1. Reliability and Safety
Therac-25

2. Case Study - Therac-25
Software controlled radiation therapy machine used to treat people with cancer
Problems:
Massive overdoses administered
Repeated overdoses due to faulty display
Death of patients
Operated in dual machine mode - electron beam or x-ray photon beam

3. Why Did Problems Arise? Lapses in good safety design
Insufficient testing
Bugs in software that controlled machines
Inadequate system of reporting and investigating accidents and deaths

4. Specific Problems
Some hardware safety features were eliminated in newer models
Software used was assumed correct form older systems
Malfunctioned frequently requiring operator intervention
Weakness in design of operator interface
Inadequate explanation of error messages if any

5. Specific Problems continued…
Machine allowed one-key intervention versus automatic shutdown
Inadequate documentation
Poor test plan

6. Software Errors - Bugs Fatal error was a simple fix
Fixes are complex, expensive, and prevents use of machine while fixing
Bugs
can be intermittent and hard to detect
importance of self checking
importance of using good programming techniques

7. Overconfidence Leaving out changes that are necessary (by designers)
Ignoring error messages (by technicians)
Not using backup devices (video or audio communication with patient)

8. Conclusion and Perspective
Irresponsibility leads to criminal charges
Responsibility leads to merit awards
Importance of good software development
Consequences of carelessness, cutting corners, unprofessional work, or attempts to avoid responsibility
Lack of appreciation for risks
Poor training
Overconfidence in systems can prove deadly