Electron beams have been used in treating malignant disease for over three decades. Although electron radiation is radiobiological^ equivalent to photon radiation, the physical characteristics of electron beams are preferred over photon beams in the treatment of certain anatomical sites. Unlike photons, electrons possess charge and so interact frequently as they penetrate tissue; the resulting nearly continuous energy loss leads to a well-defined range in tissue (radiation dose deposited beyond a certain depth in tissue in negligible). This treats the target volumes lying within a few centimetres of the skin's surface while sparing any underlying critical structures.
The frequent interactions between the penetrating electrons and the tissues have several deleterious effects. Specifically:
♦ Large-beam penumbra
♦ 'Hot' and 'cold' spots beneath surface discontinuities
♦ Significant changes in dose near inhomogeneities
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