Abstract
This study shows detailed characteristics of realistic radiotherapy photon beams: dose distribution, energy spectra, angular spread, fluence profiles and mean energy profiles. It provides more comprehensive information for radiotherapy photon beams including incident photons and primary photons as well as contaminating electrons and positrons in a radiation beam for different field sizes and beam energies. The EGSnrc Monte Carlo code, BEAMnrc has been used to simulate 4 and 10 MV photon beams from a Varian Clinac 2100C accelerator. A simulated realistic beam is stored in a phase space files, which contains details of each particle's complete history including where it has been and where it has interacted. The phase space files are used to calculate depth-dose components from different particles and surface dose and contribution from different particles to surface dose across the filed. Energy spectra, angular spread, fluence profiles and mean energy profiles at the phantom surface for each particle are also obtained using the phase space information. The accuracy of a simulated beam is validated by the excellent agreement between the Monte Carlo calculated and measured dose distributions except for 10 MV at the 40×40 cm2 field in the build-up region. Measured depth-dose curves are obtained from depth-ionization curves by accounting for the stopping-power ratios for realistic beams. At 4 MV, the contaminant charged particles contribute 6% to 26% of maximum dose at the surface when the field size increases from 10×10 to 40×40 cm2. Similarly, their contributions at 10 MV are up to 7% and 23% of maximum dose at the surface for 10×10 cm2 and 40×40 cm2 fields, respectively. However, the fluence of these contaminant charged particles is less than 1.0% of incident photon fluence in all cases.
