Therac-25 CS4001 Kristin Marsicano

Therac-25 Overview  What was the Therac-25?  How did it relate to previous models? In what ways was it similar/different?  Was the Therac-25 reliable?

Therac-25 Overview  Linear accelerator used to create high-energy electron beams to treat shallow tumors and x-ray beams to reach deeper tumors  Differed from Therac-6 and Therac-20:  computer was coupled with the system such that the hardware could not function without the computer (e.g. turntable set up)  relied on the computer for safety checks; did not include the hardware safety features of previous models (which allowed for cost savings)  Similar to Therac-6 and Therac-20:  Shared a common code base  Used a computer to augment user

Was Therac-25 reliable?

 Worked tens of thousands of times before overdosing anyone  Over course of 20 months (June 1985-July 1987) it administered massive overdoses to 6 patients, resulting in 3 deaths  Was notorious for displaying non-descript errors that had no negative side-effects (e.g. up to 40 times a day) Do not confuse reliability with safety!

Under what conditions did the lethal doses occur?

 Fast-typing operators  Race condition between magnet positioning and screen edits  Software relies on positioning of cursor to determine if edits have been made  Change from X-Ray mode to Electron mode made before magnets finish moving; software doesn’t check cursor position until after magnets have stopped  Set button  Race condition between “gun ready” variable, gun positioning, and “Set” button  0 means gun is ready and will fire; means not ready; increments as gun is moving and rolls over as necessary (which means it might be 0 when the gun is not really ready!

What parties were involved?  Patients and their families  AECL (maker of the machine)  Developers  Hospital where machine was used (and the technicians)

AECL Mistakes  Assumed error was only in software  Did not design system to be fail-safe (fail-safe means no single point of failure will lead to catastrophe); instead the Therac-25 relied 100% on the software to ensure safety of the system  Lack of software and hardware devices to detect and communicate an overdoes  Presumed correctness of reused code; assumed there were no errors in the previous code base when indeed there were  Management allowed the software to be developed without adequate documentation (e.g. no user manual for error codes)  Did not communicate fully with its customers with regards to the accidents