Re: Question Regarding Accelerator Radiation Therapy

["Ross Brown" <ross.brown@sympatico.ca>]

There are two types of electron accelerators, travelling wave and standing

wave machines. In a travelling wave accelerator, the electrons are

accelerated like a surfer riding on the crest of an ocean wave. In a

standing wave accelerator that is most common for medical accelerators, the

electrons always see an accelerating force in the wave guide by being

shielded from the decelerating part of the standing rf wave.



When the monoenergetic electrons strike a heavy metal target, they produce a

spectrum of bremsstrahlung (x-ray) radiation much the same as low energy

diagnostic machines.



Low energy machines accelerate electrons to about 4 MeV. These produce a

radiation beam that has characteristics similar to a cobalt 60 therapy

machine. As the electron energy increases, the bremsstrahlung radiation

becomes more penetrating and the position of maximum dose is pushed further

into the body. This is useful for deeper tumours but there is little use for

x-ray beams greater than about 20 MeV.



Most modern accelerators have a retractable target so that it is possible to

extract the electron beam directly. This is useful for certain types of

tumours near the skin surface.



Ross Brown

(613) 825-3398

ross.brown@sympatico.ca



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