but rather, radioactive material. It may be dispersed by

E = 1.35 mrem (13.5 µSv)

ESE = 10 mrad (100 µGy)

FIGURE 37-5 Effective dose during posterior-anterior chest radiography.

BOX 37-3 Effective Dose During PA Chest Radiography

A PA chest radiograph results in an entrance skin dose of 10 mrad, an exit dose of 0.1 mrad, and an average tissue dose of 5 mrad. What is the effective dose?

E = Σ D i W i = (5)(0.12) lung

+ (5)(0.05) breast + (5)(0.05) esophagus + (5)(0.05) thyroid

All other tissues receive essentially zero dose.

= 0.6 lung + 0.25 breast + 0.25 esophagus + 0.25 thyroid

= 1.35 mrad

Monitor dose 100 mrem (1 mSv)

Effective dose (E) E = 10 mrem (100 µSv)

FIGURE 37-6 Effective dose for occupational radiation exposure is based on the occupational radiation monitor.

Rescue and medical emergencies should be atten- ded to before radiologic concerns are addressed.

Radiologic terrorism can be addressed safely with an emergency responder's equipment kit.

BOX 37-2 Effective Dose During Computed Tomography

Computed tomography of the abdomen and pelvis results in a tissue dose of 2000 mrad (20 m Gy t ). What is the effective dose?

E = Σ D i W i = (2000)(0.2) gonads + (2000)(0.12) colon + (2000)(0.05) liver

All other organs listed in Table 37-2 receive essentially zero dose.

= 400 gonads + 240 colon + 100 liver

= 740 mrad

PART VII Radiation Protection578

hand in the form of powder, mist, or gas into a water supply or ventilation system.

An IND contains nuclear material that can pro- duce a nuclear explosion. An IND is indeed a nuclear weapon; therefore, it is unlikely to be the form of at- tack used by a terrorist. However, should an IND be employed, the death and devastation would be extreme.

Radiation Protection Guidance Protection against exposure to external radiation, ex- posure from photon and particle radiation, and internal radioactive contamination transferred from surface ra- dioactive contamination must be considered. This is ac- complished by establishing boundaries for known levels of radiation exposure and radioactive contamination.

With the use of radiation monitoring instruments, an inner boundary is established at an exposure rate of 10 R/hr (100 Gy a /hr). Inside of this boundary, one should assume that levels of radioactive contamination are high, until it is proved otherwise.

An outer boundary should be established when expo- sure exceeds 10 mR/hr (100 µ Gy a /hr) or when radioac- tive contamination is detectable.

Radiation Detection and Measurement Equipment Radiation detection equipment with specifi c capac- ity should be readily available to the fi rst responder. It is recommended that such equipment be stored in the nuclear medicine laboratory and identifi ed to all tech- nologists and radiologists who might be pressed into emergency response.

Radiation detection apparatus should be capable of measuring radiation exposure levels to 50 R/hr (500 m Gy a /hr ). Further, it is recommended that such instru- ments should emit unambiguous alarms at 10 mR/hr (100 µ Gy a /hr), 10 R/hr (100 m Gy a /hr), and 50 R/hr (500 m Gy a /hr). (Such a specially designed instrument is shown in Figure 37-7 .) An additional instrument should be available that can be used to clearly detect the pres- ence of alpha and beta radioactive contamination.

Emergency responders should have available standard protective coveralls and shoe covers to protect against radioactive contamination of the responder. Protective respiratory devices may be needed in the case of aerosol radioactive contamination. Decontamination of victims

BOX 37-4 Occupational/Radiation Effective Dose

An occupational radiation monitor records a dose of 100 mrem (1 mSv). What is the effective dose if the occupational dose is received during fl uoroscopy when a protective apron is worn?

E = Σ D i W i = (100)(0.05) thyroid

All other tissues receive essentially zero dose.

= 5 mrem

FIGURE 37-7 Radiation detection instrument designed especially for radiologic terrorism. (Courtesy Ian Hamilton, Ludlum Instruments.)

Being exposed to radiation does not make an individual radioactive.

Radioactive contamination is rarely life threatening.

C H A P T E R 37 Health Physics 579

may be necessary, and an area should be cordoned off for such activity, so that a contaminated-to-clean step- off pad is provided.

The Radiation Safety Offi cer of a hospital should assign an individual to be responsible for establishing the emer- gency response equipment store and for seeing that ade- quate continuing education is provided for those who might be called upon to perform as emergency responders.

SUMMARY Health physics is concerned with the research, teach- ing, and operational aspects of radiation control. The three cardinal principles developed for radiation work- ers are as follows: Minimize time of radiation exposure, maximize distance from the radiation source, and use shielding to reduce radiation exposure. ALARA (as low as reasonably achievable) defi nes the principal concept of radiation protection.

Effective dose is that which should be used to es- timate radiation risk to the patient or the radiologic technologist. Assuming that effective dose is 10% of a collar-positioned monitor is conservative and results in overestimation of stochastic response.

Radiologic terrorism is possible with three principal devices: a radiation exposure device, a radiologic disper- sal device, and an improvised nuclear device.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. Health physics b. TVL c. NCRP d. Effective dose e. ALARA f. Tissue weighting factor (W r ) g. First responder h. Clarence Dally i. Manhattan Project j. LNT

2. Write the equation for the radiation dose as a function of time of exposure.

3. What is the function of the 5-minute reset timer on a fl uoroscopy imaging system?

4. A fl uoroscope emits 3.5 R/mA-minute at the tabletop for every mA of operation. What is the approximate patient entrance skin exposure (ESE) after a 3.2-minute fl uoroscopic examination of 1.5 mA?

5. What are the three cardinal principles of radiation protection?

6. The output intensity of a radiographic unit is 4.2 mR/mAs. What is the total output after a 200-ms exposure at 300 mA?

7. At the exposure rate in #6, what is the approxi- mate patient skin dose after a 3.2-minute fl uoro- scopic examination of 1.5 mA?

8. How can the three cardinal principles of radiation protection be best applied in diagnostic radiology?

9. What exposure will a radiologic technologist receive when exposed for 10 minutes at 4 m from a source with intensity of 100 mR/hr at 1 m while wearing a protective apron equivalent to 2 HVLs?

10. What wartime effort coined the term health physicist?

11. The collar-positioned monitor of a fl uoroscopist records 90 mrem (0.9 mSv) over the course of a month. This represents approximately what effective dose (E)?

12. Describe the change in longevity that occurred during the 20th century and the impact of radiation on that change.

13. How many half-value layers are included in a tenth-value layer?

14. What should fi rst responders do in the event of a radiologic emergency?

15. Discuss the concept of effective dose. The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

580

C H A P T E R

38 Designing for Radiation Protection OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Name the leakage radiation limit for x-ray tubes 2. List nine radiation protection features of a radiographic imaging system 3. List nine radiation protection features of a fl uoroscopic imaging system 4. Discuss the design of primary and secondary radiation barriers 5. Describe the three types of radiation dosimeters used in diagnostic

imaging

OUTLINE Radiographic Protection Features

Protective X-Ray Tube Housing

Control Panel Source-to-Image Receptor

Distance Indicator Collimation Positive-Beam Limitation Beam Alignment Filtration Reproducibility Linearity Operator Shield Mobile X-ray Imaging System

Fluoroscopic Protection Features Source-to-Skin Distance Primary Protective Barrier Filtration Collimation Exposure Control Bucky Slot Cover Protective Curtain Cumulative Timer Dose Area Product

Design of Protective Barriers Type of Radiation Factors That Affect Barrier

Thickness Radiation Detection and Measurement

Gas-Filled Detectors Scintillation Detectors Thermoluminescence Dosimetry Optically Stimulated

Luminescence Dosimetry

C H A P T E R 38 Designing for Radiation Protection 581

RADIOGRAPHIC PROTECTION FEATURES Many radiation protection devices and accessories are associated with modern x-ray imaging systems. Two that are appropriate for all diagnostic x-ray imaging sys- tems relate to the protective housing of the x-ray tube and to the control panel.

Protective X-ray Tube Housing Every x-ray tube must be contained within protective housing that reduces leakage radiation during use.

Control Panel The control panel must indicate the conditions of exposure and must positively indicate when the x-ray tube is energized. These requirements are usually satis- fi ed with the use of kVp and mA indicators. Sometimes, visible or audible signals indicate when the x-ray beam is energized.

Source-to-Image Receptor Distance Indicator A source-to-image receptor distance (SID) indicator must be provided. This can be as simple as a tape measure attached to the tube housing, or as advanced as lasers.

Collimation Light-localized, variable-aperture rectangular collima- tors should be provided. Cones and diaphragms may replace the collimator for special examinations. Attenu- ation of the useful beam by collimator shutters must be equivalent to attenuation by the protective housing.

Question: Most radiographs are taken at an SID of 100 cm. How much difference is allowed between the projection of the light fi eld and the x-ray beam at the image receptor?

Answer: 2% of 100 cm = 2 cm

Positive-Beam Limitation Automatic, light-localized, variable-aperture collima- tors were required on all but special x-ray imaging sys- tems manufactured in the United States between 1974 and 1994. These positive-beam – limiting (PBL) devices are no longer required but continue to be a part of most new radiographic imaging systems. They must be adjusted so that with any image receptor size in use and at all standard SIDs, the collimator shutters au- tomatically provide an x-ray beam equal to the image receptor.

Beam Alignment In addition to proper collimation, each radiographic tube should be provided with a mechanism to ensure proper alignment of the x-ray beam and the image receptor. It does no good to align the light fi eld and the x-ray beam if the image receptor is not also aligned.

Filtration All general purpose diagnostic x-ray beams must have a total fi ltration (inherent plus added) of at least 2.5 mm Al when operated above 70 kVp. Radiographic tubes operated between 50 and 70 kVp must have at least 1.5 mm Al. Below 50 kVp, a minimum of 0.5 mm Al total fi ltration is required. X-ray tubes designed for mammography usually have 30 µm Mo or 60 µm Rh fi ltration.

As was discussed in Chapter 9, it is not normally possible physically to examine and measure the thickness

A NUMBER of features of modern x-ray imaging systems designed to improve radiographic quality have been discussed in previous

chapters. Many of these features are also designed to reduce patient radiation dose during x-ray ex- aminations. For instance, proper beam collimation contributes to improved image contrast and is effective in reducing patient dose.

More than 100 individual radiation protection devices and accessories are associated with modern x-ray imaging systems. Some are characteristic of either radiographic or fl uoroscopic imaging sys- tems, and some are mandated by federal regulation for all diagnostic x-ray imaging systems. A descrip- tion of the devices required for all diagnostic x-ray imaging systems follows.

A

X-ray beam on must be positively and clearly indicated to the radiologic technologist.

The SID indicator must be accurate to within 2% of the indicated SID.

The x-ray beam and the light beam must coincide to within 2% of the SID.

The PBL must be accurate to within 2% of the SID. Leakage radiation must be less than 100 mR/hr (1 mGy a /hr) at a distance of 1 m from the protective housing.

PART VII Radiation Protection582

of each component of total fi ltration. An accurate mea- surement of half-value layer (HVL) is suffi cient. If the HVL is equal to or greater than the values given in Table 9-3 at various kVp levels, total fi ltration is adequate.

Question: The following data are obtained on a

three-phase radiographic imaging system operating at 70 kVp, 100 mA, 100 ms. Is the fi ltration adequate?

Added fi ltration (mm Al)

0 0.5 1.0 1.5 2.0 3.0 4.0 5.0

Exposure (mR) 87 74 65 56 49 39 31 25

Answer: A plot of these data ( Figure 38-1 ) indicates an HVL of 2.0 mm Al. Table 9-3 shows that at 70 kVp, an HVL of 2.0 mm Al or greater is suffi cient. The fi ltration is adequate.

Reproducibility For any given radiographic technique, the output radia- tion intensity should be constant from one exposure to another. This is checked by making repeated radiation exposures through the same technique and observing the average variation in radiation intensity.

Linearity When adjacent mA stations are used, for example, 100 mA and 200 mA, and exposure time is adjusted for constant mAs, the output radiation intensity should remain con- stant. When the exposure time remains constant, causing

the mAs to increase in proportion to the increase in mA, radiation intensity should be proportional to mAs.

This takes any inaccuracy in the exposure timer out of the analysis. Radiation intensity is expressed in units of mR/mAs (mGy a /mAs).

Operator Shield It must not be possible to expose an image receptor while the radiologic technologist stands unprotected outside a fi xed protective barrier, usually the console booth. The exposure control should be fi xed to the operating con- sole and not to a long cord. The radiologic technologist may be in the examination room during exposure, but only if protective apparel is worn.

Mobile X-ray Imaging System A protective lead apron should be assigned to each mobile x-ray imaging system. The exposure switch of such an imaging system must allow the operator to remain at least 2 m from the x-ray tube during expo- sure. Of course, the useful beam must be directed away from the radiologic technologist while positioned at this minimum distance.

FLUOROSCOPIC PROTECTION FEATURES The features of fl uoroscopic imaging systems that fol- low are intended primarily to reduce patient radiation dose. Usually, when patient radiation dose is reduced, personnel exposure is reduced similarly.

Source-to-Skin Distance One would think that increasing the distance between any x-ray tube and the patient would result in reduced patient dose because of the increased distance. This is true, but to maintain exposure to the image intensifi er, the mA must be increased to compensate for the in- creased distance. Because of the divergence of the x-ray beam, the entrance skin exposure (ESE) is lessened for the required exit exposure as the source-to-skin distance (SSD) is increased.

Review Figure 38-2 , where a 20-cm abdomen is 5 HVLs thick. If the fluoroscopic x-ray tube is moved from 40 cm SSD to 20 cm SSD, the ESE is greatly increased. The exposure required at the image intensifier is 1 mR.

60

40

Exposure (mR)

0

20

100

0

80

0.5 1.0 1.5 2 3 4 5 Filtration (mmAl)

FIGURE 38-1 Measurement of x-ray beam intensity as a function of added fi ltration results in a half-value layer (HVL) of 2.0 mmAl.

The variation in x-ray intensity should not exceed 5%.

The maximum acceptable variation in linearity is 10% from one mA station to an adjacent mA station.

The SSD must be not less than 38 cm on stationary fl uoroscopes and not less than 30 cm on mobile fl uoroscopes.

C H A P T E R 38 Designing for Radiation Protection 583

The ESEs will be 2.25 mR and 4.0 mR, respectively, solely because of the divergence of the x-ray beam — the inverse square law. Add the x-ray attenuation of 5 HVLs for each geometry, and the respective ESEs become 72 mR and 128 mR ( Box 38-1 ).

Primary Protective Barrier The fl uoroscopic image receptor assembly serves as a primary protective barrier and must be 2 mm Pb equiv- alent. It must be coupled with the x-ray tube and in- terlocked so that the fl uoroscopic x-ray tube cannot be energized when the image receptor is in the parked position.

Filtration The total fi ltration of the fl uoroscopic x-ray beam must be at least 2.5 mm Al equivalent. The tabletop, patient cradle, or other material positioned between the x-ray tube and the tabletop are included as part of the total fi l- tration. When the fi ltration is unknown, the HVL should be measured. The minimum HVL reported in Table 18-3 must be met so that adequate fi ltration can be assumed.

Collimation Fluoroscopic x-ray beam collimators must be adjusted so that an unexposed border is visible on the image monitor when the input phosphor of the image inten- sifi er is positioned 35 cm above the tabletop and the collimators are fully open. For automatic collimating devices, such an unexposed border should be visible at all heights above the tabletop. The collimator shutters should track with height above the tabletop.

Exposure Control The fl uoroscopic exposure control should be of the dead man type; that is, if the operator should drop dead or just release the pressure, the exposure would be terminated — unless, of course, he or she falls on the switch! The conventional foot pedal or pressure switch on the fl uoro- scopic image receptor satisfi es this condition.

Bucky Slot Cover During fl uoroscopy, the Bucky tray is moved to the end of the examination table, leaving an opening in the side of the table approximately 5 cm wide at gonadal level. This opening should be covered automatically with at least 0.25 mm Pb equivalent.

Protective Curtain A protective curtain or panel of at least 0.25 mm Pb equivalent should be positioned between the fluoros- copist and the patient. Figure 38-3 shows the typical isoexposure distribution for a fluoroscope. Without

Exit exposure

Entrance skin

exposure

1 mR 1 mR

72 mR 128 mR

20 = cm SSD40 = cm SSD

FIGURE 38-2 Patient entrance skin exposure (ESE) is considerably higher when the fl uoroscopic x-ray tube is too close to the tabletop.

BOX 38-1 Effect of SSD on ESE

On the basis of x-ray beam divergence alone at 40 cm SSD:

I I

= d d

I 1

= 60 40

I = 2.25

20 cm SSD :

I 1

= 40 20

I

1

2

2 2

1 2

1 2

2

1

1 2

2

1

mR

at

== 4.0 mR Additive HVL due to the 20-cm patient:

at cm SID

mR at cm S SD

mR

40

2 25 2 2 25 32

72 20

4 0 2 128

5

5

:

. .

.

× = × =

× =

ESE, Entrance skin exposure; HVL, half-value layer; SID, source-to-image receptor distance; SSD, source-to-skin distance.

PART VII Radiation Protection584

the curtain and the Bucky slot cover, the exposure of radiology personnel is many times higher.

Cumulative Timer A cumulative timer that produces an audible signal when the fl uoroscopic time has exceeded 5 minutes must be pro- vided. This device is designed to ensure that the radiologist is aware of the relative beam-on time during each proce- dure. The assisting radiologic technologist should record total fl uoroscopy beam-on time for each examination.

Dose Area Product The intensity of the x-ray beam at the tabletop of a fl uo- roscope should not exceed 2.1 R/min (21 mGy a /min) for each mA of operation at 80 kVp. If there is no optional

high-level control, the intensity must not exceed 10 R/min (100 mGy a /min) during fl uoroscopy. If an optional high-level control is provided, the maximum tabletop intensity allowed is 20 R/min (200 mGy a / min). There is no limit on x-ray intensity when the image is recorded, as in cineradiography or videography.

The overall carcinogenic risk (stochastic effect) to a patient depends on effective radiation dose (E), which is related to tissue radiation dose and to the volume of tissue exposed. Tissue radiation dose, which refers to the energy deposited locally, is the quantity that best refl ects the potential for injury to that tissue (deterministic effect).

Dose area product (DAP) is a quantity that refl ects not only the dose but also the volume of tissue irradi- ated; therefore, it may be a better indicator of risk than dose. DAP is expressed in R-cm 2 (cGy-cm 2 ).

DAP increases with increasing fi eld size even if the dose remains unchanged. Smaller fi eld size results in lower DAP, and thus less risk, because a smaller amount of tissue is exposed.

DAP may be used to monitor radiation output from radiographic and fl uoroscopic imaging systems. DAP meters are becoming more common on x-ray imaging systems. Typically, the radiolucent device is placed near the x-ray source below the collimator, before the beam enters the patient.

