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How Therac-25 worked

A Short History of Therac

There were two previous versions of Therac machines, each produced by AECL in collaboration with a French company, CGR. Therac 6 and Therac 20 (each named for the MeV they could produce) were based on earlier design from CGR. By the time Therac-25 was released for sale, AECL had 13 years of experience with production of medical linear accelerators. Therac-25 was based on these previous versions. Its main innovations were (1) a "double pass" electron beam so the machine could produce more energy in less space, and (2) the addition of extensive computer control of the machine. This latter innovation allowed AECL to move much of the checking for hazardous conditions into the software.

The Therac-25's ancestors, Therac-20 and Therac-6, had used a minicomputer (a DEC PDP-11) to add some convenience to the standard hardware of a medical linear accelerator. They both could work without computer control. AECL determined to make its new model, Therac-25, a tightly-coupled combination of software and hardware. Therac-25 software was not written from scratch, but was built up from components that were borrowed from the earlier versions of Therac.

The Machine in the Room

Therac-25 is not just a machine, but an installation consisting of the machine, the PDP-11 that controlled the machine, the shielded room the machine sits in, and the monitoring and control station.

A cross section drawing of a Therac-25 facility, including technological devices and electronic switches.

The control console and printer etc. are all located outside the heavily shielded treatment room. Thus, when pressing the key to begin the treatment, the operator does not have any direct access to the machine or the patient. All the occurrences in the treatment room must be observed through the TV monitor and the intercom. The intercom works both ways, that is, the patient can hear the operator (if the operator presses a switch) and the operator can hear the patient. The patient, however, cannot see anything outside the treatment room, while the operator can look in using the TV monitor.

Switching Between Modes: The Turntable

Therac-25 is a dual mode machine. This means that it can treat the patient with relatively low energy electron beams or with X-ray beams. In addition, Therac-25 had a "field light" position that allowed a standard light beam to shine in the path of treatment to help the operator in setting up the machine. Thus there were three modes in which the Therac-25 could operate: electron beam and X-ray for treatment, and field light for setup.

Even though they are relatively low energy, the electron beams are too powerful in their raw form to treat the patient. They need to be spread thinly enough to be the right level of energy. To do this, Therac-25 placed what are called scanning magnets in the way of the beam. The spread of the beam (and also it power) could be controlled by the magnetic fields generated by these magnets. Thus for electron beam therapy, the scanning magnets needed to be placed in the path of the beam.

X-ray treatment requires a very high intensity electron beam (25 MeV) to strike a metal foil. The foil then emits X-rays (photons). This X-ray beam is then "flattened" by a device below the foil, and the X-ray beam of an appropriate intensity is then directed to the patient. Thus, X-ray therapy requires the foil and the flattener to be placed in the path of the electron beam.

The final mode of operation for Therac-25 is not a treatment mode at all. It is merely a light that illuminates the field on the surface of the patient’s body that will be treated with one of the treatment beams. This "field light" required placing a mirror in place to guide the light in a path approximating the treatment beam’s path. This allowed accurate setup of the machine before treatment. Thus, for field light setup, the mirror needed to be placed in the path where one of the treatment beams would eventually go.

A cross section drawing of the Therac-25 upper turntable components.

In order to get each of these three assemblies (scanning magnets or X-ray target or field light mirror) in the right place at the right time, the Therac-25 designer placed them on a

turntable. As the name suggests, this is a rotating assembly that has the items for each mode placed on it. The turntable is rotated to the correct position before the beam is started up. This is a crucial piece of the Therac-25 machine, since incorrect matching of the turntable and the mode of operation (e.g. scanning magnets in place but Electron beam turned on high for X-ray) could produce potentially fatal levels of radiation.

Setup and Actuation

The Therac-25 operator sets up the patient on the table using the field light to target the beam. In doing this, treatment parameters must be entered into the machine directly in the treatment room.

He or she then leaves the room and uses the computer console to confirm the treatment parameters (electron or X-ray mode, intensity, duration, etc.). The parameters initially entered in the treatment room appear on the console and the operator simply presses return to confirm each one.

The computer then makes the appropriate adjustments in the machine (moving the turntable, setting the scanning magnets, setting beam intensity etc.). This takes several seconds to do. If the operator notices an error in the input parameters, he or she can, during the setup, edit the parameters at the console without having to start all over again from inside the treatment room.

When the computer indicates that the setup has been done correctly, the operator presses the actuation switch. The computer turns the beam on and the treatment begins. There are three possible outcomes at this point, and they all depend on sensors on the machine. If the sensors indicate no trouble, the treatment concludes successfully. If the sensors indicate a minor problem, like the beam being slightly out of tune, the computer turns the beam off immediately. The operator can then press a "proceed" key to retry the treatment up to 5 times. If the sensors indicate a more serious malfunction, like the beam being significantly stronger or weaker, the computer turns the beam off immediately and requires the machine to be completely setup from the beginning.