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Therac-25 was a radiation therapy machine that was involved with at least six known accidents between 1985 and 1987, in which patients were given massive overdoses. It highlighted the dangers of software-related operation of safety-critical systems (see computer bug).
The machine had at least two operating modes, and the problem involved using the wrong one by mistake, yet recording that the right one had been used. This machine was controlled by keyboard input through a computerized control system, whereas previous machines had specific switches, knobs, and dials which required specialized construction and training, and whose settings had to be recorded separately. The problem was that the computer could not reset the instrument fast enough when the operator had typed in the command for the high power setting, then realized that this was a mistake, and erased the command to type in one for the low power setting. A capacitor had already been charged up enough for the high power setting, and the system did not provide for time for it to discharge to a lower level. Also, the computer memory stated that the low power setting had been what was applied.
These incidents have become a standard case study in the history of computing and medicine.
The machine was built by the Atomic Energy of Canada Limited (AECL).
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