Virtual source distances for electron beams between 5 and 20 MeV

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Abstract
In electron beam therapy an air space between the treatment applicator and the skin surface leads to a reduction of dose and an alteration in the depth dose curves. It has been shown (ICRU, 1972; Almond, 1976; Khan et al. 1978) that this can be corrected for by the use of an inverse square factor based on the virtual source-to-skin distance (VSD). Even with improvements in scattering foil design and reduction of scatter from applicator walls, virtual source position varies considerably with field size. Patient curvature frequently results in gaps of a few cm between the end of the applicator and the patient. A gap of 3 cm causes a drop in dose of 5.7% for 100 cm VSD, 11% for 50 cm VSD. Large errors in dosimetry can thus arise from assuming the source to be at the electron window or scattering foil. Although for large field sizes at high energies one can assume that the virtual source is at a fixed position, large errors will occur if values of VSD measured for large fields are applied to small fields. It is important that measurements of virtual source position are made over the full range of energies and field sizes required for electron therapy.

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