Therac-25: The Software Bug That Turned a Cancer Machine Lethal

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In 1985, a 61-year-old woman at a clinic in Marietta, Georgia, lay down for routine radiation after a lumpectomy and felt a searing blast of heat tear through her chest. The technician checked the screen and told her a burn was impossible. She had in fact absorbed between 15,000 and 20,000 rads instead of the prescribed 200. Her case was the first in the worst series of radiation accidents in the history of medical accelerators, all traced to one machine: the Therac-25, built by Atomic Energy of Canada Limited.

This episode reconstructs how a device designed to cure cancer began killing patients. AECL had removed the physical hardware interlocks that protected its older models and trusted software alone to guard a beam roughly a hundred times stronger than the one used for surface treatment. When patients were burned in Ontario, in Yakima, Washington, and in Tyler, Texas, the manufacturer kept insisting an overdose could not happen, and hospitals believed the screen over the people on the table.

• In Ontario, an operator saw a cryptic H-tilt error and a reading of zero dose, then pressed proceed five times. The patient died months later.
• At the East Texas Cancer Center in 1986, a fast typist corrected an X to an E within the eight seconds the turntable needed to move, triggering a race condition and Malfunction 54.
• The Tyler hospital physicist pulled the machine offline and, with the operator, spent hours recreating her exact typing rhythm until they reproduced the fault.
• All the software was written by one person with no independent review, and it reused Therac-20 code whose bugs had been silently masked by mechanical locks.
• A second flaw let a counting flag roll over to zero like an odometer, telling the computer the turntable was in place when it was not.