The therac25s software was developed from the therac20s software, which was developed from the therac6s software. The therac25 was manufactured by atomic energy of canada limited aecl. The programmer is responsible for the malfunctioning therac 25 software. The therac 25 was a computer controlled 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. These two companies had collaborated since the early 1970s in building linear accelerators for medical applications. At least four bugs were found in the therac25 software that could cause radiation overdose. The therac 25 a case study in safety failure radiation therapy machine the most serious computer related accidents to date people were killed reference. Since the therac25 events, the fda has moved to improve the reporting system and to augment their procedures and guidelines to include software. Aecl built the therac 6 and 20 in partnership with cgr, a french company. The therac 25 a case study in safety failure radiation therapy machine the most serious computerrelated accidents to date people were killed reference. A history of the introduction and shut down of therac25.
The therac 25 software disaster the therac 25 is a computerized medical radiation therapy machine for cancer patients. At the individual level, the programmer had the options of inserting the safety interlocks in the hardware, software, or both. The reactions after each overdose the creators of therac 25 were contacted. At about that time, the division of aecl that designed and manufactured therac25 became an independent company. The therac25 software disaster essay 1293 words cram. The therac25 was a computercontrolled radiation therapy machine produced by atomic. Unfortunately, he decided to add the emergency locks only in the software. The therac 6 and therac 20 were clinically tested machines with an excellent safety record. Aecl produced the first hardwired prototype of the therac 25 in 1976, and the completely computerized commercial version. Writing software can seem cool and abstracted until you realise the impact your code can have. Such incidents would not have been an issue in a singleuse machine and unlike previous models, the therac 25 relied on software rather than hardware safety interlocks. Fixing each individual software flaw as it was found did not solve the devices safety problems. A series of accidents involving the aecl therac 25 in the 1980s caused three fatalities and other serious injuries. Hardware is the computer itself, its keyboard, casing, microchips, switches rusting, dusty, fallible, and mortal.
It continued to sell the therac 25 after the fda declared it ot be defective. As a result, several people died and others were seriously injured. The machine and its predecessors, therac 6 and therac 20, was a product from the collaboration of atomic energy of canada limited aecl and a french company called cgr leveson, n. The first consisted of an electron beam targeted directly at the patient in small doses for a short amount of time. The therac6 and therac20 had been designed around machines that already had histories of clinical use without computer control. That is an ethical issue just as important as the therac25 type problem. The design of realtime computing systems is the most challenging and complex task that can be undertaken by a software engineer. The therac25 was much more of a management and engineering failure than a technical problem, though. Therac25 and the security of the computer controlled equipment. Which of these problems was not encountered by bae as it tried to install on automated baggage handling system at denver international airport. The therac 25 accidents form the basis for what is often considered the bestdocumented software safety casestudy available. The therac 25 medical radiation therapy device was involved in several cases where massive overdoses of radiation were administered to patients in 198587, a side effect of the buggy software powering the device.
The gmail outage only resulted in people not having access to their email for a few hours. Weiss on six separate occasions between june 1985 and january 1987, the therac25, a computer controlled radiation therapy machine, is known to have killed or seriously injured patients in the us and canada with massive radiation overdoses. When the time came to design the therac 25, the partnership had dissolved. The software of the therac25 also controls the positioning of the turntable, a possible hazard discussed previously, and checks the position of the turntable so that all necessary devices are in place leveson and turner, 1993, p. An investigation of the therac25 accidents nancy leveson, university of washington clark s. Yet over the years there have been numerous reports both official and unofficial of accidents and overdoses involving the improper diagnostic and therapeutic application of ionizing radiation. With the aid of an onboard computer, the device could select multiple. Thus, while the hardware interlocks on therac 20 prevented software errors from causing problems, therac 25 had no similar mechanism. The therac25 was designed to be solely computer controlled.
Older theracs relied on hardware to set the machine up for treatment, to position the beam, and to run the safety system. Sixty years later, computer bugs are still with us, and show no sign of going extinct. Fixing each individual software flaw as it was found did not solve the safety problems of the device. This occurred with the therac25 that had two prominent software errors, a failed.
The therac25 was the most computerized and sophisticated radiation therapy machine of its time. 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. The first safety analysis on the therac25 did not include software although nearly full responsibility for safety rested on the software. Computers are increasingly being introduced into safetycritical systems and, as a consequence, have been involved in accidents. A number of patients received up to 100 times the intended dose, and at. After the first incident the aecl responses was simple, after careful consideration, we are of the opinion that this damage could not have been produced by any malfunction of the therac 25 or by any. There was some base duplication of the software used from the therac 20 that carried over to the therac 25. The therac 25 software also contained several userfriendly features. While this is a serious failure, im not sure its fair to say that this is a great example of an ethical dilemma.
