AN INVESTIGATION OF THE THERAC-25 ACCIDENTS

Risk in any complex technology is unavoidable. One of the best ways to reduce risk in the future is to learn from the mistakes of the past. Between June 1985 and January 1987, the Therac-25, a computerized radi ation therapy machine, was involved in six massive radiation overdoses . As a result. several people died and others were seriously injured. These accidents have been described as the worst series of radiation a ccidents in the 35-year history of medical accelerators. Published des criptions of the Therac-25 medical electron accelerator accidents leav e out important details and are thus often misleading. The authors pre sent a detailed investigation of the factors involved in the overdoses and attempts by users, manufacturers, and government agencies to deal with the accidents. Most accidents are system accidents stemming from complex interactions between various components and activities. To at tribute a single cause to an accident is usually a mistake. The author s demonstrate (1) the complex nature of accidents and (2) the need to investigate all aspects of system development and operation in order t o prevent future accidents. The authors also present some lessons lear ned in terms of system engineering, software engineering, and governme nt regulation of safety-critical systems containing software component s.