Case Study 1
THERAC – 25
Aaron James Uy Timosa
BSIT – 4
INTRODUCTION The Therac-25 was a radiation therapy machine produced by Atomic Energy of Canada Limited (AECL) after the Therac-6 and Therac-20 units (the earlier units had been produced in partnership with CGR of France). It was involved in at least six accidents between 1985 and 1987, in which patients were given massive overdoses of radiation, approximately 100 times the intended dose. These accidents highlighted the dangers of software control of safety-critical systems, and they have become a standard case study in health informatics and software engineering.
WHAT WAS THERAC -25?
Therac-25 was used in the treatment of cancer. Its purpose was to provide radiation to a specific part of the body and hopefully kill the malignant tumor. The Therac-25 was the third system created under the Therac name by the Atomic Energy of Canada Limited (AECL). The AECL is most famous in Canada for their creation of the CANDU reactors which are world renowned. A Therac-6 and Therac-20 were both used in the treatment of cancer. The number that goes along with the word Therac stands for the maximum amount of mega electron volts (MeV) the machine can dispense. It was believed that the new Therac-25 was much more efficient than Therac-6 and Therac-20. The overall size of the machine was reduced and still allowed for two modes; photon mode and electron mode. A tungsten shield was in place for the X-ray mode and removed for the electron mode
Between June 1985 and January 1987, a computer controlled radiation therapy machine, called the Therac-25, massively overdosed six people. These accidents have been described as the worst in the 35 year history of medical accelerators.
A detailed accident investigation, drawn from publicly available documents, can be found in Leverson and Turner. The following account is take this report and includes both the factors involved in the overdoses themselves