title: 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 x 10 cm(2) and 4 x 4 cm(2) 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%.
authors.affiliation: Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada. gcranmer@uvic.ca
authors.forename: G
authors.initials: G
authors.lastname: Cranmer-Sargison
authors.forename: W A
authors.initials: WA
authors.lastname: Beckham
authors.forename: I A
authors.initials: IA
authors.lastname: Popescu
mesh_terms: D000465:Algorithms; D005373:Film Dosimetry; D006801:Humans; D008168:Lung / Q000473:pathology / Q000528:radiation effects*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012984:Software; D014867:Water / Q000737:chemistry*; D014965:X-Rays
chemical_list: D014867:Water