Reliability%20and%20Safety

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