Virtually all complex software will behave in an unexpected or undesired fashion under some conditions there will always be another bug. The therac 25 a case study in safety failure radiation therapy machine the most serious computerrelated accidents to date people were killed references. Aecl was expected to notify therac25 users of the problem, and of fdas recommendations. Once the fda got involved in the therac25, their response was impressive, especially considering how little experience they had with similar problems in computer controlled medical devices. A detailed investigation of the factors involved in the software related overdoses and attempts by users, manufacturers, and government agencies to deal with the accidents is presented. The therac25 was a computercontrolled radiation therapy machine produced by atomic energy of canada limited aecl in 1982 after the therac6 and therac20 units the earlier units had been produced in partnership with cgr of france. In addition, the therac25 software has more responsibility for maintaining safety than the software in the previous machines. The software would check if the operation was safe so no harm would come to the person. Although most of us wont work on safetycritical systems, software errors can still have a significant impact on our users. Nancy leveson and clark turner, the investigation of thetherac 25 accidents, computer, 26, 7 july 1993 pp 1841. 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. To be sure, there havent been many, but cases like the therac 25 are widely seen as warnings against the widespread deployment of software in safety critical applications.
Study 27 terms computer science flashcards quizlet. It was the third radiation therapy machine by the company, preceded by the therac 6 and therac 20. Additional resources on the therac25 and related accidents. 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. The therac25 machine was a stateoftheart linear accelerator developed by the company atomic energy canada limited aecl and a french company cgr to provide radiation treatment to cancer patients. Firstly, the software controlling the machine contained bugs which proved to be fatal. Often it takes an accident to alert people to the dangers involved in technology. The worst computer bugs in history is a mini series to commemorate the discovery of the first computer bug seventy years ago. Consider the therac 25 failure, in which several deaths occurred because of a software engineering failure. The fact that computers always have problems with their programming. A final feature was that some of the old software used in therac 6 and therac 20 was used in the therac25.
Its hard to say how much of this success was a result of the work carried out to alleviate the problem, or whether the problem had been. In 1982 a machine called therac25 created by the atomic energy of canada limited aecl appeared in the medical field for cancer treatments, using radiation and xrays. When the time came to design the therac25, the partnership had dissolved. Therac25 just like any other technology, therac25 too had its sociotechnical aspects.
This machine was an improvement of the therac20 and cost approximately 1 million dollars. The reasoning given for not including software errors was the extensive testing given to the therac 25, the fact that software, unlike hardware, does not degrade, and the general assumption that software is errorproof software errors were assumed to be caused by hardware errors, and residual software errors were not included in the analysis. The therac 25 was a machine for administering radiation therapy, generally for treating cancer patients. The therac 25 was designed to be computer controlled. Nov 12, 2015 the therac 25 was manufactured by atomic energy of canada limited aecl. Software errors bugs being programmed to solve the wrong problem programmers fail to deliver client expectations misuse a computer is provided erroneous data. Therac 25 was a tragic example of how bad code hurts people. An investigation of the therac25 accidents computer. By setting the variable to 0, instead of a fixed variable, the computer was set to ready even if it was not ready. The previous product to the therac 25 was the therac. The programmer should have used a better system to check the system after each use.
When problems started occurring, investigators assumed that hardware was the cause and focused only on the hardware. There was some base duplication of the software used from the therac20 that carried over to the therac25. Pdf importance of software quality assurance to prevent and. The therac 25 was a radiation therapy machine manufactured by aecl in the 80s, which offered a revolutionary dual treatment mode. Therac25 and industrial design engineering of socio. With the aid of an onboard computer, the device could select multiple treatment table positions and select the typestrength of the energy selected by the operating technician. Nancy leveson and clark turner, the investigation of thetherac25 accidents, computer, 26, 7 july 1993 pp 1841. The therac 25 machine was a stateoftheart linear accelerator developed by the company atomic energy canada limited aecl and a french company cgr to provide radiation treatment to cancer patients. Consider the therac25 failure, in which several deaths occurred because of a software engineering failure. Therac 25 just like any other technology, therac 25 too had its sociotechnical aspects. Feb 17, 2014 the therac 25 accidents form the basis for what is often considered the bestdocumented software safety casestudy available. The series of accidents involving the therac 25 is a good example of exactly this problem. A widely cited 1993 computer article described failures in a software controlled radiation machine that massively overdosed six people in the late 1980s, resulting in serious injury and fatalities.
The therac 25 was built by the atomic energy of canada limited and a french company called cgr. The previous product to the therac 25 was the therac 6, a 6 million electron volt accelerator. Software in the therac 6 and therac 20 was reused in the therac 25. By making the pdp 11 minicomputer an integral part of the therac 25, aecl was able to reduce costs by replacing hardware safety features with software safety features what mistake was not made by aecl, the manufactureer of the therac 25. A final feature was that some of the old software used in therac 6 and therac 20 was used in the therac 25. One programmer, over several years, revised the therac6 software into the therac25 software aecl has not released any information about the programmer or his credentials. Therac 25 ethics case study by ken enstrom on prezi. A case study of the therac 25 chuck huff1 and richard brown2 abstract almost all computer. The therac25 software also contained several userfriendly features. In therac25s case, the players at the three levels had at least two options from which to choose.