The risk for injury to the skin where the beam enters the patient can be derived by dividing the DAP measurement by the area of the beam at the skin. Using DAP to monitor radiation intensity is a good way to implement radiation management procedures and keep patient exposures low.

DESIGN OF PROTECTIVE BARRIERS In designing a radiology department or an individual x-ray examination room, it is not suffi cient to consider only general architectural characteristics. Great atten- tion must be given to the location of the x-ray imaging system in the examination room.

The use of adjoining rooms is also of great importance when the design is geared toward radiation safety. It is often necessary to include protective barriers, usually sheets of lead, in the walls of x-ray examination rooms. If the radiology facility is located on an upper fl oor, then it may be necessary to shield the fl oor as well.

A great number of factors are considered when a pro- tective barrier is designed. This discussion touches only on the fundamentals and some basic defi nitions. When- ever new x-ray facilities are being designed or old ones renovated, a medical physicist must be consulted for assistance in the design of proper radiation shielding.

Type of Radiation For the purpose of protective barrier design, three types of radiation are considered ( Figure 38-4 ). Primary radiation is the most intense and therefore the most hazardous and the most diffi cult to shield.

50 mR/hr

100 mR/hr

500 mR/hr

3� 2� 1�

5 mR/hr

3� 2� 1�

Protective curtain

Bucky slot cover

A

B

FIGURE 38-3 A, Isoexposure profi le for an unshielded fl uo- roscope demonstrates the need for protective curtains and Bucky slot covers. B, Isoexposure profi le with these protective devices.

C H A P T E R 38 Designing for Radiation Protection 585

When a chest board is positioned on a given wall, it is sometimes necessary to provide shielding directly behind the chest board, in addition to that speci- fi ed for the rest of the wall. Any wall to which the useful beam can be directed is designated a primary protective barrier.

Lead bonded to sheet rock or wood paneling is used most often as a primary protective barrier. Such lead shielding is available in various thicknesses and is speci- fi ed for architects and contractors in units of pounds per square foot (lb/ft 2 ).

Concrete, concrete block, or brick may be used in- stead of lead. As a rule of thumb, 4 inches of masonry is equivalent to 1/16 inch of lead. Table 38-1 shows available lead thicknesses and equivalent thicknesses of concrete.

There are two types of secondary radiation: scat- ter radiation and leakage radiation. Scatter radiation results when the useful beam intercepts any object, causing some x-rays to be scattered. For the purpose of protective shielding calculations, the scattering

object can be regarded as a new source of radiation. During radiography and fl uoroscopy, the patient is the single most important scattering object.

Question: The patient ESE is 410 mR (4.1 mGy a ) for a kidney, ureter, and bladder (KUB) examination. What will be the approximate radiation exposure at 1 m from the patient? At 3 m from the patient?

Answer: At mR mR

mR R

At mR

1 410 0 1 410 0 001

0 41 410

3 0 41 1 3

m

m

: . % .

.

: . ( /

× = × = =

× µ

)) . ( / )

.

2 0 41 1 9

0 046 46

= = =

mR

mR Rµ

Leakage radiation is that radiation emitted from the x-ray tube housing in all directions other than that of the useful beam. If the x-ray tube housing is designed prop- erly, the leakage radiation will never exceed the regula- tory limit of 100 mR/hr (1 mGy a /hr) at 1 m. Although in practice, leakage radiation levels are much lower than this limit, 100 mR/hr at 1 m is used for protective barrier calculations.

Protective barriers designed to shield areas from sec- ondary radiation are called secondary protective barri- ers. Secondary protective barriers are always less thick than primary protective barriers.

Often, lead is not required for secondary protective barriers because the computation usually results in less than 0.4 mm Pb. In such cases, conventional gypsum board, glass, or lead acrylic is adequate.

Many walls that are secondary protective barri- ers can be protected adequately with four thicknesses of 5/8-inch gypsum board. Operating console barri- ers are secondary protective barriers — the useful beam is never directed at the operating console booth. Four thicknesses of gypsum board and 1/2-inch plate glass may be all that is necessary. Sometimes glass walls 1/2 to 1 inch thick can be used as control booth barriers. Table 38-2 gives equivalent thicknesses for secondary protective barrier materials.

Useful beam

Leakage

Primary useful beam Secondary leakage scatter

Scatter

FIGURE 38-4 Three types of radiation — the useful beam, leakage radiation, and scatter radiation — must be considered when the protective barriers of an x-ray room are designed.

Primary radiation is the useful beam.

The intensity of scatter radiation 1 m from the patient is approximately 0.1% of the intensity of the useful beam at the patient.

LEAD CONCRETE

mm in lb/ft 2 cm in

0.4 1/64 1 2.4 1 3/8 0.8 1/32 2 4.8 1 7/8 1.2 3/64 3 7.2 2 7/8 1.6 1/16 4 9.6 3 3/4

TABLE 38-1 Lead and Concrete Equivalents for Primary Protective Barrier

PART VII Radiation Protection586

Question: What percentage of the recommended

100-mrem/wk (1-mSv/wk) public dose limit will be incident on a control booth barrier located 3 m from the x-ray tube and the patient? Assume that the x-ray output is 3 mR/mAs and that the weekly beam-on time is 5 minutes at an average 100 mA — a generous assumption.

Answer: From scatter radiation, the barrier will receive: Total primary beam = 3 mR/mAs × 10 mA ×

5 min × 60 s/min = 90,000 mR Scatter radiation = 90,000 mR × 1/1000 ×

(1/3) 2 = 10 mR

From leakage radiation, the barrier will receive:

Leakage radiation at 1 m = 100 mR/hr × 5/60 hr = 8.3 mR

Leakage radiation = 8.3 mR (1/3) 2 = 0.9 mR

Total secondary radiation = 10 mR + 0.9 mR = 10.9 mR or

11% of the recommended dose limit

This analysis is representative of the clinical envi-

ronment. The estimated exposure occurs to the control booth barrier — not to the radiologic technologist. The composition of the barrier and the additional distance reduce technologist exposure even further. This is the reason why personnel radiation exposure during radi- ography is very low.

Factors That Affect Barrier Thickness Many factors must be taken into consideration when the required protective barrier thickness is calculated. A thorough discussion of these factors is beyond the scope of this book; however, a defi nition of each is useful for an understanding of the problems involved.

Distance. The thickness of a barrier naturally depends on the distance between the source of radiation and the barrier. The distance is that to the adjacent occupied area, not to the inside of the wall of the x-ray room.

A wall along which an x-ray imaging system is posi- tioned probably requires more shielding than the other walls of the room. In such a case, the leakage radiation may be more hazardous than the scatter radiation or even the useful beam. It may be desirable to position the x-ray imag- ing system in the middle of the room because then no single wall is subjected to especially intense radiation exposure.

Occupancy. The use of the area that is being pro- tected is of principal importance. If the area were a rarely occupied closet or storeroom, the required shield- ing would be less than if it were an offi ce or laboratory that was occupied 40 hours per week.

This concept refl ects the time of occupancy factor (T). Table 38-3 reports the occupancy levels of various areas as suggested by the National Council on Radiation Pro- tection and Measurements (NCRP).

Control. An area that is occupied primarily by radi- ology personnel and patients is called a controlled area. The design limits for a controlled area are based on the recommended occupational dose limit; therefore, the barrier is required to reduce the exposure to a worker in the area to less than 100 mrem per week (1 mSv/wk).

An uncontrolled area can be occupied by anyone; therefore, the maximum exposure rate allowed is based on the recommended dose limit for the pub- lic of 100 mrem/yr (1 mSv/yr). This is equivalent to 2 mrem/wk (20 µSv/wk), which is the design limit for an uncontrolled area. Furthermore, the protective barrier should ensure that no individual will receive more than 2.5 mrem (25 µSv) in any single hour.

Workload. The shielding required for an x-ray exam- ination room depends on the level of radiation activity in that room. The greater the number of examinations performed each week, the thicker the shielding that is required.

Radiologic technologists receive most of their occupational radiation exposure during fl uoroscopy.

Design limits for a controlled area are based on the annual recommended occupational dose limit of 5000 mrem/yr (50 mSv/yr).

TABLE 38-2 Equivalent Material Thicknesses for Secondary Barriers

SUBSTITUTES

Computed Lead Required Steel (mm) Glass (mm) Gypsum (mm) Wood (mm)

0.1 0.5 1.2 2.8 19 0.2 1.2 2.5 5.9 33 0.3 1.8 3.7 8.8 44 0.4 2.5 4.8 12 53

C H A P T E R 38 Designing for Radiation Protection 587

This characteristic is called workload (W) and is expressed in units of milliampere-minutes per week (mAmin/wk). A busy, general purpose x-ray room may have a workload of 500 mAmin/wk. Rooms in private offi ces have workloads of less than 100 mAmin/wk.

Question: The plans for a community hospital call for two x-ray examination rooms. The estimated patient load for each room is 15 patients per day, and each patient will average 3 fi lms taken at 80 kVp, 70 mAs. What is the projected workload of each room?

Answer: 15 patients/day 5 days/wk

= 75 patients/wk

75 patients/wk 3 f

×

× iilms/pt = 225 films/wk

225 films/wk 70 mAs/film

= 15,750 mAs/w

× kk

15,750 mAs/wk 1min 60 sec

= 262.5 mAmin/wk

×

For combination radiographic/fl uoroscopic imaging systems, usually only the radiographic workload need be considered for barrier calculations. When the fl uoro- scopic x-ray tube is energized, a primary protective barrier in the form of the fl uoroscopic screen always intercepts the useful x-ray beam. Consequently, the primary barrier requirements are always much less for fl uoroscopic x-ray beams than for radiographic x-ray beams.

Use Factor. The percentage of time during which the x-ray beam is on and directed toward a particular protective barrier is called the use factor (U) for that barrier. The NCRP recommends that walls be assigned a use factor of 1⁄4 and the fl oor a use factor of 1.

Studies have shown these recommendations to be high and therefore very conservative. Many medi- cal physicists suggest that primary barriers in fact do

not exist. All barriers are secondary because the useful beam always is intercepted by the patient and the image receptor.

If an x-ray room has a special design, other use fac- tors may be assigned. A room designed strictly for chest radiography has one wall with a use factor of 1. All oth- ers have a use factor of zero for primary radiation and thus would be considered secondary radiation barriers.

The ceiling nearly always is considered a secondary protective barrier. For a secondary barrier, leakage and scatter radiation are present 100% of the time that the x-ray tube is energized.

kVp. The fi nal consideration in the design of an x-ray protective barrier is the penetrability of the x-ray beam. For protective barrier calculations, kVp is used as the measure of penetrability. Most modern x-ray imaging systems are designed to operate at up to 150 kVp. Most examinations, however, are conducted at an average of 75 kVp.

Usually, constant operation is assumed at a kVp greater than that actually used: 100 kVp for general radiography, 30 kVp for mammography. Therefore, it is more likely that the protective barrier will be too thick than too thin.

Alternatively, a workload distribution such as those shown in Figure 38-5 may be used. Workload distribu- tion results in a more precise determination of required barrier thickness, but it is a considerably more diffi cult computation to perform.

Measurements of radiation exposure outside the x-ray examination room always result in radiation levels far less than those anticipated by calculation. The total beam-on time is always less than that assumed. The av- erage kVp is usually closer to 75 kVp than to 100 kVp.

Calculations do not account for the fact that the patient and the image receptor always intercept the useful beam. Therefore, although the calculations are intended to result in a dose limit of 100 mrem/wk (1 mSv/wk) or 2 mrem/wk(20 µSv/wk) outside the x-ray room, rarely will the actual exposure exceed 1/10 of those dose limits. To confi rm this for yourself, keep records for 1 week of kVp, mAs, and beam direction.

RADIATION DETECTION AND MEASUREMENT Instruments are designed to detect radiation or to measure radiation, or to do both. Those designed for detection usually operate in the pulse or rate mode and are used to indicate the presence of radiation. In the pulse mode, the presence of radiation is indicated by a ticking, chirping, or beeping sound. In the rate

The use factor for secondary barriers is always 1.

Occupancy Area

Full Work areas (e.g., offi ces, laboratories, shops, wards, and nurses' stations), living quarters, children's play areas, and occupied space in nearby buildings

Frequent Corridors, restrooms, patient rooms Occasional Waiting rooms, stairways, unattended

elevators, janitors' closets, outside area

TABLE 38-3

Levels of Occupancy of Areas That May Be Adjacent to X-ray Rooms, as Suggested by the NCRP

PART VII Radiation Protection588

mode, the instrument response is expressed in mR/hr (mGy a /hr) or R/hr (Gy a /hr).

Instruments designed to measure the intensity of radiation usually operate in the integrate mode. They accumulate the signal and respond with a total exposure (mR or R). Such application is called dosimetry, and the radiation measuring devices are called dosimeters.

The earliest radiation detection device was the photo- graphic emulsion; it is still a primary means of radiation detection and measurement. However, other devices have been developed that have more favorable charac- teristics than the photographic emulsion for some appli- cations. Table 38-4 lists most of the currently available radiation detection and measurement devices, along with some of their principal characteristics and uses.

It is apparent that fi lm has two principal applications in diagnostic radiology: the making of a radiograph and the radiation monitoring of personnel (fi lm badge). The photographic process was discussed in Chapters 11 and 12. Use of fi lm as a radiation monitor is covered in Chapter 40.

Four other types of radiation detection devices are of particular importance in diagnostic radiology. The gas-fi lled radiation detector is used widely as a device to measure radiation intensity and to detect radioactive contamination. Thermoluminescence dosimetry (TLD) and optically stimulated luminescence (OSL) dosimetry are used for both patient and personnel radiation moni- toring. Scintillation detection is the basis for the gamma camera, an imaging device used in nuclear medicine; it

Workload (mA.min/patient)

kVp

40 50 60 70 80 90 100 110 120

1.0

0.5

Radiographic room workload distribution

for floor and walls

Workload distribution assuming all exposures

are made at 100 kVp

FIGURE 38-5 Workload distribution of clinical voltage.

Device Characteristics — Uses Photographic Limited range, sensitive,

energy dependent — personnel monitoring, emulsion imaging

Ionization chamber Wide range, accurate, portable — survey for radiation levels 1 mR/hr

Proportional counter Laboratory instrument, accurate, sensitive —� assay of small quantities of radionuclides

Geiger-Muller counter Limited to 100 mR/hr, portable — survey for low radiation levels and radioactive contamination

Thermoluminescence dosimetry

Wide range, accurate, sensitive — personnel monitoring, stationary, area monitoring

Optically stimulated luminescence dosimetry

Wide range, accurate, sensitive — newest person- nel monitoring device

Scintillation detection Limited range, very sensitive, stationary or portable instruments — photon spectroscopy, imaging

TABLE 38-4 Radiation Detection and Measuring Device Characteristics and Uses

C H A P T E R 38 Designing for Radiation Protection 589

is also used in computed tomography (CT) and digital radiography imaging systems.

Gas-Filled Detectors Three types of gas-fi lled radiation detectors are used: ionization chambers, proportional counters, and Geiger- Muller detectors. Although these are different in terms of response characteristics, each is based on the same principle of operation. As radiation passes through gas, it ionizes atoms of the gas. The electrons released in ion- ization are detected as an electrical signal that is propor- tional to the radiation intensity.

Consider an ideal gas-fi lled detector as shown schemati- cally in Figure 38-6 . It consists of a cylinder fi lled with air or any of a number of other gases.

Along the central axis of the cylinder a rigid wire called the central electrode is positioned. If a voltage is impressed between the central electrode and the wall such that the wire is positive and the wall negative, then any electrons liberated in the chamber by ionization will be attracted to the central electrode.

These electrons form an electrical signal, either as a pulse of electrons or as a continuous current. This electric signal then is amplifi ed and measured. Its intensity is proportional to the radiation intensity that caused it.

In general, the larger the chamber, the more gas molecules are available for ionization, and therefore, the more sensitive is the instrument. Similarly, if the chamber is pressurized, then a greater number of molecules are available for ionization, and even higher sensitivity results.

Sensitivity is not the same as accuracy. A high level of accuracy means that an instrument can detect and precisely measure the intensity of a radiation fi eld. Instrument accuracy is controlled by the overall elec- tronic design of the device.

Region of Recombination. If the voltage across the chamber of the ideal gas-fi lled detector is increased slowly from zero to a high level, the resulting electrical signal in the presence of fi xed radiation intensity will increase in stages ( Figure 38-7 ). During the fi rst stage, when the voltage is very low, no electrons are attract- ed to the central electrode. The ion pairs produced in the chamber recombine. This is known as the region of recombination, shown as stage R in Figure 38-7 .

Ion Chamber Region. As the chamber voltage is increased, a condition is reached whereby every electron released by ionization is attracted to the central elec- trode and collected. The voltage at which this occurs varies according to the design of the chamber, but for most conventional instruments, it occurs in the range of 100 to 300 V.

This portion of the gas-fi lled detector performance curve is known as the ionization region, indicated by I in the Figure 38-7 . Ion chambers are operated in this region.

Several different types of ion chambers are used in radiology; the most familiar of these is the portable sur- vey instrument ( Figure 38-8 ). This instrument is used principally for area radiation surveys. It can measure a wide range of radiation intensities, from 1 mR/hr (10 µGy a /hr) to several thousand R/hr (Gy a /hr).

The ion chamber is the instrument of choice for measuring radiation intensity in areas around a fl uoro- scope, around radionuclide generators and syringes, in the vicinity of patients with therapeutic quantities of radioac- tive materials, and outside of protective barriers. Other,

R I

P

Chamber voltage

Re la

tiv e

ou tp

ut s

ig na

l

GM

CD

FIGURE 38-7 The amplitude of the signal from a gas-fi lled detector increases in stages as the voltage across the chamber is increased.

Central electrode

Ionization

Amplifying electronics

Meter

+

– e–

+

FIGURE 38-6 The ideal gas-fi lled detector consists of a cylin- der of gas and a central collecting electrode. When a voltage is maintained between the central electrode and the wall of the chamber, electrons produced in ionization can be collected and measured.

The ionization of gas is the basis for gas-fi lled radiation detectors.

High sensitivity means that an instrument can detect very low radiation intensities.

PART VII Radiation Protection590

more accurate ion chambers are used for precise calibra- tion of the output intensity of diagnostic x-ray imaging systems ( Figure 38-9 ).

Another application of a precision ion chamber is the dose calibrator ( Figure 38-10 ). These devices fi nd daily use in nuclear medicine laboratories for the assay of radioactive material.

Proportional Region. As the chamber voltage of the ideal gas-fi lled detector is increased still farther above the ionization region, electrons of the fi lling gas released by primary ionization are accelerated more rapidly to the central electrode. The faster these electrons travel, the greater is the probability that they will produce additional ionization on their way to the central elec- trode. These additional ionizations result in additional electrons called secondary electrons.

Secondary electrons also are attracted to the central electrode and collected. The total number of electrons collected in this fashion increases with increasing cham- ber voltage. The result is a rather large electron pulse for each primary ionization. This stage of the voltage response curve is known as the proportional region.

