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The Therac-25 was a 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 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, it sometimes gave its patients radiation doses that were hundreds of times greater than normal, resulting in death or serious injury.
The machine offered two modes of radiation therapy:
- Direct electron-beam therapy, which delivered low doses of high-energy (5 MeV to 25 MeV) electrons over short periods of time;
- Megavolt X-ray therapy, which delivered X-rays produced by colliding high-energy (25 MeV) electrons into a “target”.
When operating in direct electron-beam therapy mode, a low-powered electron beam was emitted directly from the machine, then spread to safe concentration using scanning magnets. When operating in megavolt X-ray mode, the machine was designed to rotate four components into the path of the electron beam: a target, which converted the electron beam into X-rays; a flattening filter, which spread the beam out over a larger area; a set of movable blocks (also called a collimator), which shaped the X-ray beam; and an X-ray ion chamber, which measured the strength of the beam.
The accidents occurred when the high-power electron beam was activated instead of the intended low power beam, and without the beam, spreader plate rotated into place. Previous models had hardware interlocks in place to prevent this, but Therac-25 had removed them, depending instead on software interlocks for safety. The software interlock could fail due to a race condition. The defect was as follows: a one-byte counter in a testing routine frequently overflowed; if an operator provided manual input to the machine at the precise moment that this counter overflowed, the interlock would fail.