The Therac-25 Incident

Summary

A few months ago, someone noted in the comments that they hadn't heard about the Therac-25 incident. I was surprised, and went off to do an informal survey of developers I know, only to discover that only about half of them knew what it was without searching for it. I think it's important that everyone in our industry know about this incident, and upon digging into the details I was stunned by how much of a WTF there was. Today's article is not fun, or funny. It describes incidents of death and maiming caused by faulty software engineering processes. If that's not what you want today, grab a random article from our archive, instead. When you're strapping a patient to an electron gun capable of delivering a 25MeV particle beam, following procedure is vitally important. The technician operating the Therac-25 radiotherapy machine at the East Texas Cancer Center (ETCC) had been running this machine, and those like it, long enough that she had the routine down. On March 21, 1986, the technician brought a patient into the treatment room. She checked their prescription, and positioned them onto the bed of the Therac-25. Above the patient was the end-point of the emitter, a turntable which allowed her to select what kind of beam the device would emit. First, she set the turntable to a simple optical laser mode, and used that to position the patient so that the beam struck a small section of his upper back, just to one side of his spine. By Ajzh2074 - Own work, CC BY-SA 4.0, Link With the patient in the correct position, she rotated the turntable again. There were two other positions. One would position an array of magnets between the beam and the patient; these would shape and aim the beam. The other placed a block of metal between the beam and the patient. When struck by a 25MeV beam of electrons, the metal would radiate X-rays. This patient's prescription was for an electron beam, so she positioned the turntable and left the room. In the room next door, shielded from the ...