Computerized Radiation Therapy
The use of computers in the medical field is becoming more and more widely
used. Computers are obviously very beneficial in the medical field. However, in
the case of Therac-25, they can be deadly.
Therac-25 was used in the treatment of cancer. Its purpose was to provide
radiation to a specific part of the body and hopefully kill the malignant tumor.
The Therac-25 was the third system created under the Therac name by the Atomic
Energy of Canada Limited (AECL). The AECL is most famous in Canada for their
creation of the CANDU reactors which are world renowned. A Therac-6 and
Therac-20 were both used in the treatment of cancer. The number that goes along
with the word Therac stands for the maximum amount of mega electron volts (MeV)
the machine can dispense. It was believed that the new Therac-25 was much more
efficient than Therac-6 and Therac-20. The overall size of the machine was
reduced and still allowed for two modes; photon mode and electron mode. A
tungsten shield was in place for the X-ray mode and removed for the electron
mode as illustrated below:
Each mode was used in treating cancer depending on the depth of the tumor
under the skin.
The software for the Therac-25 system was created by one programmer who
revised the Therac-6 systems code. The software was basically responsible
- monitoring the machine
- accepting the input for the treatment
- setting up the machine to administer this treatment
- and finally controlling the machine to carry out the treatment
The diagram below briefly illustrates the set up of the machine. It begins
with the computer that the software is on for the technician to use. This is
sent to another minicomputer called the PDP-11. Finally the radiation machine
receives the commands and treats the patient.
The machine itself is enclosed in a radiation treatment room in order to
prevent exposure to the technicians working nearby. The patient has audio and
visual equipment, allowing communication with the technicians. A diagram of the
general layout of the Therac-25 is shown below:
The Therac-25 treatment system was first started in 1976 but was generally a
piece of machinery. The software controlled system came online in 1982. The
first incident occurred in 1985 and five more happened within 19 months of the
original. After the fifth the Federal Drug Administration recalled Therac-25
until it was "fixed". Unfortunately the sixth incident occurred before the
changes had been made.
The first incident involved a 61 year old women from Marietta, Ga. who was
receiving follow-up treatment after a tumor had been removed from her breast.
Therac-25 administered a very large overdose of radiation and essentially burnt
her. The hospital and technician denied any wrong doing and in fact continued to
send her for treatments. She had to have both breast removed and lost total use
of her right arm because of the radiation overdose.
A 2nd incident occurred closer to home when a 40 year old women
was receiving her 24th Therac-25 treatment at the Ontario Cancer
Foundation in Hamilton. The machine stopped 5 seconds into the treatment with an
error. The technician seeing that "No Dose" had been administered (according to
the computer) hit the 'P' key thus proceeding with the dose. This was done a
total of 5 times giving the patient 13 000 - 17 000 rads. To give an idea of how
much of an overdose this is; a regular treatment is around 200 rads and 1000
rads of radiation to the entire body can be fatal. The patient died 3 months
after the overdose from the original cancer, but if she had lived an entire hip
replacement would have been necessary due to the damage done by the overdose.
The 3rd case occurred in Yakima, WA. This patient received radiation burns
but many years later has fully recovered from the damages.
One of the most severe cases was the 4th in Tyler, TX. A male
required radiation treatment on his back. The machine was set to X-ray mode
instead of Electron mode so the technician just used the "cursor up" key up and
quickly changed this mistake. However, this only made things worse as a software
bug had been mistakenly stumbled upon. While administering the first treatment
an error "Malfunction 54" flashed up telling the technician an underdose had
been administered. The technician hit the 'P' key and a 2nd dose was delivered.
The patient had been given an overdose after the first treatment, and he knew
something was wrong, due to the burning sensation he felt in his back. As he
attempted to get up the 2nd dose was administered. The technician
would have known the man was in pain if the audio and visual equipment was
working. This was just something else that was wrong with the Therac-25
treatment system at this particular hospital. This man within weeks, lost the
use of both legs and his left arm. Five months later he became the first
fatality directly related to the Therac-25 system.
A month later at the same hospital, with the same technician another fatal
dosage was given. The technician made the same error of quickly changing the
mode from X-ray mode to Electron mode using the 'cursor up' key. This again
caused "Malfunction 54". The patient this time was receiving treatment on his
face. When the overdose was administered he yelled and then began to moan. The
audio equipment was working this time but the initial dose was too much for the
man. He received severe neurological damage, fell into a coma and died only 3
The 6th and last incident occurred in January of 1987 in Yakima,
WA. The patient required only a small dose and according to the machine that is
all he received. Yet again when the treatment was underway and error paused the
machine and the technician hit the 'P' key to proceed. A overdose was
administered and the man died just 3 months later. Reference:1,2,5
After each overdose the creators of Therac-25 were contacted. After the
first incident the AECL responses was simple, "After careful consideration, we
are of the opinion that this damage could not have been produced by any
malfunction of the Therac-25 or by any operator error (Leveson, 1993)."
After the 2nd incident the AECL sent a service technician to the
Therac-25 machine, he was unable to recreate the malfunction and therefore
conclude nothing was wrong with the software. Some minor adjustments to the
hardware were changed but the main problems still remained.
It was not until the fifth incident that any formal action was taken by the
AECL. However it was a physicist at the hospital where the 4th and
5th incident took place in Tyler, Texas who actually was able to reproduce the
mysterious "malfunction 54". The AECL finally took action and made a variety of
changes in the software of the Therac-25 radiation treatment system. The machine
itself is still in use today.
The general consensus is that the Atomic Energy of Canada Limited is to
blame. There was only one person programming the code
for this system and he largely did all the testing. The machine was tested for
only 2700 hours of use, but for code which controls such a critical machine,
many more hours should have been put in to the testing phase. Also Therac-25 was
tested as a whole machine rather then in separate modules. Testing in separate
modules would have discovered many of the bugs. Also, if the AECL believed that
there were problems with the Therac-25 right after the first incident then it is
possible that most of the 5 other incidents could have been avoided and possibly
the 3 fatalities.
The Therac-25 is one of the most devastating computer related engineering
disasters to date. The machine was designed to help people and largely, it did.
Yet some sloppy engineering on the part of the AECL led to the death or serious
injury of six people. These incidents could have been avoided if the AECL
reacted instead of denying responsiblity.
1. Death and Denial, The Failure of the Therac-25
2. The Downfall of the Therac-25
3. The Therac-25 Incident
4. Human Error in Medicine
5. An Investigation of the Therac-25 Accidents (part 1-5)