Accuracy of the phase space evolution dose calculation model for clinical 25 MeV electron beams

The phase space evolution (PSE) model is a dose calculation model for elect ron beams in radiation oncology developed with the aim of a higher accuracy than the commonly used pencil beam (PB) models and with shorter calculatio n times than needed for Monte Carlo (MC) calculations. In this paper the ac curacy of the PSE model has been investigated for 25 MeV electron beams of a MM50 racetrack microtron (Scanditronix Medical AB, Sweden) and compared w ith the results of a PB model. Measurements have been performed for tests l ike non-standard SSD, irregularly shaped fields, oblique incidence and in p hantoms with heterogeneities of air, bone and lung. MC calculations have be en performed as well, to reveal possible errors in the measurements and/or possible inaccuracies in the interaction data used for the bone and lung su bstitute materials. Results show a good agreement between PSE calculated do se distributions and measurements. For all points the differences-in absolu te dose-were generally well within 3% and 3 mm. However, the PSE model was found to be less accurate in large regions of low-density material and erro rs of up to 6% were found for the lung phantom. Results of the PB model sho w larger deviations, with differences of up to 6% and 6 mm and of up to 10% for the lung phantom; at shortened SSDs the dose was overestimated by up t o 6%. The agreement between MC calculations and measurement was good. For t he bone and the lung phantom maximum deviations of 4% and 3% were found, ca used by uncertainties about the actual interaction data. In conclusion, usi ng the phase space evolution model, absolute 3D dose distributions of 25 Me V electron beams can be calculated with sufficient accuracy in most cases. The accuracy is significantly better than for a pencil beam model. In regio ns of lung tissue, a Monte Carlo model yields more accurate results than th e current implementation of the PSE model.