Full forward Monte Carlo calculation of portal dose from MLC collimated treatment beams

This work deals with a full Monte Carlo (MC) simulation of a radiotherapy treatment facility including a multi-leaf collimator (MLC) and electronic portal imaging device (EPID). A method for a planar calibration of the EPID response in terms of dose using the MC technique is presented. Calibration measurements and simulations with several blocks of attenuating material are carried out down to approximatively 5% of the open field transmitted dose. A linear relationship is shown between the squared EPID signal and the MC calculated dose. The calibrated EPID was used as a dosimetric system to validate a MC model for the MLC. Computations and measurements agreed within 2% of dose difference (or 2 mm in regions of high dose gradient). The technique described herein is not significantly limited by physics transport model constraints. Therefore it can potentially provide a more accurate verification of dose delivery to inhomogeneous anatomical regions in patients undergoing complex multi-field conformal or intensity-modulated radiation therapy.