Megavoltage photons

♦ Maximum dose below skin surface

♦ Absorbed dose falls off exponentially with depth

♦ Sharp 'fall off' of dose at beam edge (penumbra)

♦ Whole-body radiation can be used

♦ Beam shape modified by metal shields or multi-leaf collimators

♦ Metal filters can be used to gradient dose

♦ Treatment from any direction can be used

♦ Crossfire technique with 2-4 beams gives higher target dose Some linear accelerators are also configured to produce beams of electrons of various energies, usually in the range 4-20 MeV. Such beams can uniformly treat from skin surface down to a certain depth (related to the energy), with a fairly rapid fall-off in dose beyond that. For

Isodose curves for open beams of 6 MV X-rays. (b)

Isodose curves for 12 MeV electrons.

example, 6 MeV electrons will treat to about 1.5 cm deep and 20 MeV to about 5.5 cm. Electrons offer a good alternative to kilovoltage X-rays for treating superficial tumours.

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