Monte carlo calculation of 6 MV varian linac photon beam spectral characteristics using the BEAM code

Abstract

In this study the BEAMnrc Monte Carlo (MC) code is used to simulate 6 MV photon beam as the MC methods are consider to be most reliable and accurate to evaluate the radiation beam characteristics. In this work the MC simulation model is used to calculate depth-dose curves and contribution of contaminant electron to it. We also calculated photon spectra, photon average energy distributions, photon energy fluence spectra, contaminant electron spectra at 100 cm source to surface distance. The sensitivity of these parameters has been analyzed in detail. The contribution of electrons to the central-axis depth-dose was calculated and it was observed that its contribution is less than 7% of maximum total dose at surface, while at the depth of maximum dose (dmax) its contribution was less than 3% of maximum total dose for 10×10 cm2 field sizes. The photon energy fluence spectra are separated into direct and scatter components from the primary collimator, flattening filter and the adjustable collimators. The contribution of direct photons to the total photon energy fluence is about 97% and the scatter contributions to the total photon energy fluence from the primary collimator and flattening filter are typically less than 3% and scatter contributions from jaws is less than 0. 30% to the total photon energy fluence for 10×10 cm2 fields size. The calculated and measured depth-dose data agree within 1% of local dose, and 1. 0 mm in depth at all depths and field sizes which gave enough confidence that MC Simulations could be used to simulate the 6 MV photon beam. Our study shows that both photon and electron fluence spectra strongly depend upon field size. Most of the scatter energy fluence of photon comes from flattening filter and primary collimator. Beam hardening effects of flattening filter have also been verified by the study of average energy distribution. © Research India Publications.