An Investigation of the Therac-25 Accidents -- Part V

Nancy Leveson, University of Washington
Clark S. Turner, University of California, Irvine

Reprinted with permission, IEEE Computer, Vol. 26, No. 7, July 1993, pp. 18-41.

Software engineering. The Therac-25 accidents were fairly unique in having software coding errors involved -- most computer-related accidents have not involved coding errors but rather errors in the software requirements such as omissions and mishandled environmental conditions and system states. Although using good basic software-engineering practices will not prevent all software errors, it is certainly required as a minimum. Some companies introducing software into their systems for the first time do not take software engineering as seriously as they should. Basic software-engineering principles that apparently were violated with the Therac-25 include:

In addition, special safety-analysis and design procedures must be incorporated into safety-critical software projects. Safety must be built into software, and, in addition, safety must be assured at the system level despite software errors.[9,10] The Therac-20 contained the same software error implicated in the Tyler deaths, but the machine included hardware interlocks that mitigated its consequences. Protection against software errors can also be built into the software itself.

Furthermore, important lessons about software reuse can be found here. A naive assumption is often made that reusing software or using commercial off-the-shelf software increases safety because the software has been exercised extensively. Reusing software modules does not guarantee safety in the new system to which they are transferred and sometimes leads to awkward and dangerous designs. Safety is a quality of the system in which the software is used; it is not a quality of the software itself. Rewriting the entire software to get a clean and simple design may be safer in many cases.

Taking a couple of programming courses or programming a home computer does not qualify anyone to produce safety-critical software. Although certification of software engineers is not yet required, more events like those associated with the Therac-25 will make such certification inevitable. There is activity in Britain to specify required courses for those working on critical software. Any engineer is not automatically qualified to be a software engineer -- an extensive program of study and experience is required. Safety-critical software engineering requires training and experience in addition to that required for noncritical software.

Although the user interface of the Therac-25 has attracted a lot of attention, it was really a side issue in the accidents. Certainly, it could have been improved, like many other aspects of this software. Either software engineers need better training in interface design, or more input is needed from human factors engineers. There also needs to be greater recognition of potential conflicts between user-friendly interfaces and safety. One goal of interface design is to make the interface as easy as possible for the operator to use. But in the Therac-25, some design features (for example, not requiring the operator to reenter patient prescriptions after mistakes) and later changes (allowing a carriage return to indicate that information has been entered correctly) enhanced usability at the expense of safety.

Finally, not only must safety be considered in the initial design of the software and it operator interface, but the reasons for design decisions should be recorded so that decisions are not inadvertently undone in future modifications.

User and government oversight and standards. Once the FDA got involved in the Therac-25, their response was impressive, especially considering how little experience they had with similar problems in computerized medical devices. Since the Therac-25 events, the FDA has moved to improve the reporting system and to augment their procedures and guidelines to include software. The problem of deciding when to forbid the use of medical devices that are also saving lives has no simple answer and involves ethical and political issues that cannot be answered by science or engineering alone. However, at the least, better procedures are certainly required for reporting problems to the FDA and to users.

The issues involved in regulation of risky technology are complex. Overly strict standards can inhibit progress, require techniques behind the state of the art, and transfer responsibility from the manufacturer to the government. The fixing of responsibility requires a delicate balance. Someone must represent the public's needs, which may be subsumed by a company's desire for profits. On the other hand, standards can have the undesirable effect of limiting the safety efforts and investment of companies that feel their legal and moral responsibilities are fulfilled if they follow the standards.

Some of the most effective standards and efforts for safety come from users. Manufacturers have more incentive to satisfy customers than to satisfy government agencies. The American Association of Physicists in Medicine established a task group to work on problems associated with computers in radiation therapy in 1979, long before the Therac-25 problems began. The accidents intensified these efforts, and the association is developing user-written standards. A report by J.A. Rawlinson of the Ontario Cancer Institute attempted to define the physicist's role in assuring adequate safety in medical accelerators:

We could continue our traditional role, which has been to provide input to the manufacturer on safety issues but to leave the major safety design decisions to the manufacturer. We can provide this input through a number of mechanisms. . . These include participation in standards organizations such as the IEC [Interna-tional Electrotechnical Commission], in professional association groups . . . and in accelerator user groups such as the Therac-25 user group. It includes also making use of the Problem Reporting Program for Radiation Therapy Devices . . . and it includes consultation in the drafting of the government safety regulations. Each of these if pursued vigorously will go a long way to improving safety. It is debatable however whether these actions would be sufficient to prevent a future series of accidents.

Perhaps what is needed in addition is a mechanism by which the safety of any new model of accelerator is assessed independently of the manufacturer. This task could be done by the individual physicist at the time of acceptance of a new machine. Indeed many users already test at least the operation of safety interlocks during commissioning. Few however have the time or resources to conduct a comprehensive assessment of safety design.

A more effective approach might be to require that prior to the use of a new type of accelerator in a particular jurisdiction, an independent safety analysis is made by a panel (including but not limited to medical physicists). Such a panel could be established within or without a regulatory framework.[1]

It is clear that users need to be involved. It was users who found the problems with the Therac-25 and forced AECL to respond. The process of fixing the Therac-25 was user driven -- the manufacturer was slow to respond. The Therac-25 user group meetings were, according to participants, important to the resolution of the problems. But if users are to be involved, then they must be provided with information and the ability to perform this function. Manufacturers need to understand that the adversarial approach and the attempt to keep government agencies and users in the dark about problems will not be to their benefit in the long run.

