The Therac-25 killed patients — software replaced the hardware safety interlock

The Therac-25, a radiation therapy machine manufactured by Atomic Energy of Canada Limited (AECL), was used to treat cancer patients. Between 1985 and 1987, the machine delivered massive radiation overdoses to at least six patients, killing at least three. The overdoses were caused by a software race condition that allowed the electron beam to fire without the proper shielding in place.

The Therac-25's predecessors (Therac-6 and Therac-20) had hardware interlocks that prevented the beam from firing without shielding. The Therac-25 removed these hardware interlocks and relied entirely on software checks. When the software failed — due to a race condition that occurred when operators typed commands too quickly — there was no hardware backup to prevent the overdose.

The case became a landmark in software engineering and medical device safety. It demonstrated that software-only safety systems are insufficient for life-critical applications: hardware interlocks must provide a backup. The Therac-25 is taught in every software engineering course as the definitive example of why safety-critical systems require defense in depth.