Nic McPhee
University of Minnesota, Morris
The following is an excerpt from the comp.risks archives. I've copied it to my web site because many students were having trouble downloading the entire issue on the weaker computers on our campus. If you stumbled across this from elsewhere, I'd encourage you to go the actual archive site (http://catless.ncl.ac.uk/Risks) for this and other comp.risks materials.
My particular concern in the story that follows is that the designers and programmers probably can't know ALL the conditions for which to check. We all know that complete testing of complex systems is impossible. All too often we are put into a position of trading risks and benefits, and at least the risks (as in this case) are not and cannot be known completely.
Of course, another difficult question here is who is responsible for what happened and what should be done about it. Clearly for those three patients involved and their families and friends no amount of placing responsibility, punishment, or compensation can make up for what was done to them. Karen Sollins
A series of accidental radiation overdoses from identical cancer therapy machines in Texas and Georgia has left one person dead and two others with deep burns and partial paralysis, according to federal investigators.
Evidently caused by a flaw in the computer program controlling the highly automated devices, the overdoses - unreported until now - are believed to be the worst medical radiation accidents to date.
The malfunctions occurred once last year and twice in March and April of this year in two of the Canadian-built linear accelerators, sold under the name Therac 25.
Two patients were injured, one who died three weeks later, at the East Texas Cancer Center in Tyler, Texas, and another at the Kennestone Regional Oncology Center in Marietta, Ga.
The defect in the machines was a "bug" so subtle, say those familiar with the cases, that although the accident occurred in June 1985, the problem remained a mystery until the third, most serious accident occurred on April 11 of this year.
Late that night, technicians at the Tyler facility discovered the cause of that accident and notified users of the device in other cities.
The US Food and Drug Administration, which regulates medical devices, has not yet completed its investigation. However, sources say that discipline or penalty for the manufacturer is unlikely.
Modern cancer radiation treatment is extremely safe, say cancer specialists. "This is the first time I've ever heard of a death" from a therapeutic rediation accident, said FDA official Edwin Miller. "There have been overtreatments to various degrees, but nothing quite as serious as this that I'm aware of."
Physicians did not at first suspect a rediation overdose because the injuries appeared so soon after treatment and were far more serious than an overexposure would ordinarily have produced.
"It was certainly not like anything any of us have ever seen," said Dr. Kenneth Haile, director of radiation oncology of the Kennestone radiation facility. "We had never seen an overtreatment of that magnitude."
Estimates are that the patients received 17,000 to 25,000 rads to very small body areas. Doses of 1,000 rads can be fatal if delivered to the whole body.
The software fault has since been corrected by the manufacturer, according to FDA and Texas officials, and some of the machines have been retured to service.
... (description of the accidents)
The Therac 25 is designed so that the operator selects either X-ray or electron-beam treatment, as well as a series of other items, by typing on a keyboard and watching a video display screen for verification of the orders.
It was revealed that if an extremely fast-typing operater inadvertently selected the X-ray mode, then used an editing key to correct the command and select the electron mode instead, it was possible for the computer to lag behind the orders. The result was that the device appeared to have made the correct adjustment but in fact had an improper setting so it focussed electrons at full power to a tiny spot on the body.
David Parnas, a programming specialist at Queens University in Kingston, Ontario, said that from a description of the problem, it appeared there were two types of programming errors.
First, he said, the machine should have been programmed to discard "unreasonable" readings - as the injurious setting presumably would have been. Second, said Parnas, there should have been no way for the computer's verifications on the video screen to become unsynchronized from the keyboard commands.
[This story was also reported by Jim Kirby. It is very rare that I get MULTIPLE copies of such a report. Statistically, that suggests that there must be many things that never get reported... PGN]
End of Therac-25 accidents
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mcphee@cda.morris.umn.eduUnless otherwise indicated, all material in these Web pages Copyright Nic McPhee, Thursday, October 2, 1997. These materials may be redistributed provided that this notice remain attached and intact. Under no circumstances can this material be sold or distributed for compensation of any kind without prior written permission from the author.