Parametrisation of depth dose for electron beams

Knowledge of the central axis depth dose curve is essential for the development of an accurate electron beam treatment planning system. The equation introduced by Kawachi (1975) and modified by Steben et al. (1976) gave general agreement with experimental data but it required a complex set of equations to extract the central axis depth dose. Even though interpolation of experimental data is one of the most common methods used in the available commercial treatment planning systems, an analytical expression might have the advantage of reducing both computer time and memory space. Unfortunately the electron beam central axis depth dose distribution has not yet been expressed accurately in an analytical form. Monte Carlo calculations are reasonably accurate, so they can be used as the standard for verifying complex experimental and theoretical work. However, they are impractical in day-to-day clinical work because of their demanding computational time. The authors have tried to reproduce the results of Monte Carlo calculations and experiments with an analytical expression. The analytical form may prove useful to centres which are now working on three-dimensional treatment planning codes for electron beam treatment planning.