AECL did not have the software code independently reviewed and chose to rely on in-house code, including the operating system.These included the following institutional causes: Researchers who investigated the accidents found several contributing causes. In particular, the software was designed so that it was realistically impossible to test it in a rigorous, automated way. Root causes Ī commission attributed the primary cause to general poor software design and development practices rather than single-out specific coding errors. Several days later, radiation burns appeared, and the patients showed the symptoms of radiation poisoning in three cases, the injured patients later died as a result of the overdose. The feeling was described by patient Ray Cox as "an intense electric shock", causing him to scream and run out of the treatment room. The high-current electron beam struck the patients with approximately 100 times the intended dose of radiation, and over a narrower area, delivering a potentially lethal dose of beta radiation. Previous models had hardware interlocks to prevent such faults, but the Therac-25 had removed them, depending instead on software checks for safety. A second fault allowed the electron beam to activate during field-light mode, during which no beam scanner was active or target was in place. One, when the operator incorrectly selected X-ray mode before quickly changing to electron mode, which allowed the electron beam to be set for X-ray mode without the X-ray target being in place. The six documented accidents occurred when the high-current electron beam generated in X-ray mode was delivered directly to patients. The electron beam current required to produce the X-rays is about 100 times greater than that used for electron therapy. The flattening filter resembles an inverted ice-cream cone, and it shapes and attenuates the X-rays. The X-rays are then passed through a flattening filter, and then measured using an X-ray ion chamber. The X-ray photons are produced by colliding a high current, narrow beam of electrons with a tungsten target. Megavolt X-ray (or photon) therapy, which delivered a beam of 25 MeV X-ray photons.
Direct electron-beam therapy, in which a narrow, low-current beam of high-energy (5 to 25 MeV (0.80 to 4.01 pJ)) electrons was scanned over the treatment area by magnets.A "field light" mode, which allowed the patient and collimator to be correctly positioned by illuminating the treatment area with visible light.The machine had three modes of operation, with a turntable moving some apparatus into position for each of those modes: either a light, some scan magnets, or a tungsten target and flattener. Additionally, the overconfidence of the engineers : 428 and lack of proper due diligence to resolve reported software bugs are highlighted as an extreme case where the engineers' overconfidence in their initial work and failure to believe the end users' claims caused drastic repercussions. These accidents highlighted the dangers of software control of safety-critical systems, and they have become a standard case study in health informatics, software engineering, and computer ethics. : 425 Because of concurrent programming errors (also known as race conditions), it sometimes gave its patients radiation doses that were hundreds of times greater than normal, resulting in death or serious injury. It was involved in at least six accidents between 19, in which patients were given massive overdoses of radiation. The Therac-25 was a computer-controlled radiation therapy machine produced by Atomic Energy of Canada Limited (AECL) in 1982 after the Therac-6 and Therac-20 units (the earlier units had been produced in partnership with Compagnie Générale Radiographique (CGR) of France). Radiotherapy machine involved in six accidents