Proportional counters are sensitive instruments that are used primarily as stationary laboratory instruments for the assay of small quantities of radioactivity. One characteristic of proportional counters that makes them particularly useful is their ability to distinguish between alpha and beta radiation. Nevertheless, proportional counters fi nd few applications in clinical radiology.

Geiger-Muller Region. The fourth region of the volt- age response curve for the ideal gas-fi lled chamber is the Geiger-Muller (G-M) region. This is the region in which Geiger counters operate.

In the G-M region, the voltage across the ionization chamber is suffi ciently high that, when a single ionizing

FIGURE 38-8 This portable ion chamber survey instrument is useful for radiation surveys when exposure levels are in excess of 1 mR/hr. (Courtesy Cardinal Health, Inc.)

FIGURE 38-9 This ion chamber dosimeter is used for accurate measurement of diagnostic x-ray beams. (Courtesy Radcal Corp.)

C H A P T E R 38 Designing for Radiation Protection 591

event occurs, a cascade of secondary electrons is pro- duced in a fashion similar to a very brief, yet violent, chain reaction. The effect is that nearly all molecules of the gas are ionized, liberating a large number of elec- trons. This results in a large electron pulse.

When sequential ionizing events occur soon after one another, the detector may not be capable of responding to a second event if the fi lling gas has not been restored to its initial condition. Therefore, a quenching agent is added to the fi lling gas of the Geiger counter to enable the chamber to return to its original condition; subse- quent ionizing events then can be detected. The mini- mum time between ionizations that can be detected is known as the resolving time.

Geiger counters are used for contamination control in nuclear medicine laboratories. As portable survey instruments, they are used to detect the presence of radioactive contamination on work surfaces and labo- ratory apparatus.

They are not particularly useful as dosimeters because they are diffi cult to calibrate for varying conditions of radiation. Geiger counters are sensitive instruments that are capable of detecting and indicating single ionizing events. If they are equipped with an audio amplifi er and a speaker, one can even hear the crackle of individual ionizations.

The Geiger counter does not have a very wide range. Most instruments are limited to less than 100 mR/hr (1 mGy a /hr).

Region of Continuous Charge. If the voltage across the gas-fi lled chamber is increased still further, a condition is reached whereby a single ionizing event completely discharges the chamber, as in operation in the G-M region. Because of the high voltage, however, electrons continue to be stripped from atoms of the fi ll- ing gas, producing a continuous current or signal from the chamber.

In this condition of continuous discharge, the instru- ment is useless for the detection of radiation, and con- tinued operation in this region results in damage. The region of continuous discharge is indicated as CD in Figure 38-7 .

Scintillation Detectors Scintillation detectors are used in several areas of radiologic science. The scintillation detector is the basis for the gamma camera in nuclear medicine and is used in the detector arrays of CT imaging systems; it is the image receptor for several types of digital imaging systems.

The Scintillation Process. Some types of material scintillate when irradiated, that is, they emit a fl ash of light immediately in response to absorption of an x-ray. The amount of light emitted is proportional to the amount of energy absorbed by the material.

Consider, for example, the two x-ray interactions di- agrammed in Figure 38-11 . If a 50-keV x-ray interacts photoelectrically in the crystal, all the energy (50 keV) will reappear as light. If, however, that same x-ray interacts through a Compton scattering event in which only 20 keV of energy is absorbed, then a proportionately lower quan- tity of light will be emitted in the scintillation.

Only those materials with a particular crystalline structure scintillate. At the atomic level, the process in- volves the rearrangement of valence electrons into traps. The return of the electron from the trap to its normal position is immediate in scintillation and delayed in luminescence. This property was considered under an earlier discussion of luminescence (see Chapter 13).

FIGURE 38-10 This confi guration of an ion chamber is called a dose calibrator. It is used in nuclear medicine to measure accurately quantities of radioactive material. (Courtesy Biodex Medical Systems, Inc.)

50 keV

30-keV Compton scatter

50 keV

Scintillation crystal

50 units of light 20 units of light

FIGURE 38-11 During scintillation, the intensity of light emit- ted is proportional to the amount of energy absorbed in the crystal.

PART VII Radiation Protection592

Types of Scintillation Phosphors. Many different types of liquids, gases, and solids can respond to ioniz- ing radiation by scintillation. Scintillation detectors are used most often to indicate individual ionizing events and are incorporated into fi xed or portable radiation detection devices. They can be used to measure radia- tion in the rate mode or the integrate mode.

Nearly all the noble gases can be made to respond to radiation by scintillation. Such applications are rare, however, because the detection effi ciency is very low and the probability of interaction therefore is small.

Liquid scintillation detectors are used frequently in the research laboratory to detect low-energy beta emis- sions from carbon-14 ( 14 C) and tritium ( 3 H). Because they present a relatively harmless radiation hazard and are incorporated easily into biologic molecules, 14 C and 3 H are useful research radionuclides.

These radionuclides emit low-energy beta particles with no associated gamma rays. This makes them dif- fi cult to detect. With liquid scintillation counting, how- ever, biologic molecules can be mixed with a liquid scintillation phosphor so that the beta emission interacts directly with the phosphor, causing a fl ash of light to be emitted. Liquid scintillation counters have nearly 100% detection effi ciency for beta radiation.

By far, the most widely used scintillation phosphors are the inorganic crystals — thallium-activated sodium iodide (NaI:Tl) and thallium-activated cesium iodide (CsI:Tl). The activator atoms of thallium are impurities grown into the crystal to control the spectrum of the light emitted and to enhance its intensity.

NaI:Tl crystals are incorporated into gamma cam- eras; CsI:Tl is the phosphor that is incorporated into image-intensifi er tubes as the input phosphor and into fl at panel digital radiography image receptors. Both types of crystals have been incorporated into CT imag- ing system detector arrays. However, many of today's CT imaging systems use cadmium tungstate (CdWO 4 ) or a ceramic as the scintillation detector.

The Scintillation Detector Assembly. Light pro- duced during scintillation is emitted isotopically, that is, with equal intensity in all directions. Consequently, when used as radiation detectors, scintillation crystals are enclosed in aluminum with a polished inner surface in contact with the crystal. This allows the light fl ash to be refl ected internally to the one face of a crystal that is not enclosed, which is called the window.

Aluminum containment is also necessary to seal the crystal hermetically. A hermetic seal is one that pre- vents the crystal from coming into contact with air or moisture. This is necessary because many scintillation crystals are hygroscopic, that is, they absorb mois- ture. When moisture is absorbed, the crystals swell and crack. Cracked crystals are not useful because the crack produces an interface that refl ects and attenuates the scintillation.

Figure 38-12 shows the basic components of a single crystal – photomultiplier (PM) tube assembly representative of the type used in the portable survey instrument. The detector portion of the assembly is the NaI:Tl crystal contained in the aluminum hermetic seal. Coupled to the window of the crystal is a PM tube that converts light fl ashes from the scintillator into an electrical signal of pulses.

The PM tube is an electron vacuum tube that con- tains a number of elements. The tube consists of a glass envelope, which provides structural support for the internal elements and maintains the vacuum inside the tube.

The portion of the glass envelope that is coupled to the scintillation crystal is called the window of the tube. The crystal window and the PM tube window are sand- wiched together with a silicone grease, which provides optical coupling, so that the light emitted by the scintil- lator is transmitted to the interior of the PM tube with minimum loss.

As light passes from the crystal into the PM tube, it is incident on a thin metal coating called a photocathode, which consists of a compound of cesium, antimony, and bismuth. Electrons are emitted from the photocathode by a process called photoemission, which is similar to thermionic emission in the fi lament of an x-ray tube, except that the stimulus is light rather than heat.

The fl ash of light from the scintillation crystal therefore is incident on the photocathode, and electrons are re- leased by photoemission. The number of electrons emit- ted is directly proportional to the intensity of the light.

These photoelectrons are accelerated to the fi rst of a series of plate-like elements called dynodes. Each dynode serves to amplify the electron pulse through secondary electron emission. For each electron incident on the dyn- ode, several secondary electrons are emitted and directed

preAMP

BaseGlass envelopeWindow

Aluminum seal

NaI

Photocathode Dynodes

Collector

FIGURE 38-12 Scintillation detector assembly characteristics of the type used in a portable survey instrument.

A photocathode is a device that emits electrons when illuminated.

C H A P T E R 38 Designing for Radiation Protection 593

to the next stage. Consequently, an electron gain occurs for each dynode in the PM tube.

The number of dynodes and the gain of each dynode determine the overall electron gain of the PM tube. Pho- tomultiplier tube gain is the dynode gain raised to the power of the number of dynodes.

Question: An eight-stage PM tube (eight dynodes) has a dynode gain of three (three electrons emitted for each incident electron). What is the PM tube gain?

Answer: PM tube gain = 3 8 = 6561

The last plate-like element of the PM tube is the col- lecting electrode or collector. The collector absorbs the electron pulse from the last dynode and conducts it to the preamplifi er. The preamplifi er provides an initial state of pulse amplifi cation. It is attached to the base of the PM tube, a structure that provides support for the glass envelope and internal structures.

The overall result of scintillation detection is that a single photon interaction produces a burst of light; this, in turn, produces photoelectron emission, which then is amplifi ed to produce a relatively large electron pulse.

It is this property of scintillation detection that pro- motes its use as an energy-sensitive device for gamma spectrometry that uses pulse height analysis. Through such an application, unknown gamma emitters can be identifi ed and more sensitive radioisotope imaging can be accomplished by counting only those pulses with energy that represents total gamma ray absorption.

Scintillation detectors are sensitive devices for x-rays and gamma rays. They are capable of measuring radia- tion intensities as low as single-photon interactions. This property of scintillation detectors results in their use as portable radiation devices in much the same manner as Geiger counters are used.

A portable scintillation detector is more sensitive than a Geiger counter because it has much higher detection

effi ciency. For this application, the scintillation detector would be used to monitor the presence of contamina- tion and perhaps low levels of radiation.

Thermoluminescence Dosimetry Some materials glow when heated, thus exhibiting ther- mally stimulated emission of visible light, called ther- moluminescence. In the early 1960s, Cameron and coworkers at the University of Wisconsin experimented with some thermoluminescent materials and were able to show that exposure to ionizing radiation caused some materials to glow particularly brightly when sub- sequently heated.

Radiation-induced thermoluminescence has been de- veloped into a sensitive and accurate method of radia- tion dosimetry for personnel radiation monitoring and for measurement of patient dose during diagnostic and therapeutic radiation procedures. Personnel and patient radiation monitoring is discussed later ; however, at this time, it is important to discuss some of the basic prin- ciples of TLD ( Figure 38-13 ).

After irradiation, the TLD phosphor is placed on a special dish or planchet for analysis in an instrument called a TLD analyzer. The temperature of the planchet can be controlled carefully. Directly viewing the plan- chet is a PM tube. The PM tube is the same type of light-sensitive and light-measuring vacuum tube that was described previously as a major component of scintillation detectors.

The PM tube – planchet assembly is placed in a cham- ber with a light-tight seal. The output signal from the PM tube is amplifi ed and displayed.

The Glow Curve. As the temperature of the planchet is increased, the amount of light emitted by the TLD

PM tube

Light

TLD phosphor Planchet

Heat

X-rays

A B C

FIGURE 38-13 Thermoluminescence dosimetry is a multi- step process. A, Exposure to ionizing radiation. B, Subse- quent heating. C, Measurement of the intensity of emitted light.

TLD is the emission of light by a thermally stimulated crystal following irradiation.

PHOTOMULTIPLIER TUBE GAIN

PM tube gain = g n where dynode gain is g, and n is equal to the number of dynodes.

The dynode gain is the ratio of secondary electrons to incident electrons.

The size of the electron pulse is proportional to the energy absorbed by the crystal from the incident photon.

PART VII Radiation Protection594

increases in an irregular manner. Figure 38-14 shows the light output from lithium fl uoride (LiF) as tempera- ture increases. Several prominent peaks can be seen on the graph; each occurs because of a specifi c electron transition within the thermoluminescent crystals.

Such a graph is known as a glow curve; each type of thermoluminescent material has a characteristic glow curve. The height of the highest temperature peak and the total area under the curve are directly proportional to the energy deposited in the TLD by ionizing radia- tion. TLD analyzers are electronic instruments that are designed to measure the height of the glow curve or the area under the curve and relate this to exposure or dose through a conversion factor.

Types of Thermoluminescence Dosimetry Material. Many materials, including some body tissues, exhibit the property of radiation-induced thermoluminescence. Ma- terials that are used for TLD, however, are somewhat limited in number and are principally types of inorganic crystals. Lithium fluoride (LiF) is the most widely used TLD material. It has an atomic number of 8.2 and there- fore exhibits x-ray absorption properties similar to those of soft tissue.

LiF is relatively sensitive. It can measure doses as low as 5 mrad (50 µGy t ) with modest accuracy, and at doses exceeding 10 rad (100 mGy t ), its accuracy is better than 5%.

Calcium fl uoride (CaF) activated with manganese (CaF 2 :Mn) has a higher effective atomic number (Z = 16.3) than LiF; this makes it considerably more sensitive to ionizing radiation. CaF 2 :Mn can mea- sure radiation doses of less than 1 mrad (10 µGy t ) with moderate accuracy. Other types of TLDs are available; Table 38-5 lists some thermoluminescent phosphors and their principal characteristics and ap- plications.

Properties of Thermoluminescence Dosimetry. A particular advantage of TLD is size. The TLD can be obtained in several solid crystal shapes and sizes. Rectan- gular rods measuring 1 × 1 × 6 mm and fl at chips measur- ing 3 × 1 mm are the most popular sizes. The TLD also can be obtained in powder form; this allows irradiation in nearly any confi guration. TLDs are also available with the phosphor matrixed with Tefl on or plated onto a wire and sealed in glass.

The TLD is reusable. With irradiation, the energy absorbed by the TLD remains stored until released as visible light by heat during analysis. Heating restores the crystal to its original condition and makes it ready for another exposure.

The TLD responds proportionately to dose. If the dose is doubled, the TLD response also is doubled.

The TLD is rugged, and its small size makes it useful for monitoring dose in small areas, such as body cavi- ties. The TLD does not respond to individual ionizing events; therefore, it cannot be used in a rate meter type of instrument. The TLD is suitable only for integral dose measurements, but it does not give immediate results. It must be analyzed after irradiation for dosimetry results.

Optically Stimulated Luminescence Dosimetry An additional radiation dosimeter especially adapted for personnel monitoring was developed by Landauer in the late 1990s ( Figure 38-15 ). The process is called opti- cally stimulated luminescence (OSL) and uses aluminum oxide (Al 2 O 3 ) as the radiation detector.

Irradiation of Al 2 O 3 stimulates some electrons into an excited state. During processing, laser light stimulates these electrons, causing them to return to their ground state with the emission of visible light. The intensity of the visible light emission is proportional to the radiation dose received by the Al 2 O 3 .

The OSL process is not unlike TLD. Both are based on stimulated luminescence. However, OSL has several advantages over TLD, especially as applied to occupa- tional radiation monitoring.

With a minimum reportable dose of 1 mrad, OSL is more sensitive than TLD. OSL has a precision of 1 mrad, which beats TLD. Other features of OSL include reanalysis for confi rmation of dose, qualitative information about exposure conditions, wide dynamic range, and excellent long-term stability.

0 50 100 150

Phosphor temperature (º C)

Emitted light

intensity

200 250

FIGURE 38-14 Thermoluminescence glow curve for lithium fl uoride (LiF).

Lithium fl uoride is a nearly tissue-equivalent radiation dosimeter.

C H A P T E R 38 Designing for Radiation Protection 595

SUMMARY Many radiation protection devices, accessories, and protocols are associated with modern x-ray imaging systems. This chapter discusses the radiation protec- tion devices that are common to all radiographic and fl uoroscopic imaging systems. Many of these devices are federally mandated; others exhibit features added by manufacturers.

Leakage radiation emitted by the x-ray tube during exposure must be contained by a protective x-ray tube housing. The limit of leakage must be no more than 100 mR per hour at a distance of 1 m from the housing. The control panel must indicate exposure by kVp and mA meters or visible and audible signals.

Great attention is given to the design of radiographic rooms, to the placement of x-ray imaging systems, and to the use of adjoining rooms. Two types of protec- tive barriers are used: primary barriers and secondary barriers. Primary barriers intercept the useful x-ray beam and require the greatest amount of lead or con- crete. Secondary barriers protect personnel from scatter and leakage radiation.

Dosimeters are instruments designed to detect and measure radiation. Other than photographic emul- sion, four types of highly accurate devices are used to

measure radiation. Gas-fi lled detectors include the ion- ization chamber, the proportional counter, and the Gei- ger-Muller counter. The scintillation detector is a very sensitive device that is used in nuclear medicine. Two other radiation detection devices used especially for occupational radiation monitoring are thermolumines- cence dosimetry and optically stimulated luminescence dosimetry.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. TLD b. Use factor c. Diagnostic protective x-ray tube housing d. Glow curve e. Primary protective barrier f. X-ray linearity g. Secondary radiation h. Occupancy factor i. Geiger-Muller region j. Resolving time 2. What do audible and visible signals indicate on

the radiographic control console? 3. List as many devices used for radiation protection

on radiographic equipment as you can. 4. What is the result if the x-ray beam and the fi lm

are not properly aligned? 5. What fi ltration is used for mammography

equipment operated below 30 kVp? 6. How are reproducibility and linearity different

when the intensity of the x-ray beam is measured? 7. What characteristics of fl uoroscopic equipment are

designed for radiation protection? 8. How can fi ltration be measured if the amount of

inherent and added fi ltration is unknown? 9. Name the three types of radiation exposure

that are of concern when protective barriers are designed.

10. List four factors that are taken into consideration when a barrier for a radiographic room is designed.

Lithium Fluoride Lithium Borate Calcium Fluoride Calcium Sulfate

Composition LiF Li 2 B 4 O 7 :Mn CaF 2 :Mn CaSO 4 :Dy Density 10 3 (kg/m 3 ) 2.64 2.5 3.18 2.61 Effective atomic

number 8.2 7.4 16.3 15.3

Temperature of main peak (° C)

195 200 260 220

Principal use Patient and personnel dose

Research Environmental monitoring

Environmental monitoring

TABLE 38-5 Some Thermoluminescent Phosphors and Their Characteristics and Uses

Photodiode

Exposed Read Analyzed

Laser

Excitation Stimulated emission

A B C

FIGURE 38-15 Optically stimulated luminescence dosim- etry is a multistep process. A, Exposure to ionizing radiation. B, Laser illumination. C, Measurement of the intensity of stimulated light emission.

PART VII Radiation Protection596

11. What is the difference between a controlled area and an uncontrolled area?

12. What are the units of workload for an x-ray examination room?

13. Explain the use factor (U) as it relates to a protec- tive barrier in an x-ray examination room.

14. Why is the use factor for secondary barriers always 1?

15. Name the three gas-fi lled dosimeters. 16. Discuss the properties of TLD that make it suitable

for personnel monitoring. 17. Which modality of diagnostic imaging uses scintil-

lation detection as a radiation detection process?

