Therac-25: The Radiation Machine That Killed People Due to a Software Bug
How removing hardware safety interlocks and trusting software alone caused a radiation therapy machine to deliver lethal doses — killing at least 6 patients.
The challenge
The Therac-25 was a radiation therapy machine produced by Atomic Energy of Canada Limited. Unlike its predecessors (Therac-6 and Therac-20), it removed hardware safety interlocks — physical mechanisms that prevented dangerous radiation levels — and replaced them entirely with software safety checks. This was considered an advancement.
The strategy
The software was written by a single developer and reused code from earlier Therac models without understanding the hardware differences. The Therac-25 software had a race condition — a timing bug where the sequence of user inputs could put the machine into a state where it believed safety checks had passed when they hadn't.
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Try the full case free →Key lessons (preview)
- Never remove hardware safety interlocks and replace with software alone — software fails in ways hardware cannot.
- Race conditions in safety-critical systems can have lethal consequences.
- When users report unusual behavior, investigate immediately — never blame the user first.