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The Therac-25 was a radiation therapy machine produced by Atomic Energy of Canada Limited (AECL) and CGR MeV of France after the Therac-6 and Therac-20 units. It was involved with at least six known accidents between 1985 and 1987, in which patients were given massive overdoses of radiation, which were in some cases on the order of hundreds of grays. At least five patients died of the overdoses. These accidents highlighted the dangers of software control of safety-critical systems, and they have become a standard case study in health informatics.
Additional recommended knowledge
The machine offered two modes of Radiation therapy:
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 equalized the x-ray beam intensity; 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 for x-ray therapy, without the target having been rotated into place. The machine's software did not detect that this had occurred, and therefore did not prevent the patient from receiving a potentially lethal dose of radiation. The high-powered electron beam directly struck the patients causing the feeling of an intense electric shock and the occurrence of thermal and radiation burns. In some cases, the injured patients died later from radiation poisoning.
Researchers who investigated the accidents found several contributing causes. These included the following institutional causes:
The researchers also found several engineering issues:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Therac-25". A list of authors is available in Wikipedia.|