18. What are the two most widely used scintillation phosphors?

19. A photomultiplier has nine dynodes, each of which has a gain of 2.2. What is the overall tube gain?

20. Given the following conditions of operation, compute the weekly workload:

20 patients per day 3.2 fi lms per patient 80 mAs per view on average

The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

597

C H A P T E R

39 Patient Radiation Dose Management OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Indicate three ways that patient dose can be reported 2. Discuss ALARA principles applied to patient radiation dose

management 3. Discuss factors that affect patient radiation dose 4. Discuss the radiosensitivity of the stages of pregnancy 5. Describe the recommended management procedures for the pregnant

patient 6. Describe the intensity and distribution of radiation dose in

mammography and computed tomography 7. Identify screening x-ray examinations that are no longer performed

routinely 8. Explain when gonad shields should be used OUTLINE Patient Dose Descriptions

Estimation of Patient Dose Patient Dose in Special Examinations

Reduction of Unnecessary Patient Dose Unnecessary Examinations Repeat Examinations Radiographic Technique Image Receptor Patient Positioning Specifi c Area Shielding

The Pregnant Patient Radiobiologic Considerations Patient Information

Patient Dose Trends

PART VII Radiation Protection598

PATIENT DOSE DESCRIPTIONS Exposure of patients to medical x-rays is commanding increasing attention in our society for two reasons.

First, the frequency of x-ray examination is increas- ing among all age groups, at a rate of approximately 18% per year in the United States. This indicates that physicians are relying more and more on x-ray diagnosis to assist them in patient care, even taking into account the newer imaging modalities.

This is to be expected. X-ray diagnosis is considered much more accurate today than in the past. More rig- orous training programs required of radiologists and radiologic technologists and improvements in diagnostic x-ray imaging systems allow for more diffi cult, but more substantive, x-ray examinations. Effi cacy and diagnostic accuracy are much improved.

Second, concern among public health offi cials and radiation scientists is increasing regarding the risk that is

associated with medical x-ray exposure. Acute effects on superfi cial tissues after angiointerventional procedures are reported with increasing frequency.

The possible late effects of diagnostic x-ray expo- sure are of concern; therefore, attention must be given to good radiation control practices. When a diagnosis can be obtained with a low radiation dose, it should be used because of reduced risk. This is in keeping with ALARA.

Estimation of Patient Dose Patient dose from diagnostic x-rays usually is reported in one of three ways. Exposure to the entrance surface, or entrance skin exposure (ESE), is reported most often because it is easy to measure.

The gonadal dose is important because of possible genetic responses to medical x-ray exposure. The dose to the gonads is not diffi cult to measure or estimate.

The dose to the bone marrow is important because bone marrow is the target organ believed responsible for radiation-induced leukemia. Bone marrow dose cannot be measured directly; it is estimated from ESE.

Table 39-1 presents some representative values of ESE and gonadal dose for various x-ray examinations. The mean marrow dose for each procedure also is presented. Note that these are only approximate values and should not be used to estimate patient dose at any facility.

In any given x-ray facility, actual doses delivered may be considerably different. Effi ciency of x-ray production and image receptor speed are the most important variables. These values provide for relative dose comparisons among various radiologic examinations. Doses during fl uoroscopy are too dependent on technique, equipment, and beam-on time to be estimated easily. Usually, such doses must be measured.

A LL MEDICAL health physics activity is directed in some way toward minimizing the radia- tion exposure of radiologic personnel and the

radiation dose to patients during x-ray examination. Radiation exposure of radiologists and radiologic technologists is measured with the use of occupa- tional radiation monitors. Patient dose usually is estimated by conducting simulated x-ray examina- tions with human phantoms and test objects.

If radiation control procedures are adopted, occupational radiation exposure and patient dose can be kept acceptably low. Health physicists sub- scribe to ALARA — keep radiation exposure as low as reasonably achievable. Radiologic technologists should follow this guide as well.

A

TABLE 39-1 Representative Radiation Quantities From Various Diagnostic X-ray Procedures

Examination Technique (kVp/mAs) Entrance Skin Exposure (mR)

Mean Marrow Dose (mrad) Gonad Dose (mrad)

Skull 76/50 200 10 <1 Chest 110/3 10 2 <1 Cervical spine 70/40 150 10 <1 Lumbar spine 72/60 300 60 225 Abdomen 74/60 400 30 125 Pelvis 70/50 150 20 150 Extremity 60/5 50 2 <1 CT (head) 125/300 4000 20 50 CT (pelvis) 125/400 2000 50 2000

CT, Computed tomography.

Patient radiation dose is expressed as entrance skin exposure, gonadal dose, and bone marrow dose.

C H A P T E R 39 Patient Radiation Dose Management 599

Entrance Skin Exposure. ESE most often is referred to as the patient dose. It is used widely be cause it is easy to measure, and reasonably accurate estimates can be made in the absence of measurements.

Thermoluminescence dosimeters (TLDs) are used most often. The size, sensitivity, and accuracy of TLDs make them very satisfactory patient radiation monitors.

A small grouping or pack of 3 to 10 TLDs can be taped easily to the patient's skin in the center of the x-ray fi eld. Because the response of the TLD is propor- tional to exposure and dose, the TLD can be used to measure all levels experienced in diagnostic radiology. With proper laboratory technique, the results of such measurements are accurate to within 5%.

Two rather straightforward methods for estimating ESE are available in the absence of patient measure- ments. The fi rst requires the use of a nomogram such as that shown in Figure 39-1 . This fi gure contains a family of curves from which one can estimate the output inten- sity of a radiographic unit if the technique is known or assumed. The output intensity of different x-ray imag- ing systems varies widely, so the use of this nomogram method is good only to perhaps 50%.

Use of this nomogram fi rst requires knowledge of the total fi ltration in the x-ray beam. This is usually available from the medical physics report, but if not,

3 mm Al is a good estimate. Next, the kVp and mAs of the intended examination should be identifi ed.

A vertical line rising from the value of total fi ltration should be drawn until it intersects with the kVp of the exam- ination. From this intersection, a horizontal line is drawn to the left until it intersects the mR/mAs axis. The resultant mR/mAs value is the approximate output intensity of the radiographic unit. This value should be multiplied with the examination mAs value to obtain the approximate patient exposure.

Question: With reference to Figure 39-1 , estimate

the ESE from a lateral cervical spine image made at 66 kVp, 150 mAs, with a radiographic unit having 2.5 mm Al total fi ltration.

Answer: Estimate the intersection between a vertical line rising from 2.5 mm Al and a horizontal line through 66 kVp. Extend the horizontal line to the y-axis and read 3.8 mR/mAs.

3.8 mR/mAs × 150 = 570 mR

A better approach requires that a medical physicist construct a nomogram such as that shown in Figure 39-2 for each radiographic unit. A straight edge between any kVp and mAs value will cross the ESE scale at the cor- rect mR value.

Question: Using the nomogram in Figure 39-2 ,

identify the ESE when a radiographic exposure is made at 66 kVp, 150 mAs.

Answer: The line is drawn as shown and crosses the ESE scale at 1000 mR.

A third method for estimating ESE requires that one know the output intensity for at least one operating condition. During the annual or special radiation control survey and calibration of an x-ray imaging system, the medical physicist measures this output intensity, usually in units of mR/mAs at 80 cm — the approximate source-to- skin distance (SSD) — or at 100 cm — the source-to-image receptor distance (SID). At 70 kVp, radiographic output intensity varies from approximately 2 to 10 mR/mAs at 80 cm SSD.

With this calibration value available, one fi rst would make adjustment for a different SSD by applying the inverse square law.

Question: The output intensity of a radiographic unit

is reported as 3.7 mR/mAs (37 µGy a /mAs) at 100 cm SID. What is the intensity at 75 cm SSD?

Answer: At 75 cm SSD, the intensity will be greater by (100/75) 2 = (1.32) 2 = 1.78

3.7 mR/mAs × 1.78 = 6.6 mR/mAs

50

10 150 kVp

125 kVp 110 kVp 100 kVp 90 kVp 80 kVp 70 kVp

60 kVp

50 kVp

40 kVp

1

1 2 3 Total filtration (mm Al)

Ex po

su re

( m

R/ m

A s)

a t 1

0 0

-c m

S ID

4 5

0.5

0.2

FIGURE 39-1 This family of curves is a nomogram for esti- mating output x-ray intensity from a single-phase radiographic unit. (Courtesy John R. Cameron, † University of Wisconsin.)

PART VII Radiation Protection600

With the ESE, one scales this according to the kVp and mAs of the examination. Output intensity varies ac- cording to the square of the ratio in terms of the change in kVp. Refer to Chapter 9 to review this relationship.

Question: The output intensity at 70 kVp and 75 cm

SSD is 6.6 mR/mAs (66 µGy a /mAs). What is the output intensity at 76 kVp?

Answer: At higher kVp, the output intensity is greater by the square of the ratio of the kVp.

(76/70) 2 = (1.09) 2 = 1.18 6.6 mR/mAs × 1.18 = 7.8 mR/mAs

The fi nal step in estimating ESE is to multiply the out- put intensity in mR/mAs by the examination mAs value because these values are proportional.

Question: If the radiographic technique for an

intravenous pyelogram calls for 80 mAs, what is the ESE when the output intensity is 7.8 mR/mAs (78 µGy a /mAs)?

Answer: 7.8 mR/mAs × 80 mAs = 624 mR

These steps can be combined into a single calculation, as illustrated in the following example.

Question: The output intensity for a radiographic

unit is 4.5 mR/mAs (4.5 µGy a /mAs) at 70 kVp and 80 cm. If a lateral skull fi lm is taken at 66 kVp, 150 mAs, what will be the ESE at an 80-cm SSD? What would be the skin dose at a 90-cm SSD?

Answer:

At 80 cm SSD

Dose = (4.5 mR/mAs) 66 kVp 70 kVp

(150 mAs)

= 6

 

 

2

000 mR

At 90 cm SSD

Dose = (600 mR) 80 90

= 474 mR  

 

2

ESE in fl uoroscopy is much more diffi cult to

estimate because the x-ray fi eld moves and sometimes varies in size. If the fi eld were of one size and stationary, ESE would be directly related to exposure time.

Question: A fl uoroscopic procedure requires 2.5 min at 90 kVp, 2 mA. What is the approximate ESE?

Answer: ESE = (4 R/min) (2.5 min) = 10 R

120 110 100 90

80

70

60

50

40 kVp

1000

Imaging Services Room #1

10

1

100

mAs

10,000

1000

100

10

1 ESE (mR)

FIGURE 39-2 This type of nomogram is very accurate but must be fashioned individually for each radiographic unit. (Courtesy Michael D. Harpen, University of South Alabama.)

For the average fl uoroscopic examination, one can assume an ESE of 4 R/min.

C H A P T E R 39 Patient Radiation Dose Management 601

Mean Marrow Dose. The hematologic effects of radiation are rarely experienced in diagnostic radiology. It is appropriate, however, that we understand the mean marrow dose, which is one measure of patient dose during diagnostic procedures.

The mean marrow dose is the average radiation dose to the entire active bone marrow. For instance, if dur- ing a particular examination, 50% of the active bone marrow were in the primary beam and received an aver- age dose of 25 mrad (250 µGy t ), the mean marrow dose would be 12.5 mrad (125 µGy t ).

Table 39-1 includes the approximate mean marrow dose in adults for various radiographic examinations. In children, these levels generally would be lower because the radiographic techniques used are considerably less . Table 39-2 shows the distribution of active bone mar- row in the adult, and this gives some clue as to which diagnostic x-ray procedures involve exposure to large amounts of bone marrow.

In the United States, the mean marrow dose from diagnostic x-ray examinations averaged over the entire population is approximately 100 mrad/yr (1 mGy t /yr). Such a dose never results in the hematologic responses described in Chapter 35. It is a dose concept, however, that is used to estimate, on a population basis, the risk of one late effect of radiation — leukemia.

Genetically Significant Dose. Measurements and esti- mates of gonad dose are important because of the suspect- ed genetic effects of radiation. Although the gonad dose from diagnostic x-rays is low for each individual, this may have some signifi cance in terms of population effects.

The population gonad dose of importance is the GSD, the radiation dose to the population gene pool. Thus, it is a weighted-average gonad dose. It takes into ac- count those persons who are irradiated and those who are not, with averaging of the results. The GSD can be estimated only through large-scale epidemiologic studies.

For computational purposes, therefore, the GSD consid- ers the age, sex, and expected number of children for each person examined with x-rays. It also acknowledges the various types of examinations and the gonadal dose per examination type.

Estimates of GSD have been conducted in many dif- ferent countries ( Table 39-3 ). The estimate reported by the U.S. Public Health Service is 20 mrad/yr (0.2 mGy t / yr). Thus, this is a genetic radiation burden over and above the existing natural background radiation level of approximately 100 mrad/yr (1 mGy t /yr). The genetic effects of this total GSD — 120 mrad/yr (1.2 mGy t /yr) — are not detectable.

Patient Dose in Special Examinations Dose in Mammography. Because of the considerable application of x-rays for examination of the female breast and concern for the induction of breast can- cer by radiation, it is imperative that we have some understanding of the radiation doses involved in such examinations.

GENETICALLY SIGNIFICANT DOSE

GSD =

DN P

N P x

T

Σ Σ

where Σ is�the�mathematical symbol meaning to sum or add values, D is the average gonad dose per examination, N X signifi es the number of persons receiving x-ray examinations, N T is the total number of persons in the population, and P (progeny) is the expected future number of children per person.

Anatomic Site Percentage of Bone Marrow

Head 10 Upper limb girdle 8 Sternum 3 Ribs 11 Cervical vertebrae 4 Thoracic vertebrae 13 Lumbar vertebrae 11 Sacrum 11 Lower limb girdle 29 Total 100

TABLE 39-2 Distribution of Active Bone Marrow in Adults

TABLE 39-3 Genetically Signifi cant Dose Estimated From Diagnostic X-ray Examination

Population Genetically Signifi cant Dose (mrad)

Denmark 22 Great Britain 12 Japan 27 New Zealand 12 Sweden 72 United States 20

The genetically signifi cant dose (GSD) is the gonad dose that, if received by every member of the population, would produce the total genetic effect on the population as the sum of the individual doses actually received.

PART VII Radiation Protection602

An ESE of approximately 800 mR/view (8 mGy a /view) is normal. Increasing the x-ray tube potential much beyond 26 kVp degrades the image unacceptably; there- fore, further dose reduction by technique manipulation is unlikely.

Radiographic grids are used in most screen-fi lm mam- mography examinations. Grid ratios of 4:1 and 5:1 are most popular. The contrast enhancement produced by the use of such grids is signifi cant, but so is the increase in patient dose. Patient dose is increased by approxi- mately two times with the use of such grids compared with the nongrid technique.

The values stated for patient dose in mammography can be misleading. Because of the low x-ray energies used in mammography, the dose falls off very rapidly as the x-ray beam penetrates the breast. If the ESE for a craniocaudad view is 800 mR (8 mGy a ), the dose to the midline of the breast may be only 100 mrad (1.0 mGy t ).

Fortunately, it is known that the risk of an adverse biologic response from mammography is small. Cer- tainly, it is nothing about which a patient should be concerned. Any possible response, however, is related to the average radiation dose to glandular tissue, and not to skin exposure. Glandular dose (D g ) varies in a complicated way, with variations noted in x-ray beam quality and quantity.

Specifi cation of an ESE can be misleading when one considers a two-view examination, such as that used for screening ( Figure 39-3 ). Consider an examination that consists of craniocaudad and mediolateral oblique views, each of which produces an ESE of 800 mR (8 mGy a ).

It would be incorrect to describe this total examination procedure as resulting in an ESE of 1.6 R (16 mGy a ). Skin exposures from different projections cannot be added. We must specify the skin exposure for each view or attempt to estimate the total D g .

Total D g can be estimated by approximating that the contribution from each view will be 15% of the ESE. Con- sequently, the total D g would be the sum of (0.15 × 800 = 120 mrad [1.2 mGy t ]) as the contribution from each of the craniocaudad and mediolateral oblique views. The total D g would, therefore, be 240 mrad (2.4 mGy t ).

From this discussion, it would seem that patient dose in mammography can be considerably reduced if the number of views is restricted. The axillary view should not be done routinely. For screening programs, no more than two views per breast are advisable. Digital mam- mography should result in lower D g than that attained with screen-fi lm mammography.

Dose in Computed Tomography Imaging. An im- portant consideration in computed tomography (CT) im- aging, as with any x-ray procedure, is not only the skin dose but also the distribution of dose to internal organs and tissues during imaging. On the basis of skin dose, CT results in a higher dose than other diagnostic x-ray pro- cedures. The skin dose delivered by a series of contiguous CT slices is much higher than that delivered by a single radiographic view. A typical radiographic head or body examination, however, often involves several views.

Because of increasing use of multislice spiral CT, CT must be considered a high-dose procedure. U.S. Public Health Service data suggest that 10% of all x-ray exami- nations are now CT, yet CT accounts for 70% of total patient effective dose. The CT tissue dose is approxi- mately equal to the average fl uoroscopic dose.

As was pointed out in Chapter 23, CT differs in many important ways from other x-ray examinations. A radiograph can be likened to a photograph taken with a fl ash in that the patient is “fl oodlighted” with x-rays to directly expose the image receptor.

On the other hand, CT images the patient with a fi ne, collimated beam of x-rays. This difference in radiation delivery also means that the dose distribution from CT is different from that in radiographic procedures.

The CT dose is nearly uniform throughout the imaging volume for a head examination. The CT dose is approximately 50% of the ESE for body CT. Radiographic and fl uoroscopic doses are high at the en- trance surface and very low at the exit surface.

Part of the dose effi ciency of CT is attributable to the precise collimation of the x-ray beam. Scatter radiation

Craniocaudad

ESE = 800 mR Mediolateral oblique

Dg = 120 mrad ESE = 800 mR

Dg = 120 mrad

Total Dg = 240 mrad

FIGURE 39-3 Two mammographic exposures result in a to- tal glandular dose that is the sum of the individual glandular doses.

Glandular dose is approximately 15% of the ESE.

Screen-fi lm and digital mammography currently are the only acceptable techniques.

Glandular dose should not exceed 100 mrad/view with contact mammography and 300 mrad/view with a grid.

C H A P T E R 39 Patient Radiation Dose Management 603

increases patient dose and reduces radiographic contrast. Because CT uses narrow, well-collimated x-ray beams, scatter radiation is reduced signifi cantly, and contrast res- olution is improved signifi cantly. Thus, a larger percent- age of the x-ray beam contributes usefully to the image.