The US Air Force has one of the most extensive programs to inform users. Contractors who build space systems for the Air Force must provide an Accident Risk Assessment Report (AFAR) to system users and operators that describes the hazardous subsystems and operations associated with that system and its interfaces. The AFAR also comprehensively identifies and evaluates the system's accident risks; provides a means of substantiating compliance with safety requirements; summarizes all system-safety analyses and testing performed on each system and subsystem; and identifies design and operating limits to be imposed on system components to preclude or minimize accidents that could cause injury or damage.

An interesting requirement in the Air Force AFAR is a record of all safety-related failures or accidents associated with system acceptance, test, and checkout, along with an assessment of the impact on flight and ground safety and action taken to prevent recurrence. The AFAR also must address failures, accidents, or incidents from previous missions of this system or other systems using similar hardware. All corrective action taken to prevent recurrence must be documented. The accident and correction history must be updated throughout the life of the system. If any design or operating parameters change after government approval, the AFAR must be updated to include all changes affecting safety.

Unfortunately, the Air Force program is not practical for commercial systems. However, government agencies might require manufacturers to provide similar information to users. If required for everyone, competitive pressures to withhold information might be lessened. Manufacturers might find that providing such information actually increases customer loyalty and confidence. An emphasis on safety can be turned into a competitive advantage.

Most previous accounts of the Therac-25 accidents blamed them on a software error and stopped there. This is not very useful and, in fact, can be misleading and dangerous: If we are to prevent such accidents in the future, we must dig deeper. Most accidents involving complex technology are caused by a combination of organizational, managerial, technical, and, sometimes, sociological or political factors. Preventing accidents requires paying attention to all the root causes, not just the precipitating event in a particular circumstance.

Accidents are unlikely to occur in exactly the same way again. If we patch only the symptoms and ignore the deeper underlying causes or we fix only the specific cause of one accident, we are unlikely to prevent or mitigate future accidents. The series of accidents involving the Therac-25 is a good example of exactly this problem: Fixing each individual software flaw as it was found did not solve the device's safety problems. Virtually all complex software will behave in an unexpected or undesired fashion under some conditions -- there will always be another bug. Instead, accidents must be understood with respect to the complex factors involved. In addition, changes need to be made to eliminate or reduce the underlying causes and contributing factors that increase the likelihood of accidents or loss resulting from them.

Although these accidents occurred in software controlling medical devices, the lessons apply to all types of systems where computers control dangerous devices. In our experience, the same types of mistakes are being made in nonmedical systems. We must learn from our mistakes so we do not repeat them.


Ed Miller of the FDA was especially helpful, both in providing information to be included in this article and in reviewing and commenting on the final version. Gordon Symonds of the Canadian Government Health Protection Branch also reviewed and commented on a draft of the article. Finally, the referees, several of whom were apparently intimately involved in some of the accidents, were also very helpful in providing additional information about the accidents.


The information in this article was gathered from official FDA documents and internal memos, lawsuit depositions, letters, and various other sources that are not publicly available. Computer does not provide references to documents that are unavailable to the public.

1. J.A. Rawlinson, "Report on the Therac-25," OCTRF/OCI Physicists Meeting, Kingston, Ont., Canada, May 7, 1987.

2. F. Houston, "What Do the Simple Folk Do?: Software Safety in the Cottage Industry," IEEE Computers in Medicine Conf., 1985.

3. C.A. Bowsher, "Medical Devices: The Public Health at Risk," US Gov't Accounting Office Report GAO/T-PEMD-90-2, 046987/139922, 1990.

4. M. Kivel, ed., Radiological Health Bulletin, Vol. XX, No. 8, US Federal Food and Drug Administration, Dec. 1986.

5. Medical Device Recalls, Examination of Selected Cases, GAO/PEMD-90-6, 1989.

6. E. Miller, "The Therac-25 Experience," Proc. Conf. State Radiation Control Program Directors, 1987.

7. W.D. Ruckelshaus, "Risk in a Free Society," Risk Analysis, Vol. 4, No. 3, 1984, pp. 157-162.

8. E.A. Ryder, "The Control of Major Hazards: The Advisory Committee's Third and Final Report," Transcript of Conf. European Major Hazards, Oyez Scientific and Technical Services and Authors, London, 1984.

9. N.G. Leveson, "Software Safety: Why, What, and How," ACM Computing Surveys, Vol. 18, No. 2, June 1986, pp. 25-69.

10. N.G. Leveson, "Software Safety in Embedded Computer Systems," Comm. ACM, Feb. 1991, pp. 34-46.

Nancy G. Leveson is Boeing professor of Computer Science and Engineering at the University of Washington. Previously, she was a professor in the Information and Computer Science Department at the University of California, Irvine. Her research interests are software safety and reliability, including software hazard analysis, requirements specification and analysis, design for safety, and verification of safety. She consults worldwide for industry and government on safety-critical systems.

Leveson received a BA in mathematics, an MS in operations research, and a PhD in computer science, all from the University of California at Los Angeles. She is the editor in chief of IEEE Transactions on Software Engineering and a member of the board of directors of the Computing Research Association.

Clark S. Turner is seeking his PhD in the Information and Computer Science Department at the University of California, Irvine, studying under Nancy Leveson. He is also an attorney admitted to practice in California, New York, and Massachusetts. His interests include risk analysis of safety-critical software systems and legal liability issues involving unsafe software systems.

Turner received a BS in mathematics from King's College in Pennsylvania, an MA in mathematics from Pennsylvania State University, a JD from the University of Maine, and an MS in computer science from the University of California, Irvine.

Readers can contact Leveson at the Department of Computer Science and Engineering, FR-35, University of Washington, Seattle, WA 98195, e-mail; or Turner at the Information and Computer Science Department, University of California, Irvine, Irvine, CA 92717, e-mail