Author Topic: The Therac 25  (Read 277 times)

Offline imp

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The Therac 25
« on: September 14, 2021, 01:06:33 AM »
"The Therac-25 was a computer-controlled radiation therapy machine produced by Atomic Energy of Canada Limited (AECL) in 1982.

It was involved in at least six accidents between 1985 and 1987, in which patients were given massive overdoses of radiation.

The machine offered two modes of radiation therapy:
Direct electron-beam therapy, in which a narrow, low-current beam of high-energy (5 MeV to 25 MeV) electrons was scanned over the treatment area by magnets;
Megavolt X-ray (or photon) therapy, which delivered a fixed-width beam of X-rays, produced by colliding a narrow beam of 25 MeV electrons with a tungsten target, then passing the emitted X-rays through both a flattening filter and a collimator.

 Because of concurrent programming errors (also known as race conditions), it sometimes gave its patients radiation doses that were hundreds of times greater than normal, resulting in death or serious injury. These accidents highlighted the dangers of software control of safety-critical systems, and they have become a standard case study in health informatics and software engineering."

A Therac-25 accident, the fourth in the series, developed at the East Texas Cancer Center in March of 1986. A male patient was to receive therapy on his upper back. The Therac-25 operator had typed in incorrect treatment information by indicating X-ray mode instead of electron mode. She merely used the "cursor up" key to edit the mode entry and then quickly pressed "enter" (one of the user-friendly features), and started treatment. The machine shut down with treatment pause, and a "malfunction 54" error message was displayed on the screen. This error message indicated that either a dose too high or a dose too low had been delivered. Since an underdose value appeared on the screen and the operator was used to quirks in the machine, she hit the "P" key to continue with the treatment. The machine repeated the "Malfunction 54" error message and indicated the same underdose was delivered. The operator had no contact with the patient, because the usual audio and video monitors were not working properly. After the first attempt at treatment, the patient felt an "electric shock" or as if "someone had poured hot coffee" on his back. He knew this was not normal and began to get up from the treatment table when the second treatment was delivered. The patient felt a tremendous shock in his arm, and felt that "his hand was leaving his body". He had to pound on the treatment room door to get the operator's attention. The patient eventually loss the use of his left arm and both legs, was unable to speak, and had several other complications. He died from complications five months later (Leveson and Turner, 1993, p. 27-28).

I have feared ionizing radiation most of my life, beginning when I assembled a functional fluoroscope in my teens. I took appropriate precautions, and have lived a long life. Studying the Therac 25 use has been nightmare-inducing.

Does anyone have other information about this machine?


« Last Edit: September 14, 2021, 01:08:15 AM by imp »

Offline TMaxElectronics

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Re: The Therac 25
« Reply #1 on: September 14, 2021, 03:41:45 AM »
I beleive the youtube channel plainly difficult has made a video about this, but I don't know hoe much effort he put into his research. He might just have the same info as you :)

Offline klugesmith

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Re: The Therac 25
« Reply #2 on: September 14, 2021, 07:34:38 AM »
The Therac technical and engineering management failure was picked apart
microscopically by forensic investigators.   Like the Boeing 737-Max fatal flaw, or the lead-up to WTC tower collapses on 9/11.   Every line of code between instrument ops and interactive control console was reviewed, the race cases exactly understood and reproduced.   Development history, as more computerized successor to previous therac, also extensively studied.

In the 1990's, the Therac story was a standard case history taught in computer science programs, and as continuing education for working programmers.

How many of you would think about Therac if you were considering laser eye surgery, especially just for a refraction change?

Now off topic, this "boomer" thinks illustrated written articles are generally more useful than videos occupying similar amount of user's time. Dramatically so when the videos are unscripted amateur Youtube presentations about how to make something or do something. IN written form you can skim the whole scope and zoom directly to the parts you want.  Similar to difference between disk access and tape access.
« Last Edit: September 14, 2021, 07:44:12 AM by klugesmith »

Offline Duane B

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Re: The Therac 25
« Reply #3 on: September 14, 2021, 07:48:28 AM »
Hi imp, welcome to the forum!

I don't know anything about the Therac 25, but I have worked with similar machines made by Varian for 40 years. The linatrons 6000 and 6000-A, and the K-15 linatrons are industrial linear accelerators used to x-ray large rocket motors. The target is fixed so they will only put out x-rays and not electrons. An unwanted byproduct of hitting a target with 15 MEV electrons is the production of neutron radiation. They mainly emit 15 MEV x-rays with a flux density of up to 13000 Rads/minute. You could get a lethal dose in less than 10 seconds. These machines would run for hours continuously in Real-Time radiography, Linear Diode camera, or Computed Tomography.

