How a Medical Linear Accelerator Works

Generating an Electron Beam

Early radiation therapy machines used a radioactive source like cobalt to produce the ionizing radiation needed to treat cancerous tissue. Some machines still use an active radiation source. But most radiation therapy today is done with a linear accelerator. In principle, a linear accelerator works just like the computer monitor you are probably using to read this web page. The electrons are accelerated by the gun in the back of the monitor and directed at the inside of the screen, where phosphors absorb the electrons and produce light. A medical linear accelerator produces a beam of electrons about 1,000 times more powerful than the standard computer monitor. The longer a linear accelerator is, the higher the energy of the beam it can produce. The innovation of Therac 25 was that the designers found a way to fold the beam back and forth so a very long accelerator could be fit into a smaller space. Thus powerful beams could be produced, but within a reasonable amount of space

Patients can be treated directly with the resulting electron beam, as long as the beam is spread out by scanning magnets to produce a safe level of radiation. The medical linear accelerator spreads and directs the beam at the appropriate place for treatment. The picture below shows a typical medical linear accelerator in operation.

But a difficulty with the electron beam is that it diffuses rapidly in tissue and cannot reach deeper tissue for treatment. The picture below is a simulation (produced by the Stanford Linear Accelerator Center) of an electron beam traveling through air and entering human tissue. You can see the beam quickly diffuses and therefore does not penetrate deeply.

To solve this problem, Therac-25 and many other machines can switch to a mode in which X-ray photons are used for treatment. These penetrate much more deeply without harming intervening tissue. To do this, the electron beam is greatly increased in intensity and a metal foil followed by a beam "flattener" is placed in the path of the electron beam. This transforms the electron beam into an X-ray (called photons in some literature). This process is inefficient and requires a high intensity electron beam to produce enough X-ray intensity for treatment. Therac-25 used a 25 MeV electron beam to produce an X-ray for treatment. 25 MeV is 25 million electron volts (eV -- an eV is the energy needed to move one electron through a potential of one volt).

Therac-25 was what was called a dual-mode machine. It could produce the low energy electron beams for surface treatment and it could also produce a very high intensity electron beam that would be transformed into an X-ray by placing the metal foil in the path of the beam. The serious danger in a dual mode machine is that the high-energy beam might directly strike the patient if the foil and flattener were not placed in its way.

Although MeVs are used to measure the strength of the electron beam, the measure used for therapeutic uses is the radiation absorbed dose (rad). This is a measure of the radiation that is absorbed by tissue in a treatment. Standard single radiation treatments are in the range of 200 rads. 500 rads is the accepted level of radation that, if the entire body is exposed to it, will result in the death of 50% of the cases. The unprotected electron beam in the Therac-25 is capable of producing between 15,000 and 20,000 rads in a single treatment. The unprotected beam is never aimed directly at a patient. It is either spread to a safe concentration by scanning magnets or turned into X-rays and reduced by a beam flattener.