Grizzly stuff, but at least he kept his sense of humour in the end:
Ray Cox of Tyler, Texas, was scheduled for his ninth radiation treatment for a cancerous tumour previously removed from his back. He was to receive a 180 rad dose from the Therac 25 Radiation Therapy System, which was set at the lower of two energy modes it could deliver. A minicomputer controlled the system through commands entered via a keyboard. The command "E" ordered the lower-energy electron beam mode. The command "X" ordered the higher-energy x-ray mode. The x-ray mode, which had an output of 25MeV (million electron volts), was attenuated to proper dose levels by mechanically inserting a beam-flattening filter into the radiation beam path. This attenuating filter was mounted on a turntable and was put into the beam path by computer control only when the system was in the high-energy x-ray moder. The attenuating filter was not usedd in the lower-energy electron beam mode. The placement of this filter is a key factor in this case.
The system operator had successfully used the Therac 25 more than 500 times. When she mistakenly entered the command "X" instead of "E", she quickly realised her error and hit the up arrow key to move the cursor to the beam selection field and then typed the correct command "E". One of the causal problems was that an 8-second timer was activated in the software by this irregular command sequence, so that any edits made before the 8 seconds elapsed were ignored. So the operator thought that the edit was successful when it was not. After she commanded "B" for beam on, the system delivered the high-power x-ray energy mode without the beam-flattening attenuating filter in place. Because the software was originally written for a different radiation therapy system that did not use a turntable to position the two power mode beams and the attenuating beam flattener, the timer software was left in the code base in error. To further compound the tragedy, the system gave the cryptic error message "Malfunction 54" after the first dose, which was unrecognisable to the operator. (This is a classic case of poor system error messages that have no meaning to typical users but do have meaning to the software designers.) So she subsequently pressed "P" to proceed. The patient screamed in pain after a second massive overdose, but was not heard or seen by the operator because the video and audio links between the control station and the patient were malfunctioning that day.
Again the cryptic error message appeared and the screen displayed no unusual amounts of delivered radiation, so the operator proceeded with a third overdose. The repeated doses of the unattenuated beam resulted in the patient receiving more than 16,000 rads instead of the intended 180 rads. The patient died about five months later from these massive overdoses, but was heard to say before his death, "Captain Kirk must have forgotten to set phasers on stun."
Israelski, EW & Muto, WH.
Human factors risk management as a way to improve medical device safety: A case study of the Therac 25 radiation therapy system. Joint Commission Journal on Quality and Safety, vol 30, no 12, p 689-695