The precise collimation used in CT means that only a well-defi ned volume of tissue is irradiated for each image. The ideal x-ray beam for CT would have sharp boundaries. No overlap between adjacent images would be seen. Thus, the dose delivered to a patient from a series of ideal contiguous CT images should be the same as that from a single slice.

Figure 39-4 illustrates, however, why this ideal situation cannot be attained in practice. The size of the focal spot of the x-ray tube blurs the sharp boundaries of the section. Also, the x-ray beam is not precisely parallel, and some spreading occurs as the beam crosses the image fi eld.

If a series of adjacent images is performed with an automatically indexed patient couch, the couch movement must be precise. If the couch moves too much between images, some tissue will be missed. If it moves too little, some tissue in each image will be doubled-exposed.

Multislice spiral CT results in lower patient dose than conventional step-and-shoot CT because fewer tails are seen on the dose profi le for a given volume of tissue. The dose profi le tail is called a penumbra.

Typical CT doses range from 3000 to 5000 mrad (30 to 50 mGy t ) during head imaging and from 2000 to

4000 mrad (20 to 40 mGy t ) during body imaging. These values are only approximate and vary widely depending on the type of CT imaging system and the examination technique used. The effective dose for each examination is approximately 1000 mrem (10 mSv).

A 64-slice CT imaging system will result in a lower pa- tient dose than fewer slices because a lower contribution is made from the penumbra for the same volume of tissue ( Figure 39-5 ). Additional patient dose saving occurs when the same beam width is imaged with combined pixel rows ( Figure 39-6 ) because the mA can be reduced without com- promising image noise and therefore contrast resolution.

Because the CT x-ray beam is well collimated, the area of irradiation can be precisely controlled. Thus, radiosensi- tive organs such as the eyes can be avoided selectively. Shields as protection from the primary x-ray beam in CT are of little use. Not only does the metal from shields produce artifacts in the image, but the rotational scheme of the x-ray source greatly reduces their effectiveness.

Patient dose during spiral CT is somewhat more diffi cult to assess than the dose during conventional step-and-shoot CT. At a pitch of 1.0:1, the patient dose is approximately the same. At a higher pitch, the dose is reduced compared with conventional CT. At a lower pitch, patient dose is increased.

As with any radiographic procedure, many factors infl uence patient dose. For CT imaging, a generalization is possible.

Ideal dose profile

Single-scan dose profile

Multiple-scan dose profile

Beam width (mm)

Radiation dose

10 20010 10 020 20 30103020

Penumbra

FIGURE 39-4 Patient dose distribution in step-and-shoot multislice spiral computed tomog- raphy is complicated because the profi le of the x-ray beam cannot be made sharp.

The higher the multislice value, the lower the patient dose will be.

It is essential that CT collimators be monitored periodically for proper adjustment.

PART VII Radiation Protection604

Note that, as with radiography, patient dose is propor- tional to x-ray beam intensity. It is also directly pro- portional to the average beam energy. Other factors are variables that are unique to CT imaging.

Sigma (σ) is noise. This is equivalent to quantum mot- tle in screen-fi lm radiography and represents random statistical variations in CT numbers. The w stands for the pixel size, one of the determinants of spatial resolu- tion. The last factor, h, is the beam width.

All other factors being equal, a low-noise, high-resolution CT image results in higher patient dose. The challenge with CT, as indeed with all x-ray imaging, is not so much to deliver fantastically good resolution and low noise

64 slice 32 slice (each twice as thick)

FIGURE 39-6 When two detector rows are combined for the same beam width, patient dose will be lower.

COMPUTED TOMOGRAPHY PATIENT DOSE

σ 2 IE

Patient Dose w h3

= k

where k is a conversion factor, I is a beam intensity in mAs, E is average beam energy in keV (approximately 1/2 kVp), σ is a system noise, w is pixel size, and h is beam width.

Penumbra64 slice 16 slice

FIGURE 39-5 Patient radiation dose is lower with higher multislice computed tomography because the beam penumbra is less for a given imaged anatomy.

A reduction in the noise or beam width, while other factors remain constant, increases patient dose.

C H A P T E R 39 Patient Radiation Dose Management 605

(because this could be achieved at the cost of very high patient dose) but to use the x-ray beam effi ciently, pro- ducing the best possible image at a reasonable dose to the patient.

REDUCTION OF UNNECESSARY PATIENT DOSE The radiologic technologist has considerable control over many sources of unnecessary patient dose. Unnec- essary patient dose is defi ned as any radiation dose that is not required for the patient's well-being or for proper management and care.

Unnecessary Examinations The radiologic technologist has practically no control over what some consider the largest source of unneces- sary patient dose, that is, the unnecessary x-ray exami- nation. This is almost exclusively the radiologist's or the clinician's responsibility. Radiologic technologists can help by asking whether the patient has had a previous x-ray examination. If so, perhaps those images should be obtained for review before any other steps are taken.

Unfortunately, this source of unnecessary patient dose presents a serious dilemma for the radiologist and the clinician. Many x-ray examinations are requested when it is known that the yield of helpful information may be extremely low or nonexistent. When such an examination is performed, the benefi t to the patient in no way compensates for the radiation dose.

If the examination is not performed, however, the cli- nician and the radiologist may be criticized severely if management of the patient's condition results in failure. Even though the examination in question would have contributed little, if anything, to effective patient man- agement, the radiologist may even be sued. In such situ- ations, the radiologist is caught between the proverbial “rock and a hard place.”

Routine x-ray examinations should not be performed when there is no precise medical indication. Substantial evidence shows that such examinations are of little ben- efi t because they are not cost-effective and the disease detection rate is very low. Examples of such cases are discussed in the following sections.

Mass Screening for Tuberculosis. General screen- ing by chest x-ray examination has not been found to be effective. Better methods of tuberculosis testing are now available. Some x-ray screening in high-risk groups (e.g., medical and paramedical personnel), in service person- nel posing a potential community hazard (e.g., food han- dlers, teachers), and in special occupational groups (e.g., miners, workers having contact with beryllium, asbestos, glass, or silica) may be appropriate.

Hospital Admission. Chest x-ray examinations should not be performed for routine hospital admission when no clinical indication of chest disease is found.

Among patients who might be candidates for such ex- amination are those admitted to the pulmonary service.

Preemployment Physicals. Chest and lower back x-ray examinations are not justifi ed because the knowl- edge gained about previous injury or disease through this approach is nil.

Periodic Health Examinations. Many physicians and health care organizations promote annual or bian- nual physical examinations. Certainly, when such an examination is conducted on an asymptomatic patient, it should not include x-ray examination, especially fl uo- roscopic examination.

Emergency Room CT. CT has passed radiography as the fi rst line of diagnostic imaging. This overutiliza- tion must be controlled because of the rapidly rising population effective dose.

Whole-Body Multislice Spiral CT Screening. Some facilities now offer this procedure to the public for self- referral. Until evidence reveals a signifi cant disease de- tection rate, this should not be done. The radiation dose is too high.

Repeat Examinations One area of unnecessary radiation exposure that the radiologic technologist can infl uence is that of repeat examinations. The frequency of repeat examinations has been estimated variously to range as high as 10% of all examinations. In the typical busy hospital facility, the rate of repeat examinations should not normally exceed 5%. Examinations with the highest repeat rates include lumbar spine, thoracic spine, and abdomen.

Some repeat examinations are performed because of equipment malfunction. However, most are caused by radiologic technologist error. Studies of causes of repeat examinations have shown that improper positioning and poor radiographic technique resulting in an image that is too light or too dark are primarily responsible for repeats.

Motion and improper collimation are responsible for some repeats. Infrequent errors that contribute to repeat examinations include dirty screens, use of im- properly loaded cassettes, light leaks, chemical fog, ar- tifacts caused by a dirty processor, wrong projection, improper patient preparation, grid errors, and multiple exposures.

Radiographic Technique In general, the use of high-kVp technique results in reduced patient dose. Increasing the kVp is always asso- ciated with a reduction in mAs to obtain an acceptable radiographic optical density; this, in turn, results in reduced patient dose.

It should never be necessary to repeat a digital radiographic examination.

PART VII Radiation Protection606

This dose reduction occurs because the patient dose is linearly related to the mAs but is related to approxi- mately the square of the kVp. An area of radiography for which high-kVp technique is widely accepted is examination of the chest.

Question: A lateral skull radiograph is obtained at

64 kVp, 80 mAs, and results in an ESE of 400 mR (4 mGy a ). If the tube potential is increased to 74 kVp (15% increase) and the mAs is reduced by half, to 40 mAs, the optical density will remain the same. What will be the new ESE?

Answer: Dose = (400 mR)

40 mAs

80 mAs

74 kVp

64 kVp

= (400 mR

 

 

 

 

2

))(0.5)(1.34)

= 268 mR

Of course, the radiologist must be the fi nal judge of radiographic quality. Increasing kVp even slightly may result in images with contrast that is too low for proper interpretation by the radiologist.

Proper collimation is essential to good radiographic technique. Positive beam limitation does not prevent the radiologic technologist from reducing fi eld size still fur- ther through collimation. With the use of collimation, not only is patient effective dose reduced, but image quality is improved with enhanced contrast resolution because scatter radiation also is reduced.

Image Receptor The image receptor should be selected fi rst for the type of examination that is being performed, and second for the radiation dose necessary to produce a good-quality image. It should be kept in mind that it is screen speed rather than fi lm speed that principally controls patient dose.

Rare Earth and other fast screens should be used when possible. The routine application of such screens in orthopedic, chest, and magnifi cation radiography is appropriate. In some applications, the use of such fast systems may result in bothersome quantum mottle, but this again must be decided by the radiologist. Usually, 400-speed systems are used now for general radiography.

Digital radiographic (DR) image receptors are inherently faster than screen-fi lm. Patient dose should be lower with the use of DR because of increased speed and increased kVp accompanied by reduced mAs.

Patient Positioning When the upper extremities or the breast is examined, especially with the patient in a seated position, care should be taken that the useful beam does not intercept the gonads. Position the patient lateral to the useful beam and provide a protective apron as a shield.

Specifi c Area Shielding X-ray examinations result in partial-body exposure, although most radiation protection guides and radiation response information are based on whole-body expo- sure. The partial-body nature of the x-ray examination is controlled by proper beam collimation and the use of specifi c area shielding.

Use of specifi c area shielding is indicated when a particularly sensitive tissue or organ is in or near the useful beam. The lens of the eye, the breasts, and the gonads frequently are shielded from the primary radia- tion beam. Two types of specifi c area shielding devices are used: the contact shield and the shadow shield.

Lens shields are always of the contact type. The con- tact shielding device is positioned directly on the patient. Gonad shields, on the other hand, can be of the contact or shadow type.

Breast shields are contact shields that are recom- mended for use during scoliosis examinations. Such ex- aminations often consist of an anterior-posterior (AP) projection, which subjects juvenile breasts to primary beam x- irradiation. The posterior-anterior (PA) projec- tion, however, is equally satisfactory because magnifi - cation is of little importance. The PA projection results in a breast dose of only approximately 1% of the AP projection.

Figure 39-7 shows some examples of contact gonad shields. When such contact shields are not purchased commercially, a properly cut piece of protective material is perfectly adequate. Shapes such as hearts, diamonds, triangles, and squares have been used effectively, espe- cially for children.

An example of the shadow shield is shown in Figure 39-8 . This type of shield is just as effective as the contact shield and is more acceptable for use with adult patients. The use of such devices, however, requires care- ful attention on the part of the radiologic technologist.

The shield must shadow the gonads without interfer- ing with the desired anatomy. Improper positioning of the shadow shield can result in a repeat examination and increased patient dose. Shadow shields are particu- larly useful during surgery for which sterile procedure is required. Box 39-1 lists the main points of gonadal shielding.

Digital radiography can be conducted at higher kVp, resulting in lower patient dose.

The fastest-speed screen-fi lm combination consistent with the nature of the examination should be used.

C H A P T E R 39 Patient Radiation Dose Management 607

THE PREGNANT PATIENT Two situations in diagnostic radiology require particu- lar care and action. Both are associated with pregnancy. Their importance is obvious from both a physical and an emotional standpoint.

Radiobiologic Considerations The severity of the potential response to radiation exposure in utero is both time related and dose related, as was discussed in Chapter 36. Unquestionably, the period most sensitive to radiation exposure occurs be- fore birth. Furthermore, the fetus is more sensitive early

in pregnancy than late in pregnancy. As a general rule, the higher the radiation dose, the more severe will be the radiation response.

Time Dependence. A grave misunderstanding is that the most critical time for irradiation is during the fi rst 2 weeks, when it is most unlikely that the expectant mother knows of her condition. In fact, this is the time during pregnancy when such irradiation is least hazard- ous. Pregnancies fail during this period for reasons other than exposure to radiation.

The most likely biologic response to irradiation during the fi rst 2 weeks of pregnancy is resorption of the embryo, and therefore no pregnancy. No other response is likely.

No concern has been expressed over the possibility of induction of congenital abnormalities during the fi rst 2 weeks of pregnancy. Such a response has not been demonstrated in experimental animals or in humans after any level of radiation dose.

The time from approximately the second week to the tenth week of pregnancy is called the period of major organogenesis. During this time, the major organ sys- tems of the fetus are developing. If the radiation dose is suffi ciently high, congenital abnormalities may result.

Contact shield

A B

FIGURE 39-7 Examples of useful contact gonad shields, which can be a piece of vinyl lead (A) or shaped (B).

A

B

FIGURE 39-8 A, Shadow shield. B, Shadow shield suspend- ed above the beam-defi ning system casts a shadow over the gonads. (Courtesy Fluke Biomedical.)

BOX 39-1 Gonad Shielding

• Gonad shielding should be considered for all patients, especially children and those who are potentially reproductive. As an administrative procedure, this would include all patients younger than 40 years of age and perhaps even older men.

• Gonad shielding should be used when the gonads lie in or near the useful beam.

• Proper patient positioning and beam collimation should not be relaxed when gonad shields are in use.

• Gonad shielding should be used only when it does not interfere with obtaining the required diagnostic information.

PART VII Radiation Protection608

Early in organogenesis, the most likely congenital abnormalities are associated with skeletal deformities. Later in this period, neurologic defi ciencies are more likely to occur.

During the second and third trimesters of preg- nancy, the responses previously noted are unlikely. The results of numerous investigations strongly suggest that if a response occurs after diagnostic irradiation during the latter two trimesters, the principal response would be the appearance of malignant disease during childhood.

These responses to irradiation during pregnancy require a very high radiation dose before the risk of occurrence is signifi cant. No such responses would occur at less than 25 rad (250 mGy).

Such dose levels are highly unlikely, yet they are possible with patients who receive multiple x-ray ex- aminations of the abdomen or pelvis. They are essen- tially impossible with radiologic technologists because their occupational exposures are so low. No other signifi cant responses have been reported after irradia- tion in utero.

Dose Dependence. As one might imagine, virtu- ally no information is available at the human level to construct dose-response relationships for irradiation in utero. However, a large body of data on animal irradiation, particularly that in rats and mice, serves as the basis from which such relationships can be estimat- ed. The statements that follow, although attributed to human exposure, represent estimates based on extrapo- lation from animal studies.

After an in utero radiation dose of 200 rad (2 Gy), it is nearly certain that each of the effects noted previ- ously will occur. The likelihood is small, however, that an exposure of this magnitude would be experienced in diagnostic radiology.

Spontaneous abortion after irradiation during the fi rst 2 weeks of pregnancy is unlikely at radiation doses less than 25 rad (250 mGy). The precise nature of the dose-response relationship is unknown, but a reasonable estimate of risk suggests that 0.1% of all conceptions would be resorbed after a dose of 10 rad (100 mGy).

The response at lower doses would be proportion- ately lower. Keep in mind, however, that the incidence of spontaneous abortion in the absence of radiation exposure is estimated to be in the 25% to 50% range.

In the absence of radiation exposure, approximately 5% of all live births exhibit a manifest congenital abnormality. A 1% increase in congenital abnormali- ties is estimated to follow a 10-rad (100-mGy) fetal dose, with a proportionately lower increase at lower doses.

The induction of a childhood malignancy after irra- diation in utero is diffi cult to assess. Risk estimates are

even lower than those reported for spontaneous abor- tion and congenital abnormalities. The best approach to assessing risk of childhood malignancy is to use a rela- tive risk estimate.

During the fi rst trimester, the relative risk of radiation-induced childhood malignancy is in the range of 5 to 10; it drops to approximately 1.4 during the third trimester. The overall relative risk is accepted to be 1.5 — a 50% increase over the naturally occurring in- cidence.

Patient Information Safeguards against accidental irradiation early in preg- nancy present complex administrative problems. This situation is particularly critical during the fi rst 2 months of pregnancy, when such a condition may not be sus- pected, and when the fetus is particularly sensitive to radiation exposure. After 2 months, the risk of irradiat- ing an unknown pregnancy becomes small because the patient is usually aware of her condition.

If the state of pregnancy is known, then under some circumstances, the radiologic examination should not be conducted. One should never knowingly examine a pregnant patient with x-rays unless a documented decision to do so has been made. When such an exami- nation does proceed, it should be conducted with all of the previously discussed techniques for minimizing patient dose.

When a pregnant patient must be examined, the examination should be done with precisely collimated beams and carefully positioned protective shields. The use of high-kVp technique is most appropriate in such situations. The administrative protocols that can be used to ensure that we do not irradiate pregnant pa- tients vary from complex (elective booking) to simple (posting).

Elective Booking. The most direct way to ensure against the irradiation of an unsuspected pregnancy is to institute elective booking. This requires that the cli- nician, radiologist, or radiologic technologist determine the time of the patient's previous menstrual cycle. X-ray examinations in which the fetus is not in or near the primary beam may be allowed, but they should be ac- companied by pelvic shielding.

Ideally, the referring physician should be respon- sible for determining the menstrual cycle and for withholding the examination request if there is any question about its necessity. This may require a ra- diologist-sponsored educational program that can be conducted easily at regularly scheduled medical staff meetings.

Patient Questionnaire. An alternative procedure is to have the patient herself indicate her menstrual cycle. In many diagnostic imaging departments, the patient must complete an information form before undergoing examination.

C H A P T E R 39 Patient Radiation Dose Management 609

These forms often include questions such as, “Are you or could you be pregnant?” and “What was the date of your last menstrual period?” Figure 39-9 is an example of such a simple, yet effective questionnaire for protecting against irradiation of a pregnant pa- tient.

Posting. If neither elective booking nor the request form seems appropriate to a diagnostic imaging service, an equally successful method is to post signs of caution in the waiting room. Such signs could read, “Are you pregnant or could you be? If so, inform the radiologic technologist,” or “Warning — special precautions are necessary if you are pregnant,” or “Caution — if there is any possibility that you are pregnant, it is very impor- tant that you inform the radiologic technologist before you have an x-ray examination.”

Figure 39-10 is a helpful poster that is available from the National Center for Devices and Radiological Health.

Such posting satisfi es our responsibility to the patient and to the health care facility.

