Many lessons can be learned from this series of accidents. During the time span of june 1985 to january 1987, it was the source of six fatal or near fatal overdoses. A thorough account of the therac 25 medical electron accelerator accidents reveals previously unknown details and suggests ways to reduce risk in the future. An investigation of the therac25 accidents computer author. During the time span of june 1985 to january 1987, it. Name instructors name course date the therac25 and its accident investigation several factors led to the thera25 accidents. Interface design as a life or death proposition cooper. Software in the therac 6 and therac 20 was reused in the therac 25. The therac 25 ion chambers could not handle the high density of ionization from the unscanned electron beam at highbeam current. The therac25 and its accident investigation case study.
Turner, university of california, irvine reprinted with permission, ieee computer, vol. System safety and computers published by addisonwesley. Stories about the therac25 have appeared in trade jour nals, newspapers, people magazine, and on televisions 20120 and mcneill lehrer news hour. Therac 6 and therac 20 had histories of clinical use without computer control therac 25 software had more responsibility for safety than in previous machines. These socalled accidents and mistakes are really just cases of human inattention.
Researchers who investigated the accidents found several contributing. Published papers deal with medical, legal, economic, educational, behavioral, theoretical or empirical aspects of. The therac25 accidents are the most serious computerrelated accidents to date at least nonmilitary and admit tcd and have even drawn the attention of the popular press. Nancy leveson and clark turner, the investigation of thetherac 25 accidents, computer, 26, 7 july 1993 pp 1841.
Feb 17, 2014 the therac 25 accidents form the basis for what is often considered the bestdocumented software safety casestudy available. A brief summary of how therac25 actually operated is given as follows. On the surface, the primary reason that therac20 killed far fewer people than therac25 was the fact that therac20 had hardware interlocks, while therac25 did not. Leveson, addison wesley, 1995 text by the same author with a 33page analysis of the therac 25 case, somewhat less technical than the ieee paper but still including thorough details, with perhaps more emphasis on the ethical. Safeware system safety and computers a guide to preventing accidents and losses caused by technology, nancy g. The therac 25 was a computerized radiation therapy machine 11 machines were installed us and canada in 19851987 there were 6 known accidents where massive overdoses were made patients died or suffered serious injuries these were traced to race conditions in reading operator input unique early investigation of safetycritical. A detailed investigation of the factors involved in the softwarerelated overdoses and attempts by users, manufacturers, and government agencies to deal with the accidents is presented.
Therac 25 aecl designed therac 25 to use computer control from the start. The therac25 was a computercontrolled radiation therapy machine produced by atomic. The investigation conclusion trial investigation scenario based test this is an example investigation designed to give an overview of how an accident investigation is conducted, what is required to be found and how you disseminate information from witness statements. The operators manual supplied with the machine does sitions the. The therac25 software disaster essay 1293 words cram. After the tyler accidents, therac 20 users who had heard informally about the tyler accidents from therac 25 users conducted informal investigations to determine whether the same problem could occur with their machines. Professionalismtherac25 wikibooks, open books for an open.
Stories about the therac25 have appeared in trade journals, newspapers, people magazine, and on televisions 2020 and mcneil lehrer news hour. Radiation therapy machine the most serious computerrelated accidents to date people were killed reference. The therac25 software disaster the therac25 is a computerized medical radiation therapy machine for cancer patients. What made therac 25 unique at the time of its use was the software. Depending on whether the tumor was close to the skin or in deeper tissue, the therac 25 would operate in an electronbeam or xray mode. A detailed accident investigation, drawn from publicly available docu ments, can. Computers are increasingly being introduced into safetycritical systems and, as a consequence, have been involved in accidents. As a result, several people died and others were seriously injured. Information and computer science, university of california, irvine, 1992 59 pages. Accident investigation and reporting intranet home. An investigation of the therac 25 accidents nancy leveson, university of washington clark s.
An investigation of the therac25 accidents abstract. This resulted in everyone from the manufacturers, to fda, to hospitals and operators assuming that it was a failsafe machine, especially since its earlier versions had been working. An investigation of the therac25 accidents cal poly computer. The therac25 and its accident investigation case study 1. Nancy leveson and clark turner, the investigation of thetherac25 accidents, computer, 26, 7 july 1993 pp 1841. All accidents, incidents, near misses or occupational ill health that occur at, or in conjunction with university activities should be investigated and reported in accordance with university policy ushp23air11 action, investigation and reporting in the event of an accident.
