A multiple source model for 6 MV photon beam dose calculations usingMonte Carlo

Abstract

A multiple source model (MSM) for the 6 MV beam of a Varian Clinac 2300 C/Dwas developed by simulating radiation transport through the acceleratorhead for a set of square fields using the GEANT Monte Carlo (MC) code. Thecorresponding phase space (PS) data enabled the characterization of 12sources representing the main components of the beam defining system. Byparametrizing the source characteristics and by evaluating the dependenceof the parameters on field size, it was possible to extend the validity ofthe model to arbitrary rectangular fields which include the central3×3 cm2 field without additional precalculated PS data.Finally, a sampling procedure was developed in order to reproduce the PSdata.To validate the MSM, the fluence, energy fluence and mean energydistributions determined from the original and the reproduced PS data werecompared and showed very good agreement. In addition, the MC calculatedprimary energy spectrum was verified by an energy spectrum derived fromtransmission measurements.Comparisons of MC calculated depth dose curves and profiles, using originaland PS data reproduced by the MSM, agree within 1% and 1 mm. Deviationsfrom measured dose distributions are within 1.5% and 1 mm. However, thereal beam leads to some larger deviations outside the geometrical beam areafor large fields. Calculated output factors in 10 cm water depth agreewithin 1.5% with experimentally determined data.In conclusion, the MSM produces accurate PS data for MC photon dosecalculations for the rectangular fields specified.