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 http://sunnyday.mit.edu/papers/therac.pdf

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 http://sunnyday.mit.edu/papers/therac.pdf

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 http://sunnyday.mit.edu/papers/therac.pdf