What were the problems
The Therac-25 Incident (1986)
Ray Cox, a Texas oil worker was over exposed to radiation treatment which caused his death.
He had been to the facility where he had been treated eight times with the Therac-25 dual
mode machine which uses low energy radiation of 200 rads and a high energy X-ray of 25 million
electron volt full capacity. Ray Cox was placed on low energy radiation of 200 rads aimed
directly at his left shoulder to remove cancerous tumor. On this fateful day, he was given
an over dose of both low and high radiation energy because the care giver slipped at work
and turn on the wrong buttons. To operate this machine, the technician needs to enter "e"
for the electron beam or "x" for x-ray radiations which works with a “beam flattener” to
avert over exposure. Unfortunately, she slipped into the "x" mode for x-ray instead of "e"
for electron beam for which Ray Cox was scheduled for. Realising her mistake, the technician
reversed the modes, pressing enter for each operation without recourse to the computer
operating system which accepted the FIFO (First in, First out) execution.
There were two problems here Human error, and system encouraged error.
- Human error: The technician who operated the
Therac-25 suffered from attention deficit; slipped on her job and placed the machine in a wrong
mode for her patient. She could have stopped the system for error, reboot and continue on the
correct mode. The machine gave a garbage-in, garbag-out response according to its operating system
for what ever action the care give placed on the computer terminal.
- System Error: Any computer operator understands that prompting a system to perform different
actions would execute consecutively. Please! perform this simple action and see what happens. On
this edit mode of your wiki page: After you have typed few lines click on the save page button
and the show preview button at the same time. You will get this error message - Remember
that this is only a preview. Your changes have not yet been saved! The two actions were executed
consecutively. The reversed switching from "x" to "e" mode never stopped one or the other; instead, the
two action executed consecutively and caused radion over exposure.
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