Therac-25

Therac-25 was a medical linear accelerator, a device used to treat cancer. What made Therac-25 unique at the time of its use was the software. Not only did the software ease the laborious set-up process, but it also monitored the safety of the machine. In this case on safety critical software, you will find that some patients received much more radiation than prescribed despite the software safety programming,
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  • Added04/24/2020

    This introduction to the Therac-25 case is for teachers of the case and provides a guide to the case from the inside or from the teacher’s perspective.

    Year 2003
  • Added04/24/2020

    The case narrative materials provide only information up until the time of the accidents. This nicely puts students in the decision maker's seat, but one is left wondering what decisions actually were made by the main actors. This document provides answers to those questions.

    Year 2003
  • Added04/24/2020

    The safety of the Therac-25 is not really a property of the machine alone. Accidents that go unreported contribute to (or at least fail to stop) later accidents. When the TV camera in the room is unplugged, the operator cannot see that the patient is in trouble. So safety is really a property of the entire technical and social system (socio-technical system). In a similar manner, an ethical analysis of the issues in this case requires an awareness of the entire socio-technical system.

    Year 2003
  • Added04/20/2020

    We use aliases for the victim names in this case. We do so because we do not want to focus on who the players are in this case, but rather on the predicaments in which they found themselves. However, if you must know who is who, we provide a key.

    Author(s) Charles Huff
    Year 2003
  • Added04/20/2020

    These excerpts are from the article Leveson published in 1993 in IEEE Computer. You can find a more current version of the article at her web site at http://sunnyday.mit.edu/papers/therac.pdf. We selected these excerpts because we felt they described the most critical issues in the case. They go into much more detail on the software problems, the design of the machine and software, and the interface on the VT100 terminal.

    Author(s) Nancy Leveson
    Year 1995
  • Added04/20/2020

    This is a transcription of the memo that the medical physicist at the Tyler, Texas produced upon discovering how to produce the "malfunction 54." Malfunction 54, produced in this way would deliver a dose 25,000 rads of 25 MeV electrons in less than two seconds. The standard therapeutic dose is about 200 rads at any one time. A dose of 500 rads over the entire body is considered lethal to 50% of individuals who receive it. Two persons were killed from the malfunction 54 overdose. One died in 5 months, the other within one month.

    Author(s) Charles Huff
    Year 2003
  • Added04/20/2020

    There are two documents in this section. Both are derived from an interview we conducted with an operator of a Therac machine. This person was trained as a linear accelerator operator just before the transition to computer controlled linear accelerators in the mid-1980s. One of the first machines she worked on was a Therac-4. This machine had similar computer controls to the Therac-25, but it was not dual mode, and so all the accidents based on dual mode operation were not a possibility. In her interview she speaks of the training (or lack thereof) that operators receive, of the financial pressures on hospitals and cancer treatment facilities, and of the production pressures that operators experience.

    Author(s) Charles Huff
    Year 2003
  • Added04/20/2020

    In addition to an attempt at a comprehensive bibliography, the authors have included annotations to those references that they think would be the most useful to students of this case.

    Author(s) Charles Huff
    Year 2003
  • 241KB

    This is a paper on teaching the Therac-25 case with particular reference to understanding the race conditions that underlie some of the errors. We contend that, given the poor understanding of race conditions in the 1970s, it is anachronistic to blame AECL programmers for not handling these with correct syncronization techniques. This does not absolve AECL of blame for many other system safety errors, but it places the case in its historical context.

    Resource AddedFriday, May 1, 2020 at 6:35 PM

Cite this page: "Therac-25" Online Ethics Center for Engineering 4/16/2020 OEC Accessed: Friday, July 24, 2020 <www.onlineethics.org/89737/Therac25>