The effect of secondary electron spread on the penumbra in high energy photon beam therapy

In photon beam therapy, the geometric penumbra width is determined by the source-size and the collimator design. The width of the physical (i.e. dosimetric) penumbra involves an additional contribution from secondary electron spread. Using a suitably defined measure of penumbra width, the separate widths due to photons and secondary electrons are additive in quadrature. Secondary electron spread functions were measured using photographic dosimetry for 60Co gamma rays and for 4, 8 and 16 MV X rays. The results suggest that, under typical treatment conditions, secondary electron spread may be the predominant contributor to the penumbra at effective generating voltages exceeding 10 MV. The implications for selection of beam energy in precision small-field radiotherapy are discussed briefly.