Therac-25 Accidents What was Therac-25? Who developed it? released in 1983 computerized radiation therapy machine used to treat cancer patients Who developed it? Atomic Energy of Canada, Ltd and GCR (French-based company) What were the key advances of it over its predecessors (Therac-6 and Therac-20)? move to more complete software-based control faster set-up safety checks were now controlled by software (instead of mechanical interlocks)

Therac 25

Therac-25 Accidents (What Happened?) Massively overdosed patients at least 6 times (3 died, 3 seriously disabled) June 1985 Marietta, Ga (Linda Knight, 61) July 1985 Hamilton, Ont (Donna Gartner, 40) December 1985 Yakima, Wash (Janis Tilman) March 1986 Tyler, Tx (Isaac Dahl, 33) April 1986 Tyler, Tx (Daniel McCarthy) January 1987 Yakima, Wash (Anders Engman)

Therac-25 Accidents (Example of Contributing UI Problems) The technician got the patient set up on the table, and went down the hall to start the treatment. She sat down at the terminal: hit “x” to start the process she realized she made a mistake, since she needed to treat the patient with the electron beam, not the X-ray beam hit the “Up” arrow, selected the “Edit” command, hit “e” for electron beam, and hit “enter” (signifying she was ready to start treatment) the system showed a “beam ready” prompt she hit “b” to turn the beam therapy on the system gave her an error message (Malfunction 54) she overrode the error message It turns out that the UI showed that it was in electron mode but it was actually in a “hybrid” mode  delivered more than 125 times the normal dose to the patient

Therac-25 Accidents (What Were the Problems?) simple programming errors inadequate safety engineering ignored the software risks (almost no unit or integration testing at all) operators were told it was impossible to overdose a patient poor HCI design lax culture of safety in the manufacturing co. problems were not reported quickly to manufacturer or FDA prompted a 1990 federal law