Basic principles

Treatment with beams of ionizing radiation produced from a source external to the patient is known as external beam radiotherapy. Superficial tumours are often treated with X-rays of low energy, in the range 80-300 kV. Electrons, emitted from a heated cathode, are accelerated across an X-ray tube, strike a tungsten anode, and undergo bremsstrahlung interactions. The beam size is selected by using metal cone-shaped applicators of different sizes. The main limitations of such beams are:

♦ Inherent delivery of high dose to the skin

♦ Relatively rapid 'fall off' of dose with depth

♦ Higher absorbed dose in bone compared with soft tissue Deeper-seated tumours are mostly treated using megavoltage photons. One option is to use a source of Co-60, emitting gamma rays of average energy 1.25 MeV. Source strengths of about 350 TBq are required to achieve a sufficiently high dose rate.

It is more common to use megavoltage X-rays produced by linear accelerators, in which electrons are accelerated to near the speed of light in a waveguide, before striking a thin transmission target. The resultant X-rays can have energies in the range 4-20 MV. Such beams offer advantages of higher penetration, higher dose rate, and better collimation than beams of Co-60.

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