Accident Accounts

Anders Engman: January 17, 1987

Anders Engman was at the Yakima Valley Memorial Hospital on January 17, 1987 to receive three sets of radiation treatment from the Therac-25.

The first two treatments went as planned. Engman received 7 rads (radiation absorbed dose), 4 rads followed by 3 rads of radiation to take pictures of internal structure. The Therac-25 operator then entered the room and used the Therac-25's hand control to verify proper beam alignment on Engman's body. Engman's final dose of the day was to be a moderate 79-rad photon treatment.

The operator pressed a button to command the Therac to move its turntable to the proper position for treatment. Outside the treatment room, the Therac-25's control console read "beam ready," and the operator pressed the "B" key to turn the beam on. The beam activated, but the Therac-25 shut down after about 5 seconds. The console indicated that no dose had been given, so the operator pressed "P" to proceed with the treatment.

The Therac-25 shut down again, listing "flatness" as the reason for treatment pause. Engman said something over the intercom, but the operator couldn't understand him. The operator went into the treatment room to speak with Engman. Engman told the operator that he had felt a "burning sensation" in the chest. The operator's console displayed only the total dose of the two earlier treatments (7 rads).

Later that day, Engman developed a skin burn over the treatment area. Four days later the burn was striped in a manner similar to that of Janis Tilman's burn after she had been treated at Yakima the year before.

AECL investigated the accident. All users were again told to visually confirm turntable setting before proceeding with any treatment. Given the information, it was suspected that the electron beam had come on when the turntable was in the field light position. AECL could not reproduce the error.

Later that week, AECL sent an engineer to Yakima to investigate. The hospital physicist had also been running tests. They eventually discovered a software flaw and fixed it. AECL engineers estimated that Engman received between 8,000 and 10,000 rads instead of the prescribed 86.

Anders Engman died in April 1987. He had been suffering from a terminal form of cancer before the Therac accident, but it was determined that his death was primarily caused by complications related to the radiation overdose, not the cancer.