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suffered acute pain and steadily lost bodily functions until he died from complications
of the overdose five months later.
SECOND AECL INVESTIGATION, MARCH 1986
After the accident, the ETCC shut down its Therac-25 and notified AECL. AECL sent
out two engineers to examine the system. Try as they might, they could not reproduce
Malfunction 54. They told the physicians it was impossible for the Therac-25 to overdose
a patient, and they suggested that the patient may have received an electrical shock due
to a fault in the hospital's electrical system.
The ETCC checked out the electrical system and found no problems with it. After
double-checking the linear accelerator's calibration, they put the Therac-25 back into
service.
TYLER, TEXAS, APRIL 1986
The second Tyler, Texas, accident was virtually a replay of the prior accident at ETCC.
The same technician was in control of the Therac-25, and she went through the same
process of entering X-ray when she meant electron beam, then going back and correcting
her mistake. Once again, the machine halted with a Malfunction 54 shortly after she
activated the electron beam. This time, however, the intercom was working, and she
rushed to the accelerator when she heard the patient moan. There was nothing she could
do to help him. The patient had received a massive dose of radiation to his brain, and he
died three weeks later.
After the accident, ETCC immediately shut down the Therac-25 and contacted
AECL again.
YAKIMA, WASHINGTON, JANUARY 1987
A second patient was severely burned by the Therac-25 at Yakima Valley Memorial
Hospital under circumstances almost identical to those of the December 1985 accident.
Four days after the treatment, the patient's skin revealed a series of parallel red stripes—
the same pattern that had perplexed the radiation staff in the case of the previous patient.
This time, the staff members were able to match the burns to the slots in the Therac-25's
blocking tray. The patient died three months later.
THERAC-25 DECLARED DEFECTIVE, FEBRUARY 1987
On February 10, 1987, the FDA declared the Therac-25 to be defective. In order for the
Therac-25 to gain back FDA approval, AECL had to demonstrate how it would make the
system safe. Five months later, after five revisions, AECL produced a corrective action
plan that met the approval of the FDA. This plan incorporated a variety of hardware
interlocks to prevent the machine from delivering overdoses or activating the beam when
the turntable was not in the correct position.
8.5.3 Software Errors
In the course of investigating the accidents, AECL discovered a variety of hardware and
software problems with the Therac-25. Two of the software errors are examples of race
 
 
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