1. Therac-25: A Lesson Learned
A medical device for combating cancerous tumours
Developed by AECL and CRG
A “superior” model in comparison to its predecessors
Medically superior
Economically superior
Much more software-reliant than previous models
Shifting responsibilities from hardware to software
Inherited software components from previous models
Therac-6 and Therac-20
George Howarth, Yi Xuehui

2. Therac-25: A Lesson Learned
Software contained major bugs
The “cursor-up” issue
The “Class3” variable bug
Software was never truly “fixed”
Developers kept patching bugs
Poor design
No formal documentation or test plans were created
Could not prove that the software met the requirements
Some tests were not even executed correctly!
Also a lack of regression testing
George Howarth, Yi Xuehui

3. Therac-25: A Lesson Learned
Naïve attitudes towards software
Operators accepted malfunctions as normal behaviour
Developers denied software was at fault
Technicians never investigated malfunctions
Software faults never seriously considered
...until it was too late
Attitudes towards software changed
FDA conceded that software could not be fully relied upon
FDA told AECL to rigorously test software modifications
George Howarth, Yi Xuehui