Monte carlo dose calculation of a six megavoltage clinical photon beam

Abstract

A photon beam of 6 MV from a Varian Clinac 600C/D accelerator was simulated using the EGSnrc user code BEAMnrc. The accelerator was modeled as a combination of component modules (CMs) consisting of a target, a primary collimator, a flattening filter, a monitor chamber, a secondary collimator and an air column. A simulated beam was stored in a phase space data file, which contained details such as each particle`s position, energy, angle, charge, weight, and complete history including where it has been and where it has interacted in the beam simulation. The phase space file was analyzed to obtain fluence and energy fluence profiles, spectral distributions, mean energy distributions and angular distributions at the phantom surface of source-to-surface distance (SSD) of 100 cm for the 10 × 10㎠ field. The phase space file also was used to calculate dose distributions using the EGSnrc user code DOSXYZnrc. Good agreement between measurements and Monte Carlo calculations was achieved in the dose distributions. It was shown that past the buildup region, the agreement between the measured and Monte Carlo calculated central axis depth dose curves was better than 2% of local absorbed dose and within the geometrical edges of the field, the agreement between measured and Monte Carlo calculated dose profiles was better than 2%. To increase the efficiency of computation, time reduction techniques such as variance reduction techniques and parallel processing were used and the effects of them were investigated. The results of this work indicates the feasibility of direct treatment planning applications of Monte Carlo methods.