Photon beam characterization and modelling for Monte Carlo treatment planning

TLDR

Photon beams of 4, 6 and 15 MV from Varian Clinac 2100C/2300C D accelerators were modeled with the EGS4/BEAM code as component modules (target, collimators, flattening filter, etc.) and a full phase‑space file was scored above the photon jaws and processed with BEAMDP to derive beam characteristics, while separate source models for the target, primary collimator and flattening filter were constructed with accurate intensity, energy, fluence and angular distributions. The multiple‑source model accurately reconstructs the original phase space, yielding dose distributions that agree within 2 % with the original phase space for field sizes 4 × 4 cm to 40 × 40 cm at SSDs of 80–120 cm and for lung and bone heterogeneous phantoms across 4, 6 and 15 MV photon beams.

Abstract

Photon beams of 4, 6 and 15 MV from Varian Clinac 2100C and 2300C/D accelerators were simulated using the EGS4/BEAM code system. The accelerators were modelled as a combination of component modules (CMs) consisting of a target, primary collimator, exit window, flattening filter, monitor chamber, secondary collimator, ring collimator, photon jaws and protection window. A full phase space file was scored directly above the upper photon jaws and analysed using beam data processing software, BEAMDP, to derive the beam characteristics, such as planar fluence, angular distribution, energy spectrum and the fractional contributions of each individual CM. A multiple-source model has been further developed to reconstruct the original phase space. Separate sources were created with accurate source intensity, energy, fluence and angular distributions for the target, primary collimator and flattening filter. Good agreement (within 2%) between the Monte Carlo calculations with the source model and those with the original phase space was achieved in the dose distributions for field sizes of 4 cm × 4 cm to 40 cm × 40 cm at source surface distances (SSDs) of 80-120 cm. The dose distributions in lung and bone heterogeneous phantoms have also been found to be in good agreement (within 2%) for 4, 6 and 15 MV photon beams for various field sizes between the Monte Carlo calculations with the source model and those with the original phase space.