November 28, 2007

Arithmetic overflow caused death of several people!

The Therac-25 was a computer controlled radiation therapy machine produced by Atomic Energy Canada Ltd. in the 1980s. The machine offered two modes of therapy, including "Megavolt X-ray therapy". The problem happened when the machine's software failed to move a component into place to convert an electron beam into X-rays, allowing a high power electron beam to directly strike patients. At least 5 patients died of radiation poisoning from dosages that were sometimes in the hundreds of Grays.
AECL did not have the machine's software independently reviewed. AECL did not assess what potential failure modes existed. The system documentation did not explain error codes. AECL ignored incident reports.
The problem turned out to be a software bug which intermittently caused an arithmetic overflow. When this happened, the software bypassed safety checks. So the machine killed people that were simply undergoing therapy.


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