A Simple Source Model for 6 MV Flattening Filter Free Photon Beams Monte Carlo Dose Calculations

The purpose of this work was to develop a simple source model for Monte Carlo dose calculations of flattening filter free (FFF) 6 MV photon beams. MCNPX2.4 Monte Carlo code was used to simulate 6 MV FFF photon beam from a Varian 2100C/D Clinac. The FFF photon beam features including energy and angular distributions were calculated in a phase space plane located 25.1 cm downstream from the accelerator target. A simple point source model (SM) was developed based on correlated histograms. Differences between measured and calculated profiles were less than 2% of the maximum dose or 2 mm distance to agreement at 100 cm source to surface distance (SSD). Using the obtained SM, percent depth dose and beam profiles were calculated for different field sizes from 3 x 3 to 40 x 40 cm(2). Differences between the SM and Clinac full simulation data were less than 2% or 2 mm distance to agreement at 100 cm SSD. This model is independent of components located above the accelerator movable jaws. Also, it has source biasing capabilities that increase the simulation speed up to 2000 times for certain field sizes. Our simulations showed that the dose rates of the FFF beams are at least two times higher for the same current of incident electrons on the target.