The Hidden Flaw of Therac-25

The Hidden Flaw of Therac-25

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 system that worked exactly as intended — while causing catastrophic harm. Therac-25 didn’t malfunction. It did what it was designed to do. This episode lays the foundation for Unsafe by Design: a series about technology failures rooted not in accidents, but in choices — and the human cost of trusting complex systems without adequate safeguards.