Modelling an extreme water–lung interface using a single pencil beam algorithm and the Monte Carlo method

Abstract

The goal of this study was to quantify, in a heterogeneous phantom, the difference between experimentally measured beam profiles and those calculated using both a commercial convolution algorithm and the Monte Carlo (MC) method. This was done by arranging a phantom geometry that incorporated a vertical solid water–lung material interface parallel to the beam axis. At nominal x-ray energies of 6 and 18 MV, dose distributions were modelled for field sizes of 10 × 10 cm2 and 4 × 4 cm2 using the CadPlan 6.0 commercial treatment planning system (TPS) and the BEAMnrc–DOSXYZnrc Monte Carlo package. Beam profiles were found experimentally at various depths using film dosimetry. The results showed that within the lung region the TPS had a substantial problem modelling the dose distribution. The (film–TPS) profile difference was found to increase, in the lung region, as the field size decreased and the beam energy increased; in the worst case the difference was more than 15%. In contrast, (film–MC) profile differences were not found to be affected by the material density difference. BEAMnrc–DOSXYZnrc successfully modelled the material interface and dose profiles to within 2%.