Determining Monte Carlo based photon beam model parameters for 160-leaf multileaf collimator system

The success of radiotherapy depends on the high accuracy of the treatment dose calculation and delivery. Accurate modeling of medical linear accelerator treatment head components is of paramount importance for Monte Carlo (MC) based treatment planning systems that calculate treatment dose distribution with the highest accuracy. The geometric parameters (transmission probability filter) of the multi-leaf collimators directly affect the amount of particles reaching the patient and the dose distribution. In this study, it was aimed to experimentally determine the transmission probability filter values of MC based photon beam model in Monaco® treatment planning system for 160-leaf Agility ™ collimator. As a result of water phantom measurements, leaf transmisson and leakage values were found to be consistent with the model data. Although diode array and gafchromic film measurements with test beams revealed minor tongue-groove effect, gamma analysis of 140 clinical IMRT and VMAT treatment plans, which were planned with default model paramters, showed above 95% compliance.