A theoretical multileaf collimator model for fast Monte Carlo dose calculation of linac 6/10 MV photon beams

A complete linac photon beam Monte Carlo dose calculation, even on a fast computer, requires generally a substantial computation time. The existing approaches developed to improve the efficiency are mainly based on the use of variance reduction techniques, phase space sources or virtual source models. The collimation of the simulated photon beam, involving particle transport through the multileaf collimator, considerably increases the calculation time especially for intensity modulation radiation therapy, where multiple small and medium field shapes are taking place. In this paper an improvement of a previous model will aim to significantly enhance the calculation efficiency for fast intensity modulation computations mainly by the use of an analytical collimator. The linac photon beam and multileaf collimator model is controlled by several parameters, such as, the geometrical dimensions of the primary collimator, the flattening filter and the number of leaves of the linac's collimator. The model uses only correlated energy spectra and particle fluence distributions derived from the International Atomic Energy Agency phase space files of Elekta Precise Varian TrueBeam.