Therac 25

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The Therac 25 Disaster
October 2012

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Introduction

The Therac-25 was a machine for cancer treatment manufactured by the Atomic Energy of Canada Limited (AECL) and went down to history as one of the world’s worst software disasters. The machine was released to the market in 1983 and was later involved in at least 6 accidents that lead to enormous radiation overdoses for the patients. One of the major innovations of the Therac-25 was the move to complete computer control, which allowed operators to set up the machines more quickly, so they could focus more on the patients. Altogether, eleven Therac-25 units were installed. The Therac-25 had two modes of operation: • Direct electron-beam therapy - a low power electron beam is directly administered (5 MeV - 25 MeV) • Megavolt X-Ray therapy - the high-energy electron beam is converted to an X-Ray beam by colliding the electron beam into a so-called ,,target“. The target consists of a filter, which expanded the beam over a certain area, a so-called collimator to shape the X-Ray beam and a device to measure the strength of the beam.

Figure 1: Sketch of the two modes of operation. [2]

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The basis of radiation therapy is, that the area of the body that is affected with a tumor is exposed to radioactivity in order to kill the malignant cells. The different modes of operation described above are needed, since the selected mode depends on the depth of the tumor within the body.

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Chronicle of the Incidents

The following is a list of the most severe incidents with the Therac-25. Three of them ended deadly. • June 3rd, 1985, Kennestone Regional Oncology Center After a treatment with 10 MeV, a patient complained about being burned. However, no burning marks were visible. Therefore, the incident was never officially investigated. Later, the patient sued AECL. The case was settled outside of court. • July 26th, 1985, Ontario Cancer Foundation When a 40 year old woman received her treatment, the machine...
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