Abstract
A simplified Monte Carlo (MC) source model for proton pencil-beam scanning (PBS) was proposed and demonstrated in this study. Three-dimensional dose distribution in target can be obtained by feeding treatment field parameters exported from a treatment planning system (TPS) into the source model. A PBS source term (initial energy and spread) as a function of proton energy (100-226 MeV) was constructed to directly imitate the measured integral depth-dose curves (IDDCs), which is independent of any component in treatment nozzle. The spot divergence and deflection were determined from the measured spot sizes at the isocenteric plane and source-to-axis distance respectively, and the absolute dose per particle at 2-cm depth for 100-226 MeV protons were used as the MU weighting correction factors. Simple geometry structures and TG-119 phantoms were created in the TPS as the planning target volumes (PTVs) in a 40×40×40 cm3 water phantom. The range shifters (water equivalent thickness of 4.0 cm and 7.5 cm) were used for the PTVs at depths <10 cm. The calculated depth-dose curves, point doses, and plane dose distributions by the simplified MC source model were compared with measurements using multi-layer ion chambers, parallel plate ion chamber, and MatriXX PT detectors (IBA dosimetry GmbH, Schwarzenbruck, Germany), as well as the TPS. The comparisons of absolute dose profiles at the isocenter depth were with the error within 2% compared in plateau region. The Gamma passing rate were showed and almost all of cases can reach the goal of 95% compared with the TPS calculations and measurements respectively, which met the criteria of 3% and 3 mm in a interpolated grid size of 1 mm. Over all, the typical PBS treatment field simulations could be completed in an hour using a 24-core computing node. The simplified Monte Carlo source model provides accurate 3D dose distribution in water with clinically-acceptable computational efficiency. The current model could be used for the commissioning of treatment planning system, independent MU check, and patient-specific quality assurance in a homogeneous water phantom.