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Therac -25

Therac -25. CS4001 Kristin Marsicano. Therac-25 Overv iew. 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.

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Therac -25

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  1. Therac-25 CS4001 Kristin Marsicano

  2. 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?

  3. 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

  4. Was Therac-25 reliable?

  5. 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!

  6. Under what conditions did the lethal doses occur?

  7. 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; 1-255 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!

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

  9. 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

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