Beam characterization and dosimetric calculation of megavoltage photon beams using EGSnrc Monte Carlo simulation code

Abstract

his work aims to improve the performance of EGSnrc Monte Carlo simulation to become more effective for dose calculations in the treatment planning system of radiation therapy. The variance reduction techniques available in the code were tested and assessed for 12 MV photon beam. A new methodology of commissioning the Monte Carlo model based on calculating the beam profiles at deeper depth in a square field 10*10cm2 for tuning the electron beam width has been proposed. Calculations have been performed with both the photon data provided with the EGSnrc code and the new library of photon cross section we have created based on recent ENDF/B-VII release 8 evaluation. The EGSnrc Monte Carlo based-BEAMnrc and DOSXYZnrc user codes are used for photon beam simulations and dose calculations, respectively; SATURNE43 and VARIAN linear accelerators have been modeled to simulate 12 MV photon beams, respectively. The obtained results showed that the combination of Directional Bremsstrahlung Splitting (DBS) with other variance reduction techniques increases the simulation efficiency by a factor of 1130. The electron beam parameters have been determined precisely which achieve good agreement with measured data within 1%/1mm accuracy in gamma index test. Furthermore, the unflattened photon beam of two Linacs has been evaluated and optimized by replacing the air gap by helium which contributes to enhance the unflattened beam quality through reduce the surface and build up uses ; the depth of maximum dose refers to the standard one. The second objective of this work is to develop and assess the ability of the EGSnrc MC code to be used in radiation protection calculation. A new parameter called Flux to Dose conversion factor (DF) has been introduced into the code. Also, the conversion coefficients Hp (3) have been calculated to evaluate the risk of external exposure of radiation on the lens of the eye, using a new phantom based on ORMED research. A good agreement between our results and those produced by other codes has been obtained.