BUGS THAT CHANGED HISTORY

Therac-25, 1985–87. A radiation therapy machine trusted its software alone. A fast typist could put it in a state no one had imagined.

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Six patients received massive overdoses; several of them died and others were left with serious injuries. They are not shown. This page follows Leveson & Turner's investigation and names no one.

FACT Leveson & Turner document six known massive overdoses between June 1985 and January 1987, in the US and Canada.
FACT The race needed the edit to land within about 8 s — the time the bending magnets took to set. Few operators were fast enough until they had used the machine for a while.
FACT The second fault: a one-byte variable incremented on every pass wrapped to zero every 256th time, and zero meant 'skip the collimator check'.
FACT Some of the software came from the earlier Therac-6 and Therac-20, which had hardware interlocks that masked its faults.
ESTIMATE The overdoses are estimated on the order of 100 times the intended dose — at Yakima, 8,000–10,000 rads against 86 prescribed.
MODEL The machine is drawn schematically: the waveguide, bending magnet and turntable are where they belong, not to the manufacturer's drawings.

SETUP A treatment room. The Therac-25 has two modes: a low-current electron beam, and X-rays — a beam about 100 times stronger that hits a tungsten target and a flattening filter on a turntable, which turn it into a safe X-ray field.

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