The quality assurance manager was apparently unaware that some therac 20 routines were also used in the therac 25. Nancy leveson and clark turner, the investigation of the therac25 accidents, computer, 26, 7 july 1993 pp 1841. The article proceeds to only skim over the plethora of other issues involved and mistakes made in the development process of the therac 25 the next article, an investigation of the therac 25 accidents by nancy leveson, delves much more into detail but it does state that while the software was the lynch pin in the therac 25, it. Between june 1985 and january 1987, the therac25 medical electron accelerator was involved in six massive radiation overdoses. Six accidents resulting in death and serious injury worst in 35year history previous theracs used hardware safety mechanisms. Oct 26, 2015 the therac25 was not a device anyone was happy to see. It was involved in at least six accidents between 1985 and 1987. What happened was the operator using a keypad would select a particular mode.
A bug that was discovered in therac25 was later also found in the therac 20. It was an extremely costly machine with high maintenance needs. First, like the therac 6 and the therac 20, the therac 25 is controlled by a pdp 11. For example if youre a chinese network engineer, and you can avoid it, dont take a job setting up tracking and database of uyghur people. A government report found that a software problem led to an inaccurate tracking calculation that became worse the longer the system operated. For several years and thousands of patients there were no problems. Overdoses occurred primarily because of the bugs in the therac 25 s software and because the manufacturer did not follow proper software engineering practices. And the therac 25 was controlled principally by software. Patriot missile defence system in saudi arabia, failed to detect an attack on an army barracks. Fatal dose radiation deaths linked to aecl computer errors. Therac25 is an extreme example of what can go wrong with software systems, and the devastating consequences that bugs can have on regular people. To be sure, there havent been many, but cases like the therac25 are widely seen as. A bug that was discovered in therac 25 was later also found in the therac 20. Multidatas software allows a radiation therapist to draw on a computer screen the placement of metal shields called blocks.
Therac25 relied on software controls to switch between modes, rather. Although these stories are more extreme than most software bugs engineers will encounter during their careers, they are worth studying for the insights they can offer into software development and deployment. Therac25 case study therac 25 is a radiation therapy machine that was used for treating patients with cancer. The therac6 and therac20 were clinically tested machines with an excellent safety record. Aug 11, 2018 way more people are going to be hurt in the near term by that than by therac25 type mistakes.
The software of the therac 25 also controls the positioning of the turntable, a possible hazard discussed previously, and checks the position of the turntable so that all necessary devices are in place leveson and turner, 1993, p. During the time span of june 1985 to january 1987, it was the source of six fatal or near fatal overdoses. By making the pdp 11 minicomputer an integral part of the therac 25, aecl was able to reduce costs by replacing hardware safety features with softwares safety features some computer experts have spoken out against the conversion to touchscreen voting machines because. What is the name of the programmer who wrote the therac25 software. Virtually all complex software will behave in an unexpected or undesired fashion under some conditions there will. After the therac25 deaths, the fda made a number of adjustments to its policies in an attempt to address the breakdowns in communication and product approval. On the surface, the primary reason that therac 20 killed far fewer people than therac 25 was the fact that therac 20 had hardware interlocks, while therac 25 did not. The main problems in the development of this software had. The series of accidents involving the therac25 is a good example of exactly this problem. My professor investigated the therac25 incident and was a part of the prosecution. Importance of software quality assurance to prevent and reduce software failures in medical devices. Therac 25 case study therac 25 is a radiation therapy machine that was used for treating patients with cancer. The developers of the software werent tempted to introduce the bug.
Its important to note that while the software was the lynch pin in the therac25, it wasnt the root cause. The therac 25 was the most computerized and sophisticated radiation therapy machine of its time. The therac 25 a case study in safety failure radiation therapy machine the most serious computer related accidents to date people were killed references. First, the bug that had caused the problems was an easy bug to fix. While the immediate cause of the deaths was a race condition in the software, it was only capable of causing harm because the hardware safety mechanism had been removed as a costsaving measure, without proper verification that the software was capable of doing the same job. The programmer is responsible for the malfunctioning therac25 software. Therac 25 aecl designed therac 25 to use computer control from the start. The software interlock could fail due to a race condition. The therac 20 has indepen 20 and therac 25 software programs analysis was in the form of a fault tree therac 25 software development and design we know that the software for the therae was devet aecl claims proprietary rights to its software design. An investigation of the therac25 accidents stanford university. These accidents highlighted the dangers of software control of safety critical systems, and.
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