Humble Pi: When Math Goes Wrong in the Real World

by Matt Parker

On January 17, 1987, in Yakima Valley Memorial Hospital in Washington (now Virginia Mason Memorial), a patient was due to receive eighty-six rads from a Therac-25 machine (the rad is an antiquated unit of radiation absorption). Before the patient was to receive the dose of X-rays, however, the metal target and collimator had been moved out of the way so the machine could be aligned using normal visible light. They were not put back. The operator hit the Set button on the machine at the exact moment Class3 had rolled over to zero, Chkcol was not run, and the electron beam fired with no target or collimator in place. Instead of eighty-six rads, the patient may have received around 8,000 to 10,000 rads. He died in April that year from complications because of this radiation overdose.