Get the latest tech news

The Therac-25 Incident (2021)

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.

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. In this bug, there was a variable shared by multiple processes, meant as a flag to decide whether or not the beam collimator in the turntable needs to be checked to ensure everything is in the correct position. While the FDA CAP process was grinding along, AECL wanted to ensure that people could still use the Therac-25 safely, and that meant publishing quick fixes that users could apply to their devices.

Get the Android app