• Systems engineering. In this complicated system, there was an almost exclusive reliance on the software to work correctly and ensure the safe operation of the machine. The lack of hardware safety systems was cited as one of the main problems with the Therac-25.
• Software engineering. Many software engineering errors were made during the development of the Therac-25, including inadequate documentation and testing of the software modules and the software.
Radiation Problems Continue Although the problems with the Therac-25 occurred in the 1980s and were well known in the industry, medical radiation equipment used for cancer therapy continues to have problems, some leading to the deaths of patients. The root cause of these problems is the increasing complexity of the machines and the technologies used for radiation therapy. This complexity is manifested in software glitches and hardware failures and can contribute to human errors that can have devastating results. A 2010 article in the New York Times [ Brogdanich, 2010 ] described in detail two cases of severe patient injury caused by radiation therapy machines using linear accelerator technology. In both of these cases, the computer control system malfunctioned, leading to huge overdoses to the patients. In one case, a man suffering from oral cancer was treated using a linear accelerator system. In this machine, the beam shape and intensity was determined by a sophisticated collimator controlled by computer software. After three treatments, the physician, working with the health physicist responsible for implementing the treatment plan, decided to alter the dosing plan. As the health physicist input the new plan to the computer, the software “froze” and failed to properly store the new program. Because the new program wasn’t stored properly, the computer instead directed the machine to leave the collimator wide open, not only greatly increasing the dose to the patient, but also allowing the dose to be given over a wide part of the patient’s head rather than just to the cancerous area. This accident severely injured the patient, leading to a very slow and painful death from radiation poisoning. Similarly, in the other case reported, a woman undergoing radiation treatment for breast cancer was overdosed. Her treatment was also being done using a linear accelerator system. In this machine, dosing was controlled using a wedge placed in the path of the beam to determine the intensity of the radiation and its location on the patient’s body. In this case, the computer controlling the machine was inadvertently programmed to leave the wedge out of the beam, thus greatly increasing the dose received by the patient. In this case, the patient received a dose 3.5 times larger than planned during each of her 28 radiation sessions. The severe burns resulting from this overdose caused a large hole in the woman’s chest that was painful and took months to heal. Ultimately, she died as a result of this overdose. The Times article reported that New York is among the states with the most stringent requirements for reporting of medical radiation overdose incidents and maintains a database of these events. A review of the New York records indicated that 621 radiation treatment mistakes had been reported between January 2001 and January 2009, including incorrect dosing, irradiation of the wrong location on the patient, and even applying the treatment to the wrong patient. These mistakes were attributed to various causes including hardware malfunctions, computer software malfunctions, and various human errors. When hardware and software malfunctions are the cause, what responsibility do engineers who designed these systems have for the accident? When designing any system with potential implications for human health and safety, engineers must be thorough in design and testing of the system, being especially concerned about anticipating potential failure mechanisms and designing to prevent these possibilities. In addition, fail-safe mechanisms should be incorporated into the design to ensure that failures are detected and do not lead to harm. For radiation therapy equipment, fail-safe means that the machine detects unsafe operating conditions and prevents patient irradiation until the problem is solved. Are engineers who design this sort of equipment also responsible for the human errors that led to patient overdoses? Not all human errors can be anticipated and designed around. However, it is incumbent on a design engineer to design systems so that they are easy to operate and make it simple for operators to use properly. While an engineer cannot always anticipate all of the misuses, or all of the mistakes that might occur on an engineered system, it is essential that engineers try to anticipate these types of problems before they occur and design the system to minimize the possibilities that mistakes can occur.
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casestudy_Therac25.docx
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