It has been estimated that less than 1% of all women referred for x-ray examination are potentially pregnant. If a pregnant patient escapes detection and is irradiated, however, what is the subsequent responsibility of the radiology service to the patient, and what should be done?

The fi rst step is to estimate the fetal dose. The medical physicist should be consulted immediately and requested to estimate the fetal dose. If a pre- liminary review of the examination techniques used (i.e., type of examination, kVp, and mAs) determines that the dose may have exceeded 1 rad (10 mGy t ), a more complete dosimetric evaluation should be conducted.

Table 39-4 presents representative fetal dose levels for many examinations. With knowledge of the types of examinations performed and the techniques and apparatus used, the medical physicist can accurately de- termine the fetal dose. Test objects and dosimetry mate- rials are available to ensure that this determination can be made with confi dence.

X-Ray Consent for Women of Childbearing Age

X-ray examinations of abdomen and pelvis exposing the uterus to radiation are:

The 10 days after onset of menstural period are generally considered safe for x-ray examinations.

I recognize that if I am pregnant and have radiation to the abdomen, there is a possibility of injury to the fetus. However, I understand that the likelihood of such injury is slight and that my physician feels that the information to be gained from this examination is important to my health. I therefore wish to have this x-ray examination performed now.

________________________________________ Name of examination

________________________________________ Signature of patient

_________________________ Witness

Abdomen (KUB) Stomach (UGI) Small Intestine (SI) All nuclear medicine studies

Onset of last menstural period I am pregnant I have had a hysterectomy I use an IUD

Colon (barium enema) Gallbladder Hips, sacrum, coccyx

Date _______________ Yes ______ No ______ Yes ______ No ______ Yes ______ No ______

Pyelograms (IVP and retrograde) Cystograms Lumbar spine and pelvis

Date today ________________ Don’t know ______ Don’t know ______ Don’t know ______

FIGURE 39-9 X-ray consent for women of childbearing age.

We meet our responsibility to the pregnant patient by posting signs in the waiting room.

PART VII Radiation Protection610

Once the fetal dose is known, the referring physi- cian and the radiologist should determine the stage of gestation at which x-ray exposure occurred. With this information, only two alternatives are possible: Allow the patient to continue to term, or terminate the preg- nancy.

Recommendation for abortion after diagnostic x-ray exposure is rarely indicated. Because the natural inci- dence of congenital anomalies is approximately 5%, no such effects can reasonably be considered a consequence of diagnostic x-ray doses. Manifest damage to the new- born is unlikely at fetal doses below 25 rad (250 mGy t ), although some suggest that lower doses may cause men- tal developmental abnormalities.

In view of the available evidence, a reasonable ap- proach is to apply a 10- to 25-rad rule. Below 10 rad (100 mGy t ), a therapeutic abortion is not indicated un- less additional risk factors are involved. Above 25 rad (250 mGy t ), the risk of latent injury may justify a thera- peutic abortion.

Between 10 and 25 rad, the precise time of irradia- tion, the emotional state of the patient, the effect an additional child would have on the family, and other so- cial and economic factors must be considered carefully.

Fortunately, experience with such situations has shown that fetal doses have been consistently low. The

fetal dose rarely exceeds 5 rad (50 mGy t ) after a series of x-ray examinations.

PATIENT DOSE TRENDS The National Council on Radiation Protection and Mea- surements (NCRP) issues scientifi c reports on various aspects of radiation control, including patient radiation dose. The data shown in the pie chart in Figure 1-23 were published in 1990. They show a total annual radiation dose of 3.6 mSv, of which 0.53 mSv results from patient diagnostic radiation exposure.

Figure 39-11 is from data in a soon-to-be published NCRP report showing the current estimated human radiation exposure profi le. Natural sources of radiation exposure remain at 3 mSv, but look what's happening to medical imaging, 3.2 mSv! The contribution from computed tomography is soaring and represents the overutilization of this imaging modality.

This increase in patient radiation dose requires that radiologic technologists and radiologists exercise more control over medical imaging, especially computed tomography, in keeping with ALARA. We must be more aware of appropriateness criteria for diagnostic imaging and gain more control over unnecessary x-ray imaging.

With the introduction of digital imaging we are in a better position to automatically estimate the patient effective dose for each x-ray examination and record that to a continuing patient dose fi le. We monitor our occupational radiation exposure for life; we will be instituting protocols to do the same for our medical radiation exposure.

FIGURE 39-10 Wall posters with warnings about radiation and pregnancy are available from the National Center for Devices and Radiological Health. (Courtesy National Center for Devices and Radiological Health.)

Examination Entrance Skin Exposure (mR)

Fetal Dose (mrad)

Skull (lateral) 70 0 Cervical spine (AP) 110 0 Shoulder 90 0 Chest (PA) 10 0 Thoracic spine (AP) 180 1 Cholecystogram (PA) 150 1 Lumbosacral

spine (AP) * 250 80

Abdomen or KUB (AP) *

220 70

Intravenous pyelogram (IVP) *

210 60

Hip * 220 50 Wrist or foot 5 0

TABLE 39-4 Representative Entrance Exposures and Fetal Doses for Radiographic Examinations Frequently Performed With a 400-Speed Image Receptor

AP, Anteroposterior; IVP, intravenous pyelogram; KUB, kidneys, ureters, bladder; PA, posterior-anterior.

* Gonadal shields should be used if possible.

C H A P T E R 39 Patient Radiation Dose Management 611

SUMMARY Patient dose from diagnostic x-rays usually is recorded in one of the following three ways: (1) ESE, (2) mean marrow dose, or (3) gonadal dose. TLDs are the moni- tor of choice for patient radiation dose. By knowing the output intensity of at least one x-ray technique and the SSD, the medical physicist can estimate the ESE for any patient examination. For fl uoroscopic examination, a good general assumption for the ESE is 4 R/min.

Patient radiation dose can be reduced easily by elimi- nating unnecessary examinations and repeat examina- tions, and by ensuring proper radiographic technique and patient positioning. The radiobiology of pregnancy requires particular attention to the pregnant patient. By posting the waiting room and the examination room with educational signs, we meet our responsibility to the pregnant patient.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. ALARA b. Fetal DL c. Major organogenesis d. Elective booking e. GSD f. penumbra g. shadow shield h. ESE i. CT beam width j. MMD 2. What is the embryo's response to irradiation

above 25 rad during the fi rst 2 weeks after conception?

3. During the fetal period of major organogenesis, what radiation responses are possible?

Nuclear medicine

Radiography

Interventional

NATURAL 3 mSv

CT scanning

Radon

Cosmic

Terrestial

Internal

MEDICAL IMAGING 3.2 mSv

Other Man-made

TOTAL 6.3 mSv

0.4 mSv 1.5 mSv

0.3 mSv

0.3 mSv

2.0 mSv

0.7 mSv

0.6 mSv

0.4 mSv

0.1 mSv

FIGURE 39-11 Current estimated levels of human radiation exposure. (Courtesy Fred Mettler, University of New Mexico.)

PART VII Radiation Protection612

4. What procedure should be followed if a patient is examined and subsequently discovers that she is pregnant?

5. List fi ve procedures that could result in a measur- able fetal dose.

6. How can the three cardinal principles of radiation protection best be applied in diagnostic radiology?

7. What estimate of patient radiation dose usually is measured and reported?

8. How does one use a radiation nomogram? 9. Estimate the entrance skin exposure for a PA chest

image conducted at 110 kVp/2 mAs. 10. What factors are required to estimate the

genetically signifi cant dose? 11. What radiation dose description is most important

for x-ray mammography? 12. How do x-ray beam width and beam penumbra

affect patient dose during CT?

13. How does the term “dose distribution” affect specifi cation of patient dose in x-ray imaging?

14. Describe how patient dose during multislice CT compares with that during step-and-shoot CT.

15. Name three screening x-ray examinations that should not be performed regularly.

16. Estimate the fetal dose after an AP abdominal image is conducted at 76 kVp/40 mAs.

17. What does the symbol Σ mean? 18. Approximately what percentage of the ESE is D g

for mammography? 19. What is the approximate contribution of CT to

total patient radiation dose? 20. What is the approximate fetal dose after a 3.5-min

barium enema fl uoroscopic examination? The answers to the Challenge Questions can be found by logging on to our website at http://evolve.elsevier.com .

613

C H A P T E R

Occupational Radiation Dose Management

OBJECTIVES At the completion of this chapter, the student should be able to do the following:

1. Discuss the units and concepts of occupational radiation exposure 2. Discuss ways to reduce occupational radiation exposure 3. Explain occupational radiation monitors and where they should be

positioned 4. Discuss personnel radiation monitoring reports 5. List the available thicknesses of protective apparel

OUTLINE Occupational Radiation Exposure

Fluoroscopy Interventional Radiology Mammography Computed Tomography Surgery Mobile Radiology

Radiation Dose Limits Whole-Body Dose Limits Dose Limits for Tissues and Organs Public Exposure Educational Considerations

Reduction of Occupational Radiation Exposure Occupational Radiation Monitoring Occupational Radiation Monitoring Report Protective Apparel Position Patient Holding Pregnant Technologist/Radiologist Management Principles

40

PART VII Radiation Protection614

OCCUPATIONAL RADIATION EXPOSURE Although the recommended dose limit for radiologic personnel is 50 mSv/yr (5000 mrem/yr), experience has shown that considerably lower exposures than this are routine. The occupational radiation exposure of radio- logic personnel engaged in general x-ray activity nor- mally should not exceed 1 mSv/yr (100 mrem/yr).

Radiologists usually receive slightly higher exposures than radiologic technologists. This is because the radiol- ogist receives most of his or her exposure during fl uoros- copy and is usually closer to the radiation source — the patient — during such procedures. Table 40-1 reports the results of an analysis of the annual occupational radiation exposure of radiologic personnel. Clearly, the radiation exposures are low.

Fluoroscopy Unquestionably, the highest occupational exposure of diagnostic x-ray personnel occurs during fl uoros- copy and mobile radiography. During radiographic exposure, the radiologist is rarely present and the radiologic technologist is behind the console protec- tive barrier.

When fi xed protective barriers are not available, such as during mobile examination, the mobile x-ray imaging system is equipped with an exposure cord long enough

to allow the technologist to leave the immediate exami- nation area. The radiologic technologist should wear a protective apron for each such mobile examination.

During fl uoroscopy, both radiologist and radiologic technologist are exposed to relatively high levels of radiation. Personnel exposure, however, is related directly to the x-ray beam-on time. With care, personnel exposures can be kept as low as reasonably achievable (ALARA).

Question: A barium enema examination requires 2.5

minutes of fl uoroscopic x-ray beam time. If the radiographer is exposed to 250 mR/hr, what will be his or her occupational radiation exposure?

Answer: Exposure = Exposure rate × Time = 250 mR/hr × 2.5 minutes

= 250 mR/hr × 0.0417 hour = 10.4 mR

Remote fl uoroscopy results in low personnel expo- sures because personnel are not in the x-ray examina- tion room with the patient. With some fl uoroscopes, the x-ray tube is over the table and the image receptor under the table. This geometry offers some advantage in terms of image quality, but personnel exposures are higher because secondary radiation (scatter and leakage) levels are higher.

This condition should be kept in mind during mobile and C-arm fl uoroscopy. It is best to position the x-ray tube under the patient during mobile and C-arm fl uo- roscopy ( Figure 40-1 ).

Interventional Radiology Personnel engaged in interventional radiology proce- dures often receive higher exposures than do those in general radiologic practice because of longer fl uoro- scopic x-ray beam-on time. The frequent absence of a protective curtain on the image-intensifi er tower and the use of cineradiography also contribute to higher personnel exposure.

Extremity exposure during interventional radiology procedures may be signifi cant. Even with protective gloves, exposure of the forearm can approach the rec- ommended dose limit of 500 mSv/yr (50 rem/yr) if care

R ADIATION DOSE is measured in units of rads (Gyt). Radiation exposure is measured in roent- gens (Gya). When the exposure is to radiologic

technologists and radiologists, the proper unit is the rem (Sv).

The rem is the unit of effective dose; it is used for radiation protection purposes. Although exposure, dose, and effective dose have precise and different meanings, they often are used interchangeably in radiology because they have approximately the same numeric value following whole-body exposure.

When properly used, exposure (R, Gya) refers to radiation intensity in air. Dose (rad, Gyt) measures the radiation energy absorbed as a result of radia- tion exposure; it is used to identify irradiation of patients. Effective dose (rem, Sv) identifi es the bio- logic effectiveness of the radiation energy absorbed. This unit is applied to occupationally exposed per- sons and to population exposure, and the SI unit the sievert (Sv) is preferred because all regulations are expressed in sievert.

R Exposure Category ValueAverage whole-body dose 0.7 mSv/yr Those receiving less than the

minimum detectable dose 53%

Those receiving <1 mSv/yr 88% Those receiving >50 mSv/yr 0.05%

TABLE 40-1 Occupational Radiation Exposure of Radiologic Personnel

C H A P T E R 40 Occupational Radiation Dose Management 615

is not taken. Without protective gloves, excessive hand exposures are possible.

Mammography Personnel exposures associated with mammography are low because the low kVp of operation results in less scatter radiation. Usually, a long exposure cord and a conventional wall or window wall are suffi cient to pro- vide adequate protection.

Rarely does a room that is used strictly for mammog- raphy require protective lead shielding. Dedicated mam- mography x-ray units have personnel protective barriers made of lead glass, lead acrylic, and even plate glass as an integral component. Usually, such barriers are totally adequate.

Computed Tomography Personnel exposures in computed tomography (CT) facilities are low. Because the CT x-ray beam is fi nely collimated and only secondary radiation is present in the examination room, radiation levels are low com- pared with those experienced in fl uoroscopy. Figure 40-2 shows the isoexposure profi les for the horizontal and vertical planes of a multislice spiral CT imaging system. These data are given as mR/360 degrees rotation, and they show that personnel can be permitted to remain in the room during imaging. However, protective apparel should always be worn in such situations.

Question: It is necessary for a radiologic technologist to remain in the CT room at midtable position during a 20-rotation examination. What would be the occupational exposure if no protective apron were worn?

Answer: From Figure 40-2 , we may assume an exposure of 0.1 mR/scan.

Occupational exposure = 0.1 mR/scan × 2 = 2 mR Of course, with a protective apron the

trunk of the body would receive essentially zero exposure.

Surgery Nursing personnel and others working in the operating room and in intensive care units are sometimes exposed to radiation from mobile x-ray imaging systems and C-arm fl uoroscopes. Although these personnel are often anxious about such exposures, many studies have shown that their occupational exposure is near zero and certainly is no cause for concern. It usually is not necessary to provide occupational radiation monitors for such personnel.

Mobile Radiology Occupational radiation monitors are not necessary during mobile radiography, except as used with the radiologic technologist and anyone who is required to immobilize or hold patients. Personnel who regularly operate or are in the immediate vicinity of a C-arm fl uoroscope should wear an occupational radiation monitor, in addition to protective apparel. During C-arm fl uoroscopy, the x-ray beam may be on for a relatively long time, and the beam can be pointed in virtually any direction.

Dose rate behind lead apron

Dose rate without lead apron

Lead apron

10 100 mrem/hr

1520 5 0

Dose rate behind lead apron

Dose rate without

lead apron

Lead apron

10 100 mrem/hr

1520 5 0

FIGURE 40-1 Scatter radiation during portable fl uoroscopy is more intense with the x-ray tube over the patient. (Courtesy Stephen Balter, Columbia University Medical Center.)

Extremity monitoring must be provided for interventional radiologists.

PART VII Radiation Protection616

It should never be necessary for radiologic personnel to exceed 50 mSv/yr (5000 mrem/yr). In smaller hospitals, emergency centers, and private clinics, occupational exposures rarely exceed 5 mSv/yr (500 mrem/yr). As Table 40-1 reported, average exposures in most facilities are less than 1 mSv/yr (100 mrem/yr).

RADIATION DOSE LIMITS A continuing effort of health physicists has been the description and identifi cation of occupational dose limits. For many years, a maximum permissible dose (MPD) was specifi ed. The MPD was the dose of radiation that would be expected to produce no signifi cant radiation effects.

At radiation doses below the MPD, no responses should occur. At the level of the MPD, the risk is not zero, but it is small — lower than the risk associated with other occupations and reasonable in light of the benefi ts derived. The concept of MPD is now obsolete and has been replaced by dose limits (DLs).

Whole-Body Dose Limits To establish DLs, the National Council on Radiation Protection (NCRP) assessed risk on the basis of data from reports of the National Academy of Sciences (Biologic Ef- fects of Ionizing Radiation [BEIR] Committee) and the National Safety Council ( Table 40-2 ). State and federal government agencies routinely adopt these recommended dose limits as law. Current DLs are prescribed for vari- ous organs as well as for the whole body, and for various working conditions. If one received the DL each year, the lifetime risk would not exceed 10 − 4 yr − 1 .

The value 10 − 4 yr − 1 represents the approximate risk of death for those working in safe industries. The DLs rec- ommended by the NCRP ensure that radiation workers have the same risk as those in safe industries.

Question: Suppose all 300,000 American radiologic

technologists receive the DL (50 mSv) this year. How many would be expected to die prematurely?

Answer: (300,000)(10 ) 30 But of course, they actually receive a

−4 =

ppproximately 0.5 mSv/yr; therefore, the expected mortalityy is as follows:

30 0.5 50

0.3; less than 1! 

 

=

Particular care is taken to ensure that no radiation worker receives a radiation dose in excess of the DL. The DL is specifi ed only for occupational exposure. It should not be confused with medical x-ray exposure

0.1 0.10.2 0.20.4 0.40.8

0.1 0.1

Values are mR/360° Values are mR/360°

0.2 0.20.4 0.40.8 0.8

0.8

FIGURE 40-2 Isoexposure profi les (in mR/360 degrees) in horizontal and vertical planes for multislice spiral computed tomography.

DLs imply that if received annually, the risk of death would be less than 1 in 10,000.

Industry Rate (10 − 4 yr − 1 )

Trade 0.4 Manufacture 0.4 Service 0.4 Government 0.9 Radiation workers 0.9 All groups 0.9 Transport 2.2 Public utilities 2.2 Construction 3.1 Mining 4.3 Agriculture 4.4

TABLE 40-2 Fatal Accident Rates in Various Industries

C H A P T E R 40 Occupational Radiation Dose Management 617

received as a patient. Although patient dose should be kept low, there is no patient DL.

The fi rst DL — 500 mSv/wk (50,000 mrem/wk) — was recommended in 1902. The current DL is 1 mSv/wk (100 mrem/wk). Through the years, a downward revi- sion of the DL has occurred. The history of these con- tinuing recommendations is given in Table 40-3 and is shown graphically in Figure 40-3 .

In the early years of radiology, the DL consisted of a single value that was considered the safe working level for whole-body exposure. It was based primarily on the known acute response to radiation exposure and pre- sumed that a threshold dose existed.

