Therac-25 Disaster: Software Errors Cause Fatal Radiation Overdoses (1985–1987)

The Therac-25 linear accelerator is the canonical case of software-induced medical harm.

What Happened

Between 1985–1987, at least 6 patients received massive overdoses (10,000–25,000 rad vs. prescribed 200 rad) at facilities in the US and Canada. Results: severe radiation burns, paralysis, amputations, 3 deaths.

Root Causes

• Race condition: Concurrent software tasks allowed beam to fire in high-power mode without target/spreader in place.
• No hardware interlocks: Relied solely on software safety — violated defense-in-depth.
• Inadequate testing: No formal verification; user-reported errors dismissed as "impossible."
• Poor UI: Cryptic error codes (MALFUNCTION 54); operators repeated treatment.
• Organizational failure: Manufacturer (AECL) ignored complaints; FDA oversight gaps; hospitals lacked medical physics QA.

Legacy

Transformed medical device regulation: IEC 62304 (software lifecycle), FDA guidance on software validation, mandatory independent safety assessment, hardware interlocks required, incident reporting (MDR).

Teaches that software safety requires rigorous engineering culture, not just code.