Therac-25 - WikiHQ

thumb|Therac 25 Machine

The Therac-25 was a computer-controlled radiation therapy machine produced by Atomic Energy of Canada Limited (AECL) in 1982 after the Therac-6 (neptune) and Therac-20 units (the earlier units had been produced in partnership with of France).

The Therac-25 was involved in at least six incident overdoses of radiation between 1985 and 1987, Because of concurrent programming errors (also known as race conditions), it sometimes gave its patients radiation doses that were up to 250 times higher than normal, resulting in death or serious injury.

In the early 1970s, CGR and the Canadian Crown corporation Atomic Energy of Canada Limited (AECL) collaborated on the construction of linear accelerators controlled by a DEC PDP-11 minicomputer: the Therac-6, which produced X-rays of up to 6 MeV, and the Therac-20, which could produce X-rays or electrons of up to 20 MeV. CGR developed the software for the Therac-6 and reused some subroutines for the Therac-20. In contrast to the Therac-25, the Therac-20's software "merely added convenience to the existing hardware, which was capable of standing alone."

After the accidents, in 1988 AECL dissolved the AECL Medical section and the company Theratronics International Ltd took over the maintenance of the installed Therac-25 machines.

Design

The machine had three modes of operation, with a turntable moving some apparatus into position for each of those modes: either a light, some scan magnets, or a tungsten target and flattener.A mirror would be placed to guide the light through the treatment beam's path. The staff at the treatment center did not believe it was possible for the Therac-25 to cause such an injury, and it was treated as a symptom of her cancer. Later, the hospital physicist consulted AECL about the incident. He calculated that the applied dose was between 15,000 and 20,000 rad (150…200 Gy) when she should have been dosed with 200 rad (2 Gy). A dose of 1000 rad (10 Gy) can be fatal. In October 1985, Yarbrough sued the hospital and the manufacturer of the machine. In November 1985, AECL was notified of the lawsuit. It was not until March 1986, after another incident involving the Therac-25, that AECL informed the FDA that it had received a complaint from the patient.

Due to the radiation overdose, her breast had to be surgically removed, an arm and shoulder were immobilized, and she was in constant pain. The treatment printout function was not activated at the time of treatment and there was no record of the applied radiation data. An out-of-court settlement was reached to resolve the lawsuit. Treatments to this particular patient continued through January 6, 1986, until the staff could not identify another way that this would occur in the patient except from a source of consistent radiation. meant that the radiation delivered was either too high or too low.

The radiation monitor (dosimeter) marked 6 units supplied when it had demanded 202 units. The operator pressed ( Proceed : continue). The machine stopped again with the message "Malfunction 54" (error 54) and the dosimeter indicated that it had delivered fewer units than required. The surveillance camera in the radiation room was offline and the intercom had been broken that day.

With the first dose the patient felt an electric shock and heard a crackle from the machine. Since it was his ninth session, he recognized that this sensation was abnormal. He started to get up from the table to ask for help. At that moment the operator pressed to continue the treatment. The patient felt a shock of electricity through his arm, as if his hand was torn off. He reached the door and began to bang on it until the operator opened it. A physician was immediately called to the scene, where they observed intense erythema in the area, suspecting that it had been a simple electric shock. He sent the patient home. The hospital physicist checked the machine and, because it was calibrated to the correct specification, it continued to treat patients throughout the day. The technicians were unaware that the patient had received a massive dose of radiation between 16,500 and 25,000 rads in less than a second over an area of one cm<sup>2</sup>. The crackling of the machine had been produced by saturation of the ionization chambers, which had the misleading consequence of indicating very low dose.

Over the following weeks the patient experienced paralysis of the left arm, nausea, vomiting, and ended up being hospitalized for radiation-induced myelitis of the spinal cord. His legs, mid-diaphragm and vocal cords ended up paralyzed. He also had recurrent herpes simplex skin infections. He died five months after the overdose.

From the day after the accident, AECL technicians checked the machine and were unable to replicate error 54. They checked the grounding of the machine to rule out electric shock as the cause. The machine was back in operation on April 7, 1986. Lack of communication and harmful assumptions made by the programmers and operators specifically in regard to the data entry values from previous information collected in the machine, was a simple yet major cause of the deaths and injuries from the Therac-25. Doctors and programmers had high confidence in the ability and accuracy of the machine resulting in them continuing the treatments that were harming patients and not questioning the pain experienced during the process. This fix was determined to be the single solution, however it took more than a year to get CMC to make a plan that invoved more systematic redesign issues.

Institute of Medicine (US) Committee on the Public Health Effectiveness of the FDA 510(k) Clearance Process; Wizemann T, editor. Public Health Effectiveness of the FDA 510(k) Clearance Process: Measuring Postmarket Performance and Other Select Topics: Workshop Report. Washington (DC): National Academies Press (US); 2011. Available from: <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK209654/</nowiki> doi: 10.17226/13020