CS 335 - 2/5/13

Therac - 25

Radiation Treatment Machines

Updated model of older machines

Used old software to save production costs
 - older version had hardware safety control

Improperly delivered radiation doses

Problem was caused by a race condition
  - 2 threads update a value at the same time.

  Lock:   initialize to 0


  while (lock == 1)
     ;
  lock = 1;
  svalue changed
  perform some tasked
  svalue get reset
  lock = 0;


Treatment centers were reporting errors
  - manufacturer didn't believe the errors
     existed

June 85 - first incident reported

July 85

Dec 85

Feb 87, FDA declared Thereac 25 defective

July 87, A corrective action plan is produced



In order for a moral agent to be responsible for a
  harmful event, two conditions must hold

- Causal Condition: the actions or inactions of the
     agent must have caused harm.

- Mental Condition: the actions or inactions must 
     have been intended or willed by the agent.