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

Abstract

Advanced electron beam dose calculation models for radiation oncology requireas input an initial phase space (IPS) that describes a clinical electron beam.The IPS is a distribution in position, energy and direction of electrons andphotons in a plane in front of the patient. A method is presented to derivethe IPS of a clinical electron beam from a limited set of measured beam data.The electron beam is modelled by a sum of four beam components: a maindiverging beam, applicator edge scatter, applicator transmission and a seconddiverging beam. The two diverging beam components are described by weightedsums of monoenergetic diverging electron and photon beams. The weight factorsof these monoenergetic beams are determined by the method of simulatedannealing such that a best fit is obtained with depth-dose curves measuredfor several field sizes at two source-surface distances. The resulting IPSsare applied by the phase-space evolution electron beam dose calculation modelto calculate absolute 3D dose distributions. The accuracy of the calculatedresults is in general within 1.5% or 1.5 mm; worst cases show differences ofup to 3% or 3 mm. The method presented here to describe clinical electronbeams yields accurate results, requires only a limited set of measurementsand might be considered as an alternative to the use of Monte Carlo methods togenerate full initial phase spaces.