A new model for computerized clinical electron beam dosimetry.

Clinical electron beams consist of primary electrons, primary bremsstrahlung generated in the regular photon and electron collimator system determining the composite beam, and some short-range contaminant photon and electron scatter arising from the lower parts of the standard or regular electron applicator. Any beam-shaping insert placed inside the applicator causes some extra ("contaminant") bremsstrahlung and electron scatter. The new dose calculation model is based on separate treatment of these components. For the calculation of the primary electron dose we use experimentally determined electron scatter functions and differential electron scatter functions. The primary bremsstrahlung is treated as an unflattened but otherwise regular x-ray beam. The contaminant components arising from the rim area of the regular electron collimator and from beam-shaping inserts are considered separately. The behavior of the in-air ionization profiles is described using the concepts of effective electron source position and effective electron source diameter. The model has been tested for several electron energies.