Today, the DL is specifi ed not only for whole-body exposure but also for partial-body exposure, organ exposure, and exposure of the general population, again excluding medical exposure as a patient and exposure from natural sources ( Table 40-4 ). The DLs included in Table 40-4 were published fi rst by the NCRP in 1987

and were refi ned in 1993. They replaced the previous MPDs, which had been in effect since 1959. These DLs have been adopted by state and federal regulatory agen- cies and are now the law of the United States. Note that International System (SI) units are preferred.

The basic annual DL is 50 mSv/yr (5000 mrem/yr). The DL for the lens of the eye is 150 mSv/yr (15 rem/yr), and that for other organs is 500 mSv/yr (50 rem/yr).

The cumulative whole-body DL is 10 mSv (1000 mrem) times age in years. The DL during pregnancy is 5 mSv (500 mrem), but once pregnancy has been declared, monthly exposure shall not exceed 0.5 mSv (50 mrem).

Current DLs are based on a linear, nonthreshold dose-response relationship; they are considered to represent an acceptable level of occupational radiation exposure.

Year Recommendation Approximate Daily Dose Limit (mrem) Source

1902 Dose limited by fogging of a photographic plate after 7-minute contact exposure

10,000 Rollins

1915 Lead shielding of tube needed (no numeric British Roentgen Society exposure levels given)

1921 General methods to reduce exposure British X-ray and Radium Protection Committee

1925 “It is entirely safe if an operator does not receive every thirty days a dose exceeding 1/100 of an erythema dose.”

200 Mutscheller

1925 10% of SED per year 200 Sievert 1926 One SED per 90,000 working hours 40 Dutch Board of Health 1928 0.000028 of SED per day 175 Barclay and Cox 1928 0.001 of SED per month 5 R per day

permissible for the hands 150 Kaye

1931 Limit exposure to 0.2 R per day 200 Advisory Committee on X-ray and Radium Protection of the United States

1932 0.001 of SED per month 30 Failla 1934 5 R per day permissible for the hands 5000 Advisory Committee on X-ray

and Radium Protection of the United States

1936 0.1 R per day 100 Advisory Committee on X-ray and Radium Protection of the United States

1941 0.02 R per day 20 Taylor 1943 200 mR per day is acceptable 200 Patterson 1959 5 rem per year, 5 (N-18) rem accumulated 20 National Council on Radiation

Protection and Measurements 1987 50 mSv per year, 10 × N mSv cumulative 20 National Council on Radiation

Protection and Measurements 1991 20 mSv per year 8 International Commission on

Radiation Protection

SED, Skin erythema dose.

TABLE 40-3 Historical Review of Dose Limits for Occupational Exposure

PART VII Radiation Protection618

In practice, at least in diagnostic radiology, it is seldom necessary to exceed even 1⁄10 the appropriate DL. How- ever, because the basis for the DL assumes a linear, non- threshold dose-response relationship, all unnecessary radiation exposure should be avoided.

Occupational exposure is described as dose equivalent in units of milliseivert (millirem). DLs are specifi ed as effective dose (E). This scheme has been adopted to afford enhanced precision in radiation protection practices.

The effective dose (E) concept accounts for different types of radiation because of their varying relative bio- logic effectiveness. Effective dose also considers the rela- tive radiosensitivity of various tissues and organs.

These are particularly important considerations when a protective apron is worn. Wearing a protective apron reduces radiation dose to many tissues and organs to near zero. Therefore, effective dose is much less than that recorded by a collar-positioned radiation monitor.

Adoption of this scheme is progressing. For our purposes, effective dose (E) is the quantity of importance. It is ex- pressed in mSv (mrem) and forms the basis for our DLs.

As can be seen in Table 40-5 , the radiation weighting factor (W r ) is equal to 1 for the types of radiation used in medicine. The value of W r for other types of radiation de- pends on the linear energy transfer (LET) of that radiation.

The tissue weighting factor (W t ) accounts for the rel- ative radiosensitivity of various tissues and organs. Tis- sues with a higher value of W t are more radiosensitive. These are shown in Table 40-6 .

Practical implementation of these DLs and weighting factors does not change our previous approach. The DL is suffi ciently high that it rarely, if ever, is exceeded in diagnostic radiology.

With a collar-positioned radiation monitor, a change in procedure is necessary to estimate effective dose (E). Because essentially all of our radiation exposure occurs during fl uoroscopy and the trunk is shielded by a lead apron, the response of the monitor overestimates the effective dose (E).

A conversion factor of 0.3 should be applied to the collar monitor – reported value to estimate effective dose (E). If a protective apron is not worn (e.g., by a radiog- rapher who does no fl uoroscopy), then the monitor re- sponse may be considered the effective dose.

Dose Limits for Tissues and Organs The whole-body DL of 50 mSv/yr (5000 mrem/yr) is an effective dose, which takes into account the weighted average to various tissues and organs. In addition, the

EFFECTIVE DOSE

Effective dose (E) = Radiation weighting factor (W r ) × Tissue weighting factor (W t ) × Absorbed dose

2500

50

25

5 2

2500

150

100

50

Year

1900 1920 1940 1960 1980 1990

Maximum permissable

dose (rem/yr–1)

2000

FIGURE 40-3 Dose limits over the past century.

C H A P T E R 40 Occupational Radiation Dose Management 619

NCRP has identifi ed several specifi c tissues and organs with specifi c recommended dose limits.

Skin. Some organs of the body have a higher DL than the whole-body DL. The DL for the skin is 500 mSv/yr (50 rem/yr).

This limit is not normally of concern in diagnostic radiology because it applies to nonpenetrating radiation such as alpha and beta radiation and very soft x-rays. Radiologic technologists exclusively engaged in mam- mography or nuclear medicine are highly unlikely to sustain radiation exposure to the skin in excess of 10 mSv/yr (1000 mrem/yr).

Extremities. Radiologists often have their hands near the primary fl uoroscopic radiation beam; therefore, extremity exposure may be of concern. The DL for the

extremities is the same as that for the skin — 500 mSv/yr (50 rem/yr).

These radiation levels are quite high and under nor- mal circumstances should not even be approached. For certain occupational groups, such as interventional radiologists and nuclear medicine technologists, extremity personnel monitors should be provided. Such devices are worn on the wrist or the fi nger.

Lens. Because radiation is known to produce cataracts, a DL is specifi ed for the lens of the eye. This DL is 150 mSv/yr (15 rem/yr) and it should never be approached, much less exceeded, in x-ray imaging. The response of a collar-positioned can be used as the lens dose.

Public Exposure Individuals in the general population are limited to 1 mSv/yr (100 mrem/yr). For hospital workers who are not radiology employees but who may regularly visit x-ray rooms, the DL is 1 mSv/yr (100 mrem/yr).

This value of 1 mSv/yr is the DL that medical physicists use when computing the thickness of protective barriers. If a barrier separates an x-ray examining room from an area occupied by the general public, the shielding is designed so that the annual exposure of an individual in the adja- cent area cannot exceed 1 mSv/yr (100 mrem/yr).

If the adjacent area is occupied by radiation workers, the shielding must be suffi cient to maintain an annual exposure level less than 10 mSv/yr (1000 mrem/yr). This approach to shielding derives from the 10 mSv × N cumulative DL.

Radiation exposure of the general public or of indi- viduals in this population is measured rarely because this process is not necessary. Most radiology personnel do not receive even this level of exposure.

The DL established for nonoccupationally exposed persons is 1⁄10 of that established for the radiation worker.

TABLE 40-4 Dose Limits Recommended by the National Council on Radiation Protection and Measurements

A. Occupational exposures 1. Effective dose

a. Annual: 50 mSv (5000 mrem) b. Cumulative: 10 mSv × age (1000 mrem × age)

2. Equivalent annual dose for tissues and organs a. Lens of the eye: 150 mSv (15 rem) b. Thyroid, skin, hands, and feet: 500 mSv (50 rem)

B. Public exposures (annual) 1. Effective dose, frequent exposure: 1 mSv

(100 mrem) 2. Equivalent dose for tissues and organs

a. Lens of eye: 15 mSv (1500 mrem) b. Skin, hands, and feet: 50 mSv (5000 mrem)

C. Education and training exposures (annual) 1. Effective dose: 1 mSv (100 mrem) 2. Equivalent dose for tissues and organs

a. Lens of eye: 15 mSv (1500 mrem) b. Skin, hands, and feet: 50 mSv (5000 mrem)

D. Embryo – fetus exposures 1. Total equivalent dose: 5 mSv (500 mrem) 2. Equivalent dose in 1 month: 0.5 mSv (50 mrem)

E. Negligible individual dose (annual): 0.01 mSv (10 mrem)

TABLE 40-5 Weighting Factors for Various Types of Radiation

Type of Energy Range Radiation Weighting Factor (W r )

X- and gamma rays, electrons

1

Neutrons, energy < 10 keV

5

10 keV to 100 keV 10 >100 keV to 2 MeV 20 > 2 MeV to 20 MeV 10 >20 MeV 5 Protons 2 Alpha particles 20

TABLE 40-6 Weighting Factors for Various Tissues

Tissue Tissue Weighting Factor (W t )

Gonad 0.20 Active bone marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Esophagus 0.05 Liver 0.05 Thyroid 0.05 Bone surface 0.01 Skin 0.01

PART VII Radiation Protection620

Educational Considerations Several special situations are associated with whole- body occupational DL. Students younger than 18 years of age may not receive more than 1 mSv/yr (100 mrem/yr) during the course of their educational activities. This is included in and is not added to the 1 mSv (100 mrem) permitted each year as a nonoccupational exposure.

Consequently, student radiologic technologists younger than 18 years of age may be engaged in x-ray imaging, but their exposure must be monitored and must remain below 1 mSv/yr (100 mrem/yr). Because of this, it is general practice not to accept underage persons into schools of radiologic technology unless their 18th birthday is within sight.

In keeping with ALARA, even more changes in DL are on the way. The International Commission on Ra- diological Protection (ICRP) has issued several recom- mendations, including an annual whole-body DL of 20 mSv (2000 mrem). Such a reduction is currently under consideration in the United States.

REDUCTION OF OCCUPATIONAL RADIATION EXPOSURE The radiologic technologist can do much to minimize occupational radiation exposure. Most exposure control procedures do not require sophisticated equipment or especially rigorous training, but simply a conscientious attitude regarding the performance of assigned duties. Most equipment characteristics, technique changes, and administrative procedures designed to minimize patient dose also reduce occupational exposure.

In diagnostic radiology, at least 95% of the radio- logic technologist's occupational radiation exposure comes from fl uoroscopy and mobile radiography. Attention to the cardinal principles of radiation pro- tection (time, distance, and shielding) and ALARA are the most important aspects of occupational radia- tion control.

During fl uoroscopy, the radiologist should minimize x-ray beam-on time. This can be done through care- ful technique, which includes intermittent activation of fl uoroscopic views rather than one long period of x-ray beam-on time. It is a common radiation protec- tion practice to maintain a log of fl uoroscopy time by recording x-ray beam-on time with the 5-minute reset timer.

During fl uoroscopy, the radiologic technologist should step back from the table when his or her immedi- ate presence and assistance are not required. The radio- logic technologist also should take maximum advantage of all protective shielding, including apron, curtain, and Bucky slot cover, as well as the radiologist.

The radiologic technologist should wear a protective apron during all mobile examinations and should main- tain maximum distance from the source. The primary beam should never be pointed at the radiologic tech- nologist or other nearby personnel.

During radiography, the radiologic technologist is positioned behind a control booth barrier. Such barri- ers usually are considered secondary barriers because they intercept only leakage and scatter radiation. Con- sequently, leaded glass and leaded gypsum board are often unnecessary for such barriers.

Other work assignments in diagnostic imaging, such as scheduling, darkroom duties, and fi ling, result in essen- tially no occupational radiation exposure.

Occupational Radiation Monitoring The level of occupational exposure to radiologists and radiologic technologists depends on the type and fre- quency of activity in which they are engaged. Deter- mining the quantity of radiation they receive requires a program of occupational radiation monitoring. Occupational radiation monitoring refers to procedures instituted to estimate the amount of radiation received by individuals who work in a radiation environment.

Most clinical diagnostic imaging personnel must be monitored; however, it usually is not necessary to monitor diagnostic radiology secretaries and file clerks. Furthermore, it usually is not necessary to monitor operating room personnel, except perhaps those rou- tinely involved in cystoscopy and C-arm fluoroscopy.

The occupational radiation monitor simply measures the quantity of radiation to which the monitor was exposed; therefore, it is simply an indicator of exposure to the wearer. Basically, three types of personnel monitors

The exposure cord on a portable x-ray unit must be at least 2 m long.

The useful beam should never be directed toward the operating console.

Occupational radiation monitoring is required when there is any likelihood that an individual will receive more than 1⁄10 of the recommended dose limit.

The occupational radiation monitor offers no protection against radiation exposure!

Each mobile x-ray unit should have a protective apron assigned to it.

C H A P T E R 40 Occupational Radiation Dose Management 621

are used in diagnostic radiology: film badges, thermolu- minescence dosimeters (TLDs), and optically stimulated luminescence dosimeters (OSLs).

Regardless of the type of monitor used, it is es- sential that it be obtained from a certifi ed laboratory. In-house processing of radiation monitors should not be attempted.

Film Badges. Film badges came into general use dur- ing the 1940s and have been used widely in diagnostic radiology ever since. Film badges are specially designed devices in which a fi lm similar to dental radiographic fi lm is sandwiched between metal fi lters inside a plastic holder. Figure 40-4 presents a view of two typical oc- cupational radiation monitors.

The fi lm incorporated into a fi lm badge is special radiation dosimetry fi lm that is particularly sensitive to x-rays. The optical density on the exposed and pro- cessed fi lm is related to the exposure received by the fi lm badge.

Carefully controlled calibration, processing, and an- alyzing conditions are necessary for the fi lm badge to measure accurately occupational radiation exposure. Usually, exposures less than 10 mR (100 µGy a ) are not measured by fi lm badge monitors, and the fi lm badge vendor will report only that a minimum exposure (M) was received. When higher exposures are received, they can be reported accurately.

The metal fi lters, along with the window in the plas- tic fi lm holder, allow estimation of the x-ray energy. The usual fi lters are made of aluminum and copper.

When the radiation exposure is a result of pen- etrating x-rays, the image of the fi lters on the pro- cessed fi lm is faint, and there may be no image at all of the window in the plastic holder. If the badge is exposed to soft x-rays, the fi lters are well imaged and the optical densities under the fi lters allow estimation of x-ray energy.

Often, the fi lters to the front of the fi lm badge differ in shape from the fi lters to the back of the fi lm badge. Radi- ation that had entered through the back of the fi lm badge normally would indicate that the person wearing the badge received considerably higher exposure than indi- cated, because the x-rays would have penetrated through the body before interacting with the fi lm badge.

Several advantages of fi lm badge occupational radia- tion monitors continue to make them popular. They are inexpensive, easy to handle, easy to process, and rea- sonably accurate, and they have been in use for several decades.

Film badge monitors also have disadvantages. They cannot be reused, and because they incorporate fi lm as

the sensing device, they cannot be worn for longer than 1 month because of possible fogging caused by tempera- ture and humidity.

Film badge monitors should never be left in an enclosed car or other area where excessive temperatures may occur. The fogging produced by elevated tempera- ture and humidity results in a falsely high evaluation of radiation exposure.

Thermoluminescence Dosimeters. The sensitive material of the TLD monitor ( Figure 40-5 ) is lithium fl uoride (LiF) in crystalline form, either as a powder or more often as a small chip approximately 3 mm square and 1 mm thick. When exposed to x-rays, the TLD absorbs energy and stores it in the form of excited electrons in the crystalline lattice.

When heated, these excited electrons fall back to their normal state with the emission of visible light. The intensity of visible light is measured with a photomulti- plier tube or photodiode and is proportional to the ra- diation dose received by the crystal. This sequence was described in Chapter 38.

The TLD occupational radiation monitor offers several advantages over fi lm. It is more sensitive and more accu- rate than a fi lm badge monitor. Properly calibrated TLD monitors can measure exposure as low as 5 mR (50 µGy a ).

FIGURE 40-4 Some representative radiation monitors. In many, metal fi lters are incorporated to help identify the type of radiation and its energy. (Courtesy Landauer, Inc.)

Film badges must be worn with the appropriate side to the front.

PART VII Radiation Protection622

The TLD monitor does not suffer from loss of informa- tion after it is exposed to excessive heat or humidity.

The primary disadvantage of TLD personnel monitoring is cost. The price of a typical TLD monitoring service is perhaps twice that of fi lm badge monitoring. If the frequency of monitoring is quarterly, however, the cost is about the same.

Optically Stimulated Luminescence. OSL dosim- eters ( Figure 40-6 ) are worn and handled just as fi lm badges and TLDs are, and they are approximately the same size. OSL dosimeters have one advantage over TLDs. They are more sensitive, measuring as low as 1 mR (10 µGy a ).

Where to Wear the Occupational Radiation Monitor. Much discussion and research in health phys- ics have gone into providing precise recommendations about where a radiologic technologist should wear the occupational radiation monitor. Offi cial publications of

the NCRP offer suggestions that have been adopted as regulations in most states.

Many radiologic technologists wear their personnel monitors in front at waist or chest level because it is convenient to clip the badge over a belt or a shirt pocket. If the technologist is not involved in fl uoroscopic proce- dures, these locations are acceptable.

The recommended dose limit of 50 mSv/yr (5000 mrem/yr) refers to the effective dose (E). It has been shown that during fl uoroscopy, when a protective apron is worn, exposure to the collar region is approximately 20 times greater than that to the trunk of the body beneath the protective apron. So, if the occupational radiation monitor is worn beneath the protective apron, it will record a falsely low exposure and will not indicate what could be excessive exposure to unprotected body parts.

FIGURE 40-5 Thermoluminescence dosimeters are available as chips, discs, rods, and powder. These are used for area and environmental radiation monitoring, and especially for occupational radiation monitoring. (Courtesy Bicron.)

TLDs can be worn for intervals up to 1 year.

If the radiologic technologist participates in fl uoros- copy, the occupational radiation monitor should be positioned on the collar above the protective apron.

C H A P T E R 40 Occupational Radiation Dose Management 623

In some clinical situations, for example, during preg- nancy and with extremity monitoring, it may be advisable to wear more than one radiation monitor. The abdomen should be monitored during pregnancy. The extremities should be monitored during interventional procedures when the radiologist's hands are in close proximity to the useful beam. Nuclear medicine technologists should wear extremity monitors when handling millicurie quantities of radioactive material.

Occupational Radiation Monitoring Report State and federal regulations require that results of the occupational radiation monitoring program be recorded in a precise fashion and maintained for review. Annual, quarterly, monthly, or weekly monitoring periods are acceptable.

The occupational radiation monitoring report must con- tain a number of specifi c items of information ( Figure 40-7 ). These various items are identifi ed in the headers of the columns.

Exposure data that must be included on the form include current exposure and cumulative annual exposure. Separate radiation monitors, such as extremity moni- tors or fetal monitors, are identifi ed separately from the whole-body monitor.

