Characteristics of bremsstrahlung in electron beams

Most electron beam algorithms ignore the bremsstrahlung components. This becomes increasingly important for high energies and for small fields. The difficulty of modeling lies with the lack of knowledge of the bremsstrahlung component. Bremsstrahlung is produced in the head (scattering foils, etc.), in the cerrobend or lead cutout, and in the tissue. These components were separated using measurements in water with variable collimation (diameter 0-5 cm) by 12-cm thick Lucite or by cerrobend blocks for clinical electron beams (6-22 MeV). Bremsstrahlung from the head dominates and increases with beam radius. Bremsstrahlung from cerrobend decreases with increasing radius because the amount of cerrobend intersecting beam is decreased. EGS4 Monte-Carlo simulation confirms that the bremsstrahlung from tissue is less than 1.5% of the total electron dose for 22 MeV monoenergenic electrons. The energy of the bremsstrahlung is softer than X-ray beams of corresponding nominal energy since the latter is hardened by the flattening filter. The effective attenuation coefficient was 0.057, 0.050, and 0.043 cm/sup -1/ and the depth of maximum dose 0.5, 1.7, and 2.8 cm for the 6, 12, and 22 MeV electrons, respectively. The lateral dose distribution is more forward-peaked for higher electron energy. Since the main contribution of bremsstrahlung is from the head, it is not accounted for in all state-of-art electron dose-calculation algorithms, such as Voxel Monte Carlo. This can be overcome by calculating separately for electrons and bremsstrahlung photons. The bremsstrahlung component can also be described by data from a set of simple measurements.