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

• Published in: Physics in medicine & biology Vol. 49; no. 8; pp. 1557 - 1567
• Main Authors: Cranmer-Sargison, G, Beckham, W A, Popescu, I A
• Format: Journal Article
• Language: English
• Published: England IOP Publishing 21.04.2004
• Subjects: Algorithms; Film Dosimetry; Humans; Lung - pathology; Lung - radiation effects; Monte Carlo Method; Phantoms, Imaging; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods; Radiotherapy, Conformal - methods; Software; Water - chemistry; X-Rays
• ISSN: 0031-9155; 1361-6560
• DOI: 10.1088/0031-9155/49/8/013

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%.