Validation of Monte