The linatron 6000 series machines were older without a computer but the K-15 linatron ran with 3 PLC's. Being in an industrial environment you usually did not have to worry about hurting a patient. I know of only one accident with this type of machine. Some engineers were trying to align the laser at a different facility and they bypassed the interlocks for some reason. Anyway, one of the engineers got into the x-ray bay when the machine was on. Luckily he just got his had into where the beam was. He ended up losing most of his fingers and much of his hand. He was in a lot of pain also for many years. I think we wished he would have just cut his hand off early on and not suffer,
Duane Bylund

Offline Max

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Re: The Therac 25
« Reply #4 on: September 14, 2021, 11:08:16 AM »
Does anyone have other information about this machine?
You seem to have copied parts of the wikipedia article into your post. Why stop there? Why not look at the sources (pdf and others) given at the bottom? They seem to cover it about as good as it gets ("An Investigation of the Therac-25 Accidents",  "Safeware: System Safety and Computers. Appendix A: Medical Devices: The Therac-25").

Kind regards,
Max

Offline MRMILSTAR

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Re: The Therac 25
« Reply #5 on: September 14, 2021, 04:05:16 PM »

Now off topic, this "boomer" thinks illustrated written articles are generally more useful than videos occupying similar amount of user's time. Dramatically so when the videos are unscripted amateur Youtube presentations about how to make something or do something. IN written form you can skim the whole scope and zoom directly to the parts you want.  Similar to difference between disk access and tape access.

I heartily concur with this. I'll take a well-written and illustrated how-to over some Youtube video any day.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline imp

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Re: The Therac 25
« Reply #6 on: September 15, 2021, 02:21:55 AM »
Hi imp, welcome to the forum!

I don't know anything about the Therac 25, but I have worked with similar machines made by Varian for 40 years. The linatrons 6000 and 6000-A, and the K-15 linatrons are industrial linear accelerators used to x-ray large rocket motors. The target is fixed so they will only put out x-rays and not electrons. An unwanted byproduct of hitting a target with 15 MEV electrons is the production of neutron radiation. They mainly emit 15 MEV x-rays with a flux density of up to 13000 Rads/minute. You could get a lethal dose in less than 10 seconds. These machines would run for hours continuously in Real-Time radiography, Linear Diode camera, or Computed Tomography.

The linatron 6000 series machines were older without a computer but the K-15 linatron ran with 3 PLC's. Being in an industrial environment you usually did not have to worry about hurting a patient. I know of only one accident with this type of machine. Some engineers were trying to align the laser at a different facility and they bypassed the interlocks for some reason. Anyway, one of the engineers got into the x-ray bay when the machine was on. Luckily he just got his had into where the beam was. He ended up losing most of his fingers and much of his hand. He was in a lot of pain also for many years. I think we wished he would have just cut his hand off early on and not suffer,

Thank you for your informative answer! It was positive in nature, in that it revealed more to me than the rest combined, which seemed "picky" only.

I have often wondered about the physiological effects of very intense ionizing radiation on the human body. The individual you described with the affected hand must have suffered horribly. I wonder that such intense exposure did not cause a blood dyscrasia involving blood cells, white or red.

For the rest of the posters: I wrote in such a way as to relate only to the existence of the Therac 25, primarily for those who may have never heard of it. Forgive me for being thoughtless.......imp

Offline johnnyzoo

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Re: The Therac 25
« Reply #7 on: September 15, 2021, 08:09:01 PM »
How about some some concurrent programming assignment with an actual Therac-25? :o

http://web.mit.edu/6.033/2007/wwwdocs/assignments/handson-therac.html

Offline TMaxElectronics

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Re: The Therac 25
« Reply #8 on: September 16, 2021, 10:22:13 AM »
Quote
How about some some concurrent programming assignment with an actual Therac-25?

"There you will find instructions for the remainder of the hands-on, along with liability release forms"

Good to see they have the most important things covered ;D
« Last Edit: September 16, 2021, 03:38:28 PM by TMaxElectronics »

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Re: The Therac 25
« Reply #8 on: September 16, 2021, 10:22:13 AM »

 


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