Monte Carlo simulation of Varian Linac for 6 MV photon beam with BEAMnrc code

The purpose of this study is to investigate the effects of the initial electron beam parameters on the absorbed dose distribution calculated with EGSnrc Monte Carlo code, for 6MV photon beam. A proposed methodology for benchmarking the BEAMnrc model of Varian Linac has been used. Also, a new photon cross section data based on ENDF/B-VII release 8 evaluation has been employed. The parameters tested include mean energy, radial intensity distribution and angular spread of the initial electron beam. Mean energy and angular spread were tested for a square irradiation field 10 × 10cm2, whereas beam width of the electron beam was studied for 10 × 10cm2 at different depths and 30 × 30cm2 at depth of 10cm. The results obtained are compared with measurement data to select the optimal electron beam parameters. The differences between MC calculation and measurements data are analyzed using gamma index criteria which fixed within 1% −1mm accuracy.