Modelling clinical accelerator beams: a review

Radiotherapy dose calculation algorithms currently under development (and some in use) require knowledge of the characteristics of particles in linear accelerator-generated radiation beams. A range of techniques have been developed which allow determination of those characteristics requiring minimal interaction with the physical beam. Such techniques may be analytical in nature, making use of analytical transport results and cross sections, or they may be numerical, employing the increasing utility of Monte Carlo techniques. These techniques provide us with an extensive description of clinical beams and the ability to refine beam production and collimation systems. This review details several published analytical and numerical approaches to megavoltage photon and electron beam modelling, the characteristics that they provide, and their relative accuracy and utility.