THERAC 25

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THERAC 25

• RAM MOHAN BOPPUDI
• EE 585 FAULT TOLERANT
• COMPUTING

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BACKGROUND

• Therac 25 was a medical linear accelerator , a
  linac developed by Atomic Energy Of Canada Ltd.
• Therac 25 is a radio therapy machine used to
  destroy tumours using high energy beams.
• 11 Therac 25s were installed 5 in US , 6 in
  Canada.

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Background ( contd)

• Therac 25 was derived from its previous version
  Therac 6 and Therac 20.
• Differences from Therac 20
• - Uses double pass technique which is absent
  in previous versions
• - Software is responsible for safety
• - Hardware safety interlocks removed
• - Less space and economic

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Therac-25 Turntable
Field Light Mirror
Counterweight
Beam Flattener (X-ray Mode)
Turntable
Scan Magnet (Electron Mode)
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Accidents

• 3 June 1985 patient at Marietta GA received
  overdose
• 26 July 1985 Hamilton ONT patient severely
  burned , died November 1985
• December 1985 patient in Yakima Wa receives
  overdose
• 21 March 1986- Tyler TX accident
• 11 March 1986 2nd Tyler TX accident
• 17 January 1987- Second Yakima WA Accident

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Why?

• The turntable was in the wrong position.
• Patients were receiving x-rays without
  beam-scattering.
• No hardware safety interlocks
• Non descriptive error messages
• User override able error modes
• Software designed by only one person

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Cost of the Bug

• To users (patients)
• Four deaths, two other serious injuries.
• To developers (AECL)
• One lawsuit
• Settled out of court
• Time/money to investigate and fix the bugs
• To product owners (11 hospitals)
• System downtime

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Corrective Action Plan

• Numerous hardware and software changes
• All interruptions related to dosimetry not
• continuable
• independent hardware software shutdowns
• potentiometer on turntable
• hardware interlocks
• dead man switch motion enable
• Fix documentation, messages, user manuals

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REFERENCES

• An investigation of the Therac-25 Accidents
• Nancy Leveson
• Clark S.Turner
• Wikipedia on Therac 25