Concept
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 de radiologie (CGR) of France). It was involved in at least six accidents between 1985 and 1987, in which patients were given massive overdoses of radiation. 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. 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. Additionally, the overconfidence of the engineers 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. The French company CGR manufactured the Neptune and Sagittaire linear accelerators. In the early 1970s, CGR and the Canadian public company Atomic Energy Commission Limited (AECL) collaborated on the construction of linear accelerators controlled by a DEC PDP-11 minicomputer: the Therac-6, which produced X-rays of up to 6 MeV, and the Therac-20, which could produce X-rays or electrons of up to 20 MeV. The computer added some ease of use because the accelerator could operate without it. CGR developed the software for the Therac-6 and reused some subroutines for the Therac-20. In 1981, the two companies ended their collaboration agreement. AECL developed a new double pass concept for electron acceleration in a more confined space, changing its energy source from klystron to magnetron. In certain techniques, the electrons produced are used directly, while in others they are made to collide against a tungsten anode to produce X-ray beams.