FIGURE 40-6 Optically stimulated luminescence dosimeters. (Courtesy Landauer, Inc.)

FIGURE 40-7 Occupational radiation monitoring report must include the items of information shown here. (Courtesy Landauer, Inc.)

PART VII Radiation Protection624

Occasionally, if occupational exposure involves low energy radiation, the dose to the skin might be greater than the dose of penetrating radiation. In such cases, the skin dose is separately identifi ed. Areas on the report are provided for neutron radiation exposure to accommo- date nuclear reactor and particle accelerator workers.

When a radiologic technologist changes employment, the total radiation exposure history must be transferred to the records of the new employer. Consequently, when one leaves a job, one should automatically receive a report of the total radiation exposure history at that facility. Such a report should be given automatically; if it is not, it must be requested.

When an occupational radiation monitoring pro- gram is established, the supplier of the monitor should be informed of the type of radiation facility involved. This information infl uences the method of calibration of monitors and control monitors.

The control monitor should never be stored in or adjacent to a radiation area. It should be kept in a dis- tant room or offi ce. After processing, the response of the control monitor is subtracted from each individual mon- itor. In this way, the report for each individual monitor represents only occupational radiation exposure.

All monitors should be returned to the supplier together and in a timely fashion, so they can be processed to- gether. Lost or inadvertently exposed monitors must be evaluated, and an estimate of true exposure should be made by the medical physicist.

Protective Apparel The operating console usually is positioned behind fi xed protective barriers during diagnostic radiographic pro- cedures. During fl uoroscopy or mobile radiography, radiologic personnel are in the examination room and near the x-ray source.

Protective gloves and aprons are available in many sizes and shapes. These usually are constructed of lead-impregnated vinyl. Some protective garments are impregnated with tin or other metals because other metals have some advantages over lead as a shielding material in the diagnostic x-ray energy range.

Normal thicknesses for protective apparel are 0.25, 0.5, and 1 mm of lead equivalent. The garments them- selves are much thicker than these dimensions, but they provide shielding equivalent to these thicknesses of lead ( Table 40-7 ). Protection of at least 0.25 mm Pb is re- quired; 0.5 mm Pb is normal.

Maximum exposure reduction is obtained with the 1 mm lead equivalent garment, but an apron of this material can weigh as much as 10 kg (22 lb). The wearer could be exhausted by the end of the fl uoros- copy schedule just from having to carry the protective apron. X-ray attenuation at 75 kVp for 0.25 mm lead equivalent and 1 mm lead equivalent is 66% and 99%, respectively.

Protective aprons for interventional radiology should be of the wrap-around type. During these procedures, a lot of personnel movement can occur, and some personnel, such as anesthesiologists, may even have their backs to the radiation source.

When not in use, protective apparel must be stored on properly designed racks. If they are continually folded or heaped in the corner, cracks can develop. At least once a year, aprons and gloves should be fl uoroscoped to ensure that no such cracks appear. If fl uoroscopy is not available, high-kVp radiography (e.g., 120 kVp/10 mAs) may be used.

Position During fl uoroscopy, all personnel should remain as far from the patient as possible, keeping the front of the apron facing the radiation source at all times. After loading spot fi lms, the radiologic technologist should take a step or two backward from the table when his or her presence is not required. The radiologist should use

The control monitor measures background expo- sure during transportation, handling, and storage.

It is known that 0.5 mm lead equivalent protective aprons represent a workable compromise between unnecessary weight and desired protection.

Protective apparel must be worn during fl uoros- copy and mobile radiology.

PERCENTAGE X-RAY ATTENUATION

Equivalent Thickness (mm Pb) Weight (lb) 50 kVp 75 kVp 100 kVp

0.25 3 to 10 97 66 51 0.50 6 to 15 99.9 88 75 1.00 12 to 25 99.9 99 94

TABLE 40-7 Some Physical Characteristics of Protective Lead Aprons

C H A P T E R 40 Occupational Radiation Dose Management 625

the dead man foot switch sparingly. Naturally, when x-ray beam-on time is high, the radiation exposure to patient and personnel will be proportionately high.

Patient Holding Many patients referred for x-ray examination, includ- ing infants, the elderly, and the incapacitated, are not physically able to support themselves. Mechanical immobilization devices should be available for such patients. Otherwise, a relative or a friend who accompa- nies the patient should be asked to help. As a last resort, other hospital employees such as nurses and orderlies may be used occasionally to hold patients.

When it is necessary to have another person hold the patient, protective apparel must be provided to that per- son. An apron and gloves are necessary, and the holder should be positioned and instructed carefully, so that he or she is not exposed to the useful beam. Because the holder is often the mother of a child patient, be sure to ask whether she could be pregnant.

Pregnant Technologist/Radiologist When a radiologic technologist becomes pregnant, she should notify her supervisor. The pregnancy then is declared, and the DL becomes 0.5 mSv/mo (50 mrem/mo). The supervisor then should review her previous radiation exposure history because this facilitates decisions re- garding what protective actions are necessary.

The DL for the fetus is 5 mSv (500 mrem) for the period of pregnancy — a dose level that most radiologic technol- ogists will not reach regardless of pregnancy. Although some may receive doses that exceed 5 mSv/yr (500 mrem/ yr), most receive less than 1 mSv/yr (100 mrem/yr).

This usually is indicated with the personnel monitor- ing device positioned at the collar above the protective apron. Exposure at the waist under the protective apron normally does not exceed 10% of these values; there- fore, under normal conditions, specifi c protective action is not necessary.

Most lead protective aprons are 0.5 mm lead equiv- alent. These provide approximately 90% attenuation at 75 kVp, which is suffi cient. One millimeter lead equivalent protective aprons are available, but such thickness is not necessary, particularly in view of the additional weight of the apron. Back problems during pregnancy constitute a greater hazard than radiation exposure.

The length of the apron need not extend below the knees, but wrap-around aprons are preferred during pregnancy. If necessary, a special effort should be made to provide an apron of proper size because of its weight.

An additional radiation monitor should be positioned under the protective apron at waist level. The exposure reported on this second monitor should be maintained on a separate record and identifi ed as exposure to the fetus.

Do not allow the monitors to be switched and the record confused. Try color-coding — red for the collar badge (red neck!) and yellow for the waist badge (yel- low belly!). Additional or thicker lead aprons normally are not necessary ( Figure 40-8 ).

Experience with the use of an additional monitor shows consistently that exposures to the fetus are zero. Suppose, for instance, that a pregnant radiologic tech- nologist wearing a single radiation monitor at collar level receives 1 mSv (100 mrem) during the 9-month period. The dose at waist level under a protective apron would be less than 10% of the collar dose, or 0.1 mSv (10 mrem). This is the dose to the monitor; the dose to the fetus is near zero.

Attenuation by maternal tissues overlying the fetus reduces the dose to the fetus to approximately 30% of the abdominal skin dose, or 30 µSv (3 mrem). Conse- quently, when normal protective measures are taken, it is nearly impossible for a radiologic technologist even to approach the fetal DL of 5 mSv (500 mrem).

Management Principles It should be clear that the probability of a harmful effect after any occupational radiation exposure in diagnostic imaging is highly unlikely. A biologic response is expected very rarely and has not been observed in radiologic personnel for the past 50 years or so.

Nevertheless, it is essential for the director of radiol- ogy to incorporate three steps into the radiation protec- tion program: new employee training, periodic in-service training, and counseling during pregnancy.

New Employee Training. The initial step in any administrative protocol involving pregnant employees involves orientation and training. During these ori- entation discussions, all female employees should be instructed as to their responsibility regarding pregnancy and radiation.

Each radiologic technologist should be provided with a copy of the facility radiation protection manual and other appropriate materials. This material might include a one-page summary of doses, responses, and proper radiation control working habits ( Table 40-8 ).

The new employee then should read and sign a form ( Figure 40-9 ) to indicate that she has been instructed in this area of radiation protection. An important point to

The pregnant radiologic technologist should be provided with a second personnel monitoring device.

Radiology staff should never hold patients.

PART VII Radiation Protection626

New Employee Notification

This is to certify that __________________________________ , a new employee of this radiologic facility, has received instructions regarding mutual responsibilities should she become pregnant during this employment.

In addition to personal counseling by __________________________ , she has been given to read several documents dealing with pregnancy in diagnostic radiology. Furthermore, the additional reading material that follows is available in the departmental office:

1. Review of NCRP radiation dose limit for embryo and fetus in occupationally-exposed women, NCRP Report No 53, Washington, DC, 1977, National Council on Radiation Protection and Measures. 2. Medical radiation exposure of pregnant and potentially pregnant women, NCRP Report No 54, Washington, DC, 1977, National Council on Radiation Protection and Measures. 3. Wagner, LK et al: Exposure of the pregnant patient to diagnostic radiation, Philadelphia, 1985, JB Lippincott. 4. The effects on populations of exposure to low levels of ionizing radiation, Washington, DC, 1990, National Academy of Sciences.

I understand that should I become pregnant and I decide to declare my pregnancy, it is my responsibility to inform my supervisor of my condition so that additional protective measures can be taken.

________________________________________ _____________________________________ Supervisor Employee

______________ Date

FIGURE 40-9 Form for new employee notifi cation.

Baby badge (under apron)

FIGURE 40-8 When the fl uoroscopist is pregnant, a second “baby monitor” should be positioned under the protective apron.

C H A P T E R 40 Occupational Radiation Dose Management 627

be made by signing this document is that the employee will notify her supervisor voluntarily when she is preg- nant or suspects she is pregnant.

In-Service Training. Every well-run radiology service maintains a regular schedule of in-service training. Usu- ally, this training is conducted at monthly intervals, but sometimes it occurs more often. At least twice each year, such training should be devoted to radiation protection, and a portion of these sessions should be directed at the potentially pregnant employee.

The material to be covered in such sessions is outlined in Table 40-8 . Although it is good to review doses and responses, it is probably more appropriate to emphasize radiation control procedures. These, of course, affect the radiation safety of all radiologic technologists — not only pregnant technologists.

A review of personnel monitoring records is particu- larly important. A helpful procedure is to post the most recent radiation monitoring report for all to see. The year-end report should be initialed by each radiologic technologist, and the director of radiology should en- sure that technologists understand the nature and mag- nitude of their annual exposure.

Through such training, radiologic personnel will real- ize that their occupational exposure is minimal — usually at less than 10% of the DL.

Counseling During Pregnancy. The director of radiology takes the next action when the radio- logic technologist declares her pregnancy. First, the director should counsel the employee after review- ing her radiation exposure history and considering any future modifi cations to her schedule that may be appropriate.

In all likelihood, a review of the employee's previous radiation exposure history will show a low exposure profi le. Those who wear the radiation monitor positioned at the collar, as recommended, and who are heavily in- volved in fl uoroscopy, may receive an exposure greater than 5 mSv/yr (500 mrem/yr). Such employees, how- ever, are protected by lead aprons, so that exposure to the trunk of the body normally would not exceed 500 µSv (50 mrem/yr).

During this review of occupational radiation expo- sure, it is appropriate to emphasize that the DL during pregnancy is 5 mSv (500 mrem) and 0.5 mSv/mo (50 mrem/mo). Furthermore, it should be shown that this DL refers to the fetus and not to the radiologic technolo- gist. The level of 5 mSv (500 mrem) to the fetus dur- ing gestation is considered an absolutely safe radiation exposure level.

In view of this discussion, the director of radiol- ogy should point out to the radiologic technologist that an alteration in her work schedule normally is not required.

HUMAN RESPONSE TO LOW-LEVEL EXPOSURE Life span shortening 10 days/rad Cataracts None below 200 rad Leukemia 10 cases/10 6 /rad/yr Cancer 2 cases/10 4 /rad Genetic effects Doubling dose = 50 rad Death from all causes 2 deaths/10 4 /rad

EFFECTS OF IRRADIATION IN UTERO 0 to 14 days Spontaneous abortion: 25% natural incident; 0.1% increase/10 rad 2 to 10 weeks Congenital abnormalities: 5% natural incidence; 1% increase/10 rad 2nd to 3rd trimester Cell depletion: no effect at <50 rad

Latent malignancy: 4:10,000 natural incidence; 6:10,000/rad 0 to 9 months Genetic effects: 10% natural incidence; 5 × 10 − 7 mutations/rad

PROTECTIVE MEASURES FOR THE PREGNANT RADIOLOGIC TECHNOLOGIST Two occupational radiation monitors Dose limit: 5 mSv/9 mo, 0.5 mSv/mo

TABLE 40-8 Pregnancy in Diagnostic Radiology

Under no circumstance should termination or an involuntary leave of absence occur as a con- sequence of pregnancy.

EMPHASIZE

The effective DL is 50 mSv/yr (5000 mrem/yr). Environmental background radiation is approximately 1 mSv/yr (100 mrem/yr). Occupational exposures are closer to the latter than the former.

PART VII Radiation Protection628

For radiologic technologists involved in radiation oncology, nuclear medicine, or ultrasonography, simi- lar consultation and level of modifi cation as previously discussed are appropriate. In radiation oncology, the pregnant technologist may continue her normal work- load but should be advised not to participate in brachy- therapy applications.

In nuclear medicine, the pregnant technologist should handle only small quantities of radioactive material. She should not elute radioisotope generators or inject mil- licurie quantities of radioactive material.

Ultrasound technologists normally are not classifi ed as radiation workers. A sizable number of ultrasound patients, however, are nuclear medicine patients and therefore become a potential source of exposure to the ultrasonographer. This situation presents a re- mote risk because the quantity of radioactivity used is so low. It may be advisable for the ultrasonogra- pher to be provided with a radiation monitor during pregnancy.

Finally, the pregnant technologist should be required to read and sign a form ( Figure 40-10 ) that attests to the fact that she has been given proper attention to the subject, and that she understands that the level of risk

associated with her employment is much less than that experienced by nearly all occupational groups.

SUMMARY DLs are prescribed by the NCRP for various organs, the whole body, and various working conditions, so that the lifetime risk of each year's occupational exposure does not exceed 10 − 4 per year.

The NCRP recommends a cumulative whole-body DL of 10 mSv times age in years. The DL during pregnancy is 5 mSv. In diagnostic imaging, however, it is seldom necessary to exceed 1/10 the appropriate DL.

Occupational radiation exposure is measured in mil- liseiverts (millirems), and the description of such expo- sure is effective dose (E). Effective dose accounts for type of radiation and the relative radiosensitivity of tissues and organs.

Although the dose limit for occupational workers is 50 mSv/yr, most radiologic personnel receive less than 0.5 mSv/yr. Radiologists may receive a higher dose if engaged in a heavy fl uoroscopy schedule.

Because 95% of occupational exposure comes from fl uoroscopy and mobile radiography, the radiologic

Acknowledgment of Radiation Risk During Pregnancy

I, __________________________________ , do acknowledge that I have received counseling from _________________________________ regarding my employment responsibilities during my pregnancy.

It is clear to me that there is a vanishingly small probability that my employment will in any way adversely affect my pregnancy. The reading material listed below has been made available to me to demonstrate that the additional risk during my pregnancy is much less than that for most occupational groups. I further understand that, although I may be assigned to low-exposure duties and provided with a second radiation monitor, these are simply added precautions and do not in any way convey that any assignment in this department is especially hazardous during pregnancy.

1. Review of NCRP radiation dose limit for embryo and fetus in occupational-exposed women, NCRP Report No 53, Washington, DC, 1977, National Council on Radiation Protection and Measures. 2. Medical radiation exposure of pregnant and potentially pregnant women, NCRP Report No 54, Washington, DC, 1977, National Council on Radiation Protection and Measures. 3. Wagner, LK et al: Exposure of the pregnant patient to diagnostic radiation, Philadelphia, 1985, JB Lippincott. 4. The effects of populations of exposure to low levels of ionizing radiation, Washington, DC, 1990, National Academy of Sciences.

________________________________________ _____________________________________ Supervisor Employee

______________ Date

FIGURE 40-10 Form for acknowledgement of radiation risk during pregnancy.

C H A P T E R 40 Occupational Radiation Dose Management 629

technologist should follow these guidelines for reducing occupational exposure: • During mobile radiography, wear an apron, maintain

maximum distance from the source, and never direct the primary beam toward oneself or others.

• During fl uoroscopy, step back from the table if not needed, and use shielding, including an apron, a curtain, a Bucky slot cover, and the radiologist.

• During radiography, stand behind the control booth and never direct the primary beam toward the control booth barrier. Personnel monitoring is required when there is any

likelihood that an individual will receive more than 1/10 the dose limit. The various available personnel radiation monitors include (1) fi lm badges, (2) TLDs, and (3) OSL dosimeters. The OSL is very sensitive and accurate and may be worn for up to 1 year. For general use, the ra- diographer should wear the personnel monitor at waist or chest level; however, during fl uoroscopy, the monitor is worn on the collar outside the protective apron.

Radiographers and occupational workers should never be used to hold patients during an exposure.

The radiobiology of pregnancy requires particular atten- tion to the pregnant radiologic technologist and the preg- nant patient. The pregnant radiologic technologist should be provided with a second radiation monitoring device to be worn under the protective apron at waist level.

CHALLENGE QUESTIONS 1. Defi ne or otherwise identify the following: a. NCRP b. ALARA c. Tissue weighting factor (W r ) d. Extremity monitor e. Personnel monitor f. Units of x-radiation output intensity g. Extremity DL h. Effective dose i. Threshold dose j. OSL 2. What is the dose limit for diagnostic imaging per-

sonnel? 3. During what two examinations can occupational

radiation exposure be high?

4. What does the value 10 − 4 yr − 1 mean with regard to the NCRP recommended dose limits?

5. How do some radiation occupational groups such as nuclear medicine technologists monitor their extremity doses?

6. What is the whole-body occupational DL for radiography students younger than 18 years of age?

7. State the management protocol for the pregnant radiologic technologist.

8. What information regarding radiation protection should be covered in regularly scheduled in-service training classes?

9. What exposure will a radiologic technologist receive while wearing a protective apron equiva- lent to 2 HVLs and exposed for 10 minutes at 4 m from a source with intensity of 100 mR/hr at 1 m?

10. The collar-positioned monitor of a fl uoroscopist records 0.9 mSv (90 mrem) during a month. This represents approximately what effective dose (E)?

11. What is the required length of the exposure cord on the mobile radiographic unit?

12. When must occupational radiation monitoring be provided?

13. Describe the design of occupational radiation monitors. How are they to be worn, and where on the body are they placed?

14. List the exposure data that must be included in the personnel monitoring report.

15. What is an appropriate thickness for protective apparel?

16. What procedure is used for holding patients during an x-ray examination?

17. Describe the features of optically stimulated dosim- etry that make it particularly effective for occupa- tional radiation monitoring.

18. What is the DL for the lens of the eye and the requirement for protective eyewear?

19. What is the approximate protective value of an occupational radiation monitor?

20. Describe an appropriate radiation protection program for nursing and surgical personnel.

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