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Accident Accounts

Isaac Dahl: March 22, 1986

At the East Texas Cancer Center (ETCC) in Tyler, Texas, 33-year old Isaac Dahl was to receive his ninth Therac-25 radiation therapy session after a tumor had been successfully removed from his left shoulder. By this time the Therac 25 had been in successful operation at Tyler for two years, and 500 patients had been treated with it.

The Therac-25 operator left the radiation room to begin the treatment as usual. As she was typing in values, she made a mistake and used the "cursor up" key to correct it. Once the values were correct, she hit the "B" key to begin treatment, but the Therac-25 machine shut down after a moment, and the message "Malfunction 54" showed on the control room monitor. The machine indicated that only 6 of the prescribed 202 units of radiation had been delivered. The screen of the console showed that this shut down was a "treatment pause" which indicated a problem of low priority (since little radiation had been delivered). The operator hit the "P" key to proceed with the therapy, but after a moment of activity, "Malfunction 54" appeared on the Therac control screen again.

The operator was isolated from Dahl because the Therac-25 operates from within a shielded room. On this day at the ETCC, the video monitor was unplugged and the audio monitor was broken, leaving no way for the operator to know what was happening inside. Isaac Dahl had been lying on the treatment table, waiting for the usually uneventful radiation therapy, when he saw a bright flash of light, heard a frying, buzzing sound, and felt a thump and heat like an electric shock.

Dahl, knowing from his previous 8 sessions that this was not normal, began to get up from the treatment table when the second "attempt" at treatment occurred. This time the electric-like jolt hit him in the neck and shoulder. He rolled off the table and pounded on the treatment room door until the surprised Therac-25 operator opened it. Dahl was immediately examined by a physician, who observed reddening of the skin but suspected only an electric shock. Dahl was discharged and told to return if he suffered any further complications.

The hospital physicist was called in to examine the Therac-25, but no problems were found. The Therac-25 was shut down for testing the next day, and two AECL engineers, one from Texas and one from the home office in Canada, spent a day at the ETCC running tests on the machine but could not reproduce a Malfunction 54. The home office engineer explained that the Therac-25 was unable to overdose a patient and also said that AECL had no knowledge of any overexposure accidents by Therac-25 machines. No electrical problems were found with the ETCC's Therac machine, and it was put back into use on April 7, 1986.

Isaac Dahl's condition worsened as he lost the use of his left arm and had constant pain and periodic nausea and vomiting spells. He was later hospitalized for several major radiation-induced symptoms (including vocal cord paralysis, paralysis of his left arm and both legs, and a lesion on his left lung). Dahl died in August of 1986 due to complications from the radiation overdose.