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The linear accelerator (LINAC)

The linear accelerator is a device for accelerating charged particles (either electrons or protons) to high energies (see 16-19/Accelerators). It has been in use in hospitals since the 1970s.

In medicine the LINAC is used in two forms:
(a) accelerating electrons to high energies where they collide with a heavy metal target to produce high energy X rays (photo and Figure 1)
(b) using protons as the accelerated particles and using those to collide directly with the patients body

Advantages of very high energy X rays produced using the linac:
1. The source of radiation is smaller than the cobalt 60 radioactive source and so gives sharper images
2. There are less skin effects
3. It is therefore less painful for the patient
4. The large penetrating power of the high energy X rays allows treatment of deep seated tumours

(b) linear accelerators using protons with energies up to 250 MeV are also used. The radiation produced is very penetrating and because they produce a fine beam of particles they can be used to treat very small areas of the body. Changing the accelerating voltage allows different depths to be treated for example 200 MeV protons have a range of 27 cm in tissue while a beam of energy 140 MeV will only reach a depth of 15 cm.

© Keith Gibbs 2011