1. 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)

2. Therac 25

3. 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)

4. 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

5. 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