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Accident Accounts

Daniel McCarthy : April 11,1986

Technicians could find nothing wrong with the Therac-25 unit at the East Texas Cancer Center (ETCC), after the "Malfunction 54" incident that had injured Isaac Dahl. The machine was reinstated.

Four days later, Daniel McCarthy was being treated for skin cancer on the side of his face. The same Therac operator who had treated Isaac Dahl was treating McCarthy. As the operator prepared to administer the Therac treatment from the control room, she used the "cursor up" key to correct an error in the treatment settings. She then began treatment using the "B" key.

The Therac-25 shut down within a few seconds, making a noise audible through the newly repaired intercom. The Therac monitor read "Malfunction 54." The operator rushed into the treatment room and found McCarthy moaning for help. He said that his face was on fire. The hospital physicist was called. McCarthy said that something had hit the side of his face, and that he had seen a flash of light and heard a sizzling sound.

After this second accident at the hospital, the ETCC physicist took the Therac-25 out of service and called AECL. He worked with the Therac operator who had been administering treatment to both Dahl and McCarthy when the accidents occurred. The physicist and the operator were eventually able to reproduce a Malfunction 54. They found that the malfunction occurred only if the Therac-25 operator rapidly corrected a mistake.

The ETCC physicist notified AECL of this discovery and AECL was eventually able to reproduce the error. AECL advised Therac-25 users to physically remove the up-arrow key as a short-term solution. AECL also filed a report with the United States FDA as required by law, and began work on fixing the software bug.

The FDA worked in conjunction with AECL to identify the software problem and correct it. The FDA also requested that AECL change the machine in several other ways to clarify the meaning of malfunctions error messages and to shut down treatment after any single large radiation pulse or interrupted treatment so that multiple overdoses were less likely.

Over the next three weeks Daniel McCarthy became very disoriented and then fell into a coma. He had a fever as high as 104 degrees and had suffered neurological damage. He died on May 1, 1986.