6.1 Case analysis module: therac-25  (Page 3/3)

Scenario: You are an engineer working for AECL sent to investigate an alleged overdosing incident at theOntario Cancer Foundation in Hamilton. Ontario. The following is the description provided to you of what happened:

On July 26, 1985, a forty-year old patient came to the clinic for her twenty-fourth Therac-25 treatment forcarcinoma of the cervix. The operator activated the machine, but the Therac shut down after five seconds with an HTILT errormessage. The Therac-25’s console display read NO DOSE and indicated a TREATMENT PAUSE

Since the machine did not suspend and the control display indicated no dose was delivered to the patient, theoperator went ahead with a second attempt at a treatment by pressing the Proceed Command Key, expecting the machine to deliverthe proper dose this time. This was standard operating procedure, and Therac-25 operators had become accustomed to frequentmalfunctions that had no untoward [bad] consequences for thepatient. Again the machine shut down in the same manner. The operator repeated this process four times after the originalattempt—the display showing NO DOSE delivered to the patient each time. After the fifth pause, the machine went into treatmentsuspend, and a hospital service technician was called. The technician found nothing wrong with the machine. According to aTherac-25 operator, this scenario also was not unusual.

After treatment, the patient complained of a burning sensation, described as an “electric tingling shock” to thetreatment area in her hip….She came back for further treatment on July 29 and complained of burning, hip pain, and excessive swellingin the region of treatment. The patient was hospitalized for the condition on July 30, and the machine was taken out of service.(Description taken from Nancy Leveson, Safeware, pp 523-4)

You give the unit a thorough examination and are able to find nothing wrong. Working with the operator, you tryto duplicate the treatment procedure of July 26. Nothing out of the ordinary happens. Your responsibility is to make a recommendationto AECL and to the Ontario Cancer Foundation. What will it be?

1. Identify key components of the STS

2. Specify the problem:

2a. Is the problem a disagreement on facts? What are the facts? What are cost and time constraints onuncovering and communicating these facts?

2b. Is the problem a disagreement on a critical concept? What is the concept? Can agreement be reached byconsulting legal or regulatory information on the concept? (For example, if the concept in question is safety, can disputantsconsult engineering codes, legal precedents, or ethical literature that helps provide consensus? Can disputants agree on positive andnegative paradigm cases so the concept disagreement can be resolved through line-drawing methods?

2c. Use the table to identify and locate value conflicts within the STS. Can the problem be specified as amismatch between a technology and the existing STS, a mismatch within the STS exacerbated by the introduction of the technology,or by overlooked results?

3. Develop a general solution strategy and then brainstorm specific solutions:

3a. Is problem one of integrating values, resolving disagreements, or responding to situationalconstraints?

3b. If the conflict comes from a value mismatch, then can it be solved by modifying one or more of thecomponents of the STS? Which one?

4. Test solutions:

5. Implement solution over feasibility constraints

Appendix

Therac decision point presentation

Therac-25 decision point

Therac-25 case summary

Free and informed consent, safety, and dimensions of risk

References

• Nancy G. Leveson. Safeware: System Safety and Computers . New York: Addison-Wesley Publishing Company, 515-553.
• Nancy G. Leveson and Clark S. Turner. An Investigation of the Therac-25 Accidents. Computers, Ethics, and Social Values , Johnson, D.G. and Nissenbaum, H., eds.: 478.
• Nancy G. Leveson and Clark S. Turner. An Investigation of the Therac-25 Accidents. IEEE Computer . 26(7): 18-41, July 1993.
• Computing Cases website. See above link. Materials on case including interviews and supporting documents.
• Sara Baase. A Gift of Fire: Social, Legal, and Ethical Issues in Computing . Upper Saddle River, NJ: Prentice-Hall, 125-129.
• Chuck Huff. Good Computing: A Virtue Approach to Computer Ethics . Draft for course CS-263. June 2005.
• Chuck Huff and Richard Brown. Integrating Ethics into a Computing Curriculum: A Case Study of the Therac-25. Available at Computing Cases website. See above link.
• For time line see: http://computingcases.org/case_materials/therac/supporting_docs/therac_resources/Timeline.html
• Leveson in Safeware provides an excellence summary of the literature on system safety. For two further excellent resources consult the next two references.
• Perrow, C. (1984) Normal Accidents: Living with high-risk technologies. Basic Books, NY,NY.
• Reason, J. (1990/1999) Human Error Cambridge University Press: London.