LINEAR ACCELERATOR

Typical Number in Hospital: 1

Cost Bands: 5,6

References: 3,4

Electrons can be accelerated to high velocities (up to 99% of the speed of light) in a straight wave guide driven by a microwave (e.g. 10 GHz) generator. Electrons are fed from a thermionic source into the evacuated wave guide which contains a series of metal diaphragms with a hole in the middle so that with each wave the electrons move from one diaphragm to the next. The spacing of the diaphragms is increased down the length of the tube as the velocity of the electrons increases. At the far end they strike an X-ray target from which high energy X-rays (4-15 MeV) are produced, or pass through a window to allow direct electron treatment.

The resultant X-ray beam is attenuated by a beam-flattening filter, a circular piece of metal, thicker in the centre than at the edge, which is introduced into the beam to produce a uniform X-ray intensity. The beam size is controlled by metal shutters, usually adjustable over the range 4-30 cm. Beam intensity may be modified by a variety of metal filters, the commonest being wedge shaped, which attenuate uniformly from one side of the beam to the other.

Such devices are used in the radiotherapy department for the treatment of deep cancers since these high-energy rays have good penetration properties. The machine is designed so that the beam can be rotated about the patient to provide maximum dose to a point at the centre of rotation and less dose at all radial sites.