Atomic Energy of Canada Limited’s Therac-25, introduced commercially in late 1982, was involved in at least six documented radiation-overexposure incidents between June 1985 and January 1987. The computer-controlled accelerator could deliver an electron beam in the wrong operating mode, producing doses up to roughly 100–250 times the prescription over a narrow area. The design had removed or reduced hardware interlocks used on earlier Therac models and relied on software checks, including code reused from earlier systems. Concurrent programming errors, a race condition during rapid data entry, and an integer-overflow bug could leave the beam improperly configured while the interface reported misleading errors such as “Malfunction 54.”
Patients suffered severe radiation burns, paralysis, tissue damage, and radiation poisoning. Three documented victims later died: an Ontario patient treated on July 26, 1985; an East Texas Cancer Center patient overdosed on March 21, 1986; and another East Texas patient overdosed on April 11, 1986. In the June 3, 1985, Georgia incident, Katie Yarbrough reportedly received an estimated 15,000–20,000 rad instead of 200 rad and underwent surgery after extensive injury. AECL initially dismissed or failed to reproduce several complaints, and the FDA was not informed of Yarbrough’s complaint until March 1986. The accidents became a foundational case study in safety-critical software engineering and medical ethics; AECL dissolved its medical division in 1988, transferring maintenance to Theratronics International Ltd.
