An investigation of the therac 25 accidents

An investigation of the therac 25 accidents

Companies must understand that for safety-critical software design rigorous testing and failure analyses are essential and that trained software engineers, not simply any reasonably experienced engineers, should implement the software design. Although there were no marks on the patient at the time, the treatment area felt "warm to the touch. The regulations required that manufacturers and importers report deaths, serious injuries, or malfunctions that could result in those consequences. The Therac relied more heavily on software. After undergoing a lumpectomy to remove a malignant breast tumor, a year-old woman was receiving follow-up radiation treatment to nearby lymph nodes on a Therac at the Kennestone facility in Marietta. Industry-standard hardware safety features and interlocks in the underlying machines were retained. The feeling was described by patient Ray Cox as "an intense electric shock", causing him to scream and run out of the treatment room.

The consequences of an overdose to a smaller part of the body depend on the tissue's radiosensitivity. Occasionally, major accidents draw the attention of the US Congress or President and result in formal accident investigations for instance, the Rogers commission investigation of the Challenger accident and the Kemeny commission investigation of the Three Mile Island incident.

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The patient went home, but shortly afterward she developed a reddening and swelling in the center of the treatment area. The Therac, a predecessor of the Therac, employed independent protective circuits and mechanical interlocks to protect against overdose.

Relatively shallow tising healthy tissue.

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These models had hardware interlocks that masked their software defects. AECL is an arms-length entity, called a crown corporation, of the Canadian government. The lack of follow-up in this case appears to be evidence of such a problem in the organization. We have tried to be very careful to present only what we could document from original sources, but there is no guarantee that the documentation itself is correct. On June 3, , the patient was set up for a MeV electron treatment to the clavicle area. Doses of 1, rads can be fatal if delivered to the whole body; in fact, the accepted figure for whole-body radiation that will cause death in 50 percent of the cases is rads. The modifications recommended in the Symonds report included redesigning the microswitch and changing the way the computer handled malfunction conditions. Six other patients were treated later that day without incident. When the equipment is regulated by government agencies, some information may be available. Industry-standard hardware safety features and interlocks in the underlying machines were retained. The second in this series of accidents occurred at this Hamilton, Ontario, Canada, clinic about seven weeks after the Kennestone patient was overdosed. AECL did not consider the design of the software during its assessment of how the machine might produce the desired results and what failure modes existed. But the facts available support previous hypotheses about the proper development and use of software to control dangerous processes and suggest hypotheses that need further evaluation.

Immediately after the Hamilton accident, the Ontario Cancer Foundation hired an independent consultant to investigate. In particular, the software was designed so that it was realistically impossible to test it in a clean automated way.

leveson ng & turner cs 1993 an investigation of the therac 25 accidents

This is an abstract of a article from IEEE Computer about the Therac computerized radiation therapy machine and its software flaws, which caused massive overdoses to patients.

She was admitted to West Paces Ferry Hospital in Atlanta, but her oncologists continued to send her to Kennestone for Therac treatments. Unfortunately, the previous accounts of the Therac problems have been oversimplified, with misleading omissions.

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In an effort to remedy this, we have obtained information from a wide variety of sources, including lawsuits and the US and Canadian government agencies responsible for regulating such equipment.

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An investigation of the Therac accidents