From: msb@sq.sq.com (Mark Brader)
Subject: Therac 25 accidents (Was: Computers killing people? (WAS Re: 9 track data storage???))
Organization: SoftQuad Inc., Toronto, Canada
Date: Tue, 28 Jul 92 06:47:05 GMT

> >  Then I got to wondering if ANYONE had ever been killed by computer. ...
 
> Well, I don't have the references, although any regular reader of comp.risks
> could tell you about the radiation treatment machine (for treating cancer
> patients) that killed several people due to a software bug.
 
Which is exactly the sort of thing that the original poster said he didn't
mean.  But now that it's been mentioned...

> It turned out that the programmers hadn't considered the case were the
> operator might push one button before another operation was finished, as so
> the shield that limits the amount of radiation could be completely out of
> the path, allowing a fatal dose of radiation to hit the patient.

This is only approximately right.  Here's the relevant part of the Boston
Globe article excerpted by Karen Sollins in comp.risks issue 3.09.
The Globe printed it on June 20, 1986.

#  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.

In comp.risks issue 3.11, Jon Jacky cites a story the following day in
the New York Times, and his commentary includes this clarification of
how the error could be physically possible.

|  The New York Times story did not mention the x-ray/electron confusion,
|  and that is the key to this accident.  A modern radiation therapy machine
|  is based on a linear accelerator that produces an electron beam with an
|  energy of 25 MeV or so.  You may direct the electrons directly into the
|  patient (at this energy electrons are ionizing radiation), or, to produce
|  X-rays, you put a heavy metal target in the electron beam, and when the
|  electrons hit the target X-rays come out the other side.  The target is
|  moved in and out of the beam automatically.  Here is my speculation of
|  what happened: I suspect that the current in the electron beam is
|  probably much greater in X-ray mode (because you want similar dose rates
|  in both modes, and the production of X-rays is more indirect).  So when
|  you select X-rays, I'll bet the target drops into place and the beam
|  current is boosted.  I suspect in this case, the current was boosted
|  before the target could move into position, and a very high current
|  electron beam went into the patient.

(Obviously there should have been some form or another of interlock
to prevent this combination of modes.)
-- 
Mark Brader			"'You wanted it to WORK?  That costs EXTRA!'
SoftQuad Inc., Toronto		 is probably the second-place security hole
utzoo!sq!msb, msb@sq.com	 after simple carelessness."	-- John Woods