A model to determine the initial phase space of a clinical electron beam from measured beam data

Advanced electron beam dose calculation models for radiation oncology requi re as input an initial phase space (IPS) that describes a clinical electron beam. The IFS is a distribution in position, energy and direction of elect rons and photons in a plane in front of the patient. A method is presented to derive the IFS of a clinical electron beam from a limited set of measure d beam data. The electron beam is modelled by a sum of four beam components : a main diverging beam, applicator edge scatter, applicator transmission a nd a second diverging beam. The two diverging beam components are described by weighted sums of monoenergetic diverging electron and photon beams. The weight factors of these monoenergetic beams are determined by the method o f simulated annealing such that a best fit is obtained with depth-dose curv es measured for several field sizes at two source-surface distances. The re sulting IPSs are applied by the phase-space evolution electron beam dose ca lculation model to calculate absolute 3D dose distributions. The accuracy o f the calculated results is in general within 1.5% or 1.5 mm; worst cases s how differences of up to 3% or 3 mm, The method presented here to describe clinical electron beams yields accurate results, requires only a limited se t of measurements and might be considered as an alternative to the use of M onte Carlo methods to generate full initial phase spaces.