From: frank@rover.uchicago.edu (Frank - Hardware Hacker - Borger)
Subject: Re: Therac 25 accidents (Was: Computers killing people? (WAS Re: 9 track data storage???))
News-Software: VAX/VMS VNEWS 1.41    
Reply-To: frank@rover.uchicago.edu
Organization: Joint Center for Radiation Therapy
Date: Tue, 28 Jul 1992 14:59:00 GMT

In article <1992Jul28.064705.28380@sq.sq.com>, msb@sq.sq.com (Mark Brader) writes...
>|  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.)
>-- 
	There was a second problem. To get uniform distribution in electron
	mode, the electron beam is scanned just like the electron beam in
	the CRT tube you are reading this on. Note that larger tv's have
	shutdown circuits that turn off the beam if scanning stops since you
	don't want to blow your CRT tube.

	The software bug also did not turn on the electron beam scanning, so
	the unscanned beam with an intensity about 100 times normal, was not
	scanned, resulting in another 100 times or so intensity factor.

	The predecessor to the Therac 25, the Therac 20 also has electron beam
	scanning, but it is simple sawtooth type scanning. Separate electronic
	circuits continuously monitor this pattern to make sure the scanning
	is running, TOTALLY INDEPENDANT OF THE COMPUTER.

	The newer, fancier machine, allowed fancy scanning on a point by point
	basis, with a look up table determining each and every point in the
	scan. Unfortunately, when they designed this, they dropped the older
	monitoring circuits. (ooops.)

	Things then got even worse. Basically because this was a brand new
	machine, (first or second one produced by Atomic Energy of Canada, ltd,)
	installed in a brand spanking new department, with hospital adminis-
	trators very interested in starting to get income from a ballpark $10 
	million investment. As a result, the machine was pressed into use before
	proper shakedown was done, in a department still coping with the work
	of getting everything going at once.

	1/ The operator turns on the machine, and it immediately delivers dose
	   about 5 to 10k times greater than normal. Circuits in the machine do
	   detect the problem, and shut down the machine, but the computer dis-
	   plays the cryptic error message, "Malfunction 54." (Stupid 
	   programming error #43, not having clear error messages.)

	2/ The operator didn't know what malfunction 54 was, (and had not been
	   properly instructed as to what to do when things go wrong,) so
	   she kept trying to run the machine again and again, zapping the
	   patient several times.

	3/ Because the closed circuit tv system wasn't working, (even though
	   cameras are mandated by all states,) the tech didn't see the patient
	   jump when they essentially received a 25 million volt electric shock.

	4/ Because the intercom system was also not working, the tech didn't
	   hear the patient yelling, and only knew something was wrong when she
	   heard the patient pounding on the door trying to get out.

	Moral of the story.

	1/ Don't rely on the computer too much.

	2/ Be very careful of what your error messages say.

	3/ Don't install brand new machine number 1 in a brand spanking new
	   radiation therapy department.

	4/ Train your people well in what to do if something goes wrong.

	5/ Never run in a mode where ANY monitoring cameras, intercoms, etc
	   are not working. (Any unit I spec, I always specify redundant
	   CCTV systems. Another $400 bucks added on to a $2-million machine
	   does nothing to the bottom line.)

	6/ If anything happens that you do NOT UNDERSTAND, STOP IMMEDITELY
	   UNTIL YOU FIGURE OUT WHAT IS GOING ON.

   Frank R. Borger - Physicist    __       Internet:  Frank@rover.uchicago.edu
Michael Reese - Univ. of Chicago |___      Phone : 312-791-8075 fax : 567-7455
  Center for Radiation Therapy   | |_) _   Last night I discovered a new form
    2929 South Ellis Avenue        | \|_)  of oral contraceptive. I asked a girl
    Chicago, Illinois 60616           |_)  to go to bed and she said no. W Allen