In manual mode, a radiotherapy technician would physically set up various parts of the machine, including the turntable to place one of three devices in the path of the electron beam. Both were designed to operate as standalone devices.
Therac-6 and 20 didn’t need that computer, though. All three machines used a PDP-11 computer. The Therac-20 codebase was developed from the Therac-6. However, both companies maintained access to the designs and source code of the earlier machines. When the time came to design the Therac-25, the partnership had dissolved. AECL built the Therac-6 and 20 in partnership with CGR, a French company. It was the third radiation therapy machine by the company, preceded by the Therac-6 and Therac-20. The Therac-25 was manufactured by Atomic Energy of Canada Limited (AECL). has since published updated information in a second paper which is also included in her book. Much of the information published about the Therac (including this article) is based upon her research and 1993 paper with entitled “ An Investigation of the Therac-25 Accidents”.
Much of this is due to the work of, a software safety expert who exhaustively researched the incidents and resulting lawsuits. Several universities use the case as a cautionary tale of what can go wrong, and how investigations can be lead astray. The case of the Therac-25 has become one of the most well-known killer software bugs in history. There were no hardware interlocks or supervisory circuits to ensure that software bugs couldn’t result in catastrophic failures. Secondly, the design of the machine relied on the controlling computer alone for safety. Firstly, the software controlling the machine contained bugs which proved to be fatal. During the investigation, it was determined that the root cause of the problem was twofold. For six unfortunate patients in 19, the Therac-25 did the unthinkable: it exposed them to massive overdoses of radiation, killing four and leaving two others with lifelong injuries. As with chemotherapy, the hope is that the net effect heals the patient more than it harms them. These room-sized medical devices would always cause some collateral damage to healthy tissue around the tumors. Using X-rays or a beam of electrons, radiation therapy machines kill cancerous tissue, even deep inside the body. In layman’s terms it was a “cancer zapper” a linear accelerator with a human as its target. The Therac-25 was not a device anyone was happy to see.
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