The Fatal Flaw of Therac-25

In the early 1980s, a state-of-the-art radiation therapy machine began injuring and killing cancer patients — without alarms, without mechanical safeguards, and without anyone believing the machine could be wrong.

In this pilot episode of Unsafe by Design, we examine the Therac-25 radiation overdoses, one of the most important and chilling case studies in the history of software safety. What was marketed as a technological leap forward quietly removed critical hardware interlocks and replaced them with software — software that contained hidden race conditions, inadequate error handling, and assumptions that proved fatal.

Through expert analysis and historical investigation, this episode explores how design decisions, reused code, and organizational denial combined to create a ...