An investigation of the therac25 accidents stanford university. Leveson et al reference is a very complete account of these accidents, although it is rather heavily slanted towards procedure and legality rather than concentrating on the technological lessons that are to be learned. Aug 08, 2010 the therac 25 is a radiation therapy machine used during the mid80s. Therac 25 was a medical linear accelerator, a device used to treat cancer. An updated version of the original accident investigation paper by nancy leveson i have updated and changed slightly the original accident report. An investigation of the therac25 accidents nancy g. Citeseerx document details isaac councill, lee giles, pradeep teregowda. Therac25 radiation therapy accidents ariane 5 rocket failure patriot missile system failure the therac25 accidents computerized radiation therapy machine designed in early 1980s resulted in six major accidents between june 1985 and january 1987 at least two deaths several serious injuries therac25 overview. Turner, university of california, irvine reprinted. Essay on the therac25 and its accident investigation instructor name school coursenumber june 2, 2015 introduction in 1983, a machine was released to help in the treatment of cancerous tumors through the use of high energy laser beams. The science of aircraft accident investigation is a very small field and hopefully getting smaller every year. Well worth a read, if you need knowledge in this area.
An investigation of the therac25 accidents computer. Between june 1985 and january 1987, six known accidents involved massive overdoses by the therac25 with resultant deaths and serious injuries. An investigation of the therac25 accidents essay 10546. The experience illustrates a number of principles that are vital to understanding how and why the design and analysis of safetycritical systems must be done in a methodical way according to established principles. What we can learn from the therac25 what does nancy levesons classic analysis of the therac25 recommend.
A common mistake in engineering, in this case and in many others, is to put too much confidence in software. The second, higher energy mode, used the full power of the machine at 25 million electron volts. Apr 20, 20 an investigation of the therac 25 accidents part iii nancy leveson, university of washington clark s. Some of the most widely cited softwarerelated accidents in safetycritical systems involved a computerized radiation therapy machine called the therac25. The first mode consisted of an electron beam of 200 rads that was aimed at the patient directly. Therac25, however, was developed by aecl alone when the two companies drifted apart after the joint development of therac20. A regional distribution of the number of road accidents reveals that over half of all crashes occurred in khomas region 8,590 or 55. An investigation of the therac 25 accidents nancy g. The user manual did not explain or even address the error codes, so the.
It delivered two types of radiation beams, a lowpower electron beam and a highpower xray. Between june 1985 and january 1987, the therac25 medical electron accelerator was involved in six massive radiation overdoses. An investigation of the therac25 accidents part iv. Resulted in 3 deaths and 3 cases of severe radiation related injuries. Citeseerx an investigation of the therac25 accidents. As noted earlier, the software for the therac 25 and therac 20 both evolved from the therac 6 software. Aecl sold eleven therac25 machines that were used in the united states and canada beginning in 1982. The therac 25, like other medical linear accelerators including its predecessors therac 6 and therac 20, used highenergy electron beams to destroy tumors without damaging nearby healthy tissue. An investigation of the therac25 accidents part iii nancy leveson, university of washington clark s.
The therac 25 was a computercontrolled radiation therapy machine produced by atomic energy of canada limited aecl in 1982 after the therac 6 and therac 20 units the earlier units had been produced in partnership with cgr of france. This paper presents the results of an investigation into medical errors in. The therac 25 software disaster the therac 25 is a computerized medical radiation therapy machine for cancer patients. The therac 25 software lied to the operators, and the machine itself could not detect that a massive overdose had occurred. Worst series of radiation overdoses in over 35 years. Turner, an investigation of the therac25 accidents, ieee computer, vol. The therac25 accidents are the most serious computerrelated accidents to date at least nonmilitary and admitted and have even drawn the attention of the popular press.
Turner, university of california, irvine a thorough account of the therac 25 medical electron accelerator accidents reveals previously unknown details and suggests ways to reduce risk in the future. An investigation of the therac25 accidents, by nancy leveson, university of washington and clark s. The machine was developed by aecl,a canadian company that had worked which the french company, cgr, on the earlier therac models therac6 and therac20. Turner, university of california, irvine, ieee computer, vol. This provided the economic advantage of delivering two kinds of therapeutic radiation with one machine.
Pdf investigation of alarmrelated accidents and incidents. Thus, while the hardware interlocks on therac20 prevented software errors from causing problems, therac25 had no similar mechanism. Therac 25 accidents adapted from a proprietary report from the aecl research co may,1994 the n. An investigation of the therac25 accidents essay 10546 words. Yet consider this chilling description of an event involving the therac25. It was an extremely costly machine with high maintenance needs. The therac25 accidents are the most healthy tissue. The therac 25 a case study in safety failure radiation therapy machine the most serious computerrelated accidents to date people were killed reference. Therac25 radiation overdoses your expert root cause. An investigation of the therac25 accidents nancy leveson, university of washington clark s.