On the initial angular variances of clinical electron beams

Electron beam radiotherapy treatment planning systems need to be fed with t he characteristics of the high-energy electron beams (4-50 MeV) from the sp ecifically 'applied accelerator. Beams can be characterized by their mean i nitial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This infor mation is the only input to pencil beam dose calculation models. Newer calc ulation models like macro Monte Carlo, voxel Monte Carlo and phase space ev olution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam compone nt and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to i nvestigate to what extent standard values can be used both for the effectiv e initial angular variance as input to pencil beam models and for the prima ry beam initial angular variance. Comprehensive benchmark data were obtaine d on the initial angular variance of various types of accelerator, for vari ous energies and field sizes. The initial angular variance as, has been der ived from penumbra measurements in air by means of film dosimetry at variou s distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for sigma(theta)(x2)/T( E) of around 13 cm where T(E) is the ICRU-35 linear angular scattering powe r in air. This value can be chosen as standard value for the primary beam i nitial angular variance, only slightly compromising the dose calculation ac curacy. As input to pencil beam models, an effective sigma(theta)(x2)/T(E) should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-1 0 cm) an effective sigma(theta x)(2)/T(E) of 20 cm has been found and is re commended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 5 0% higher for the effective sigma(theta x)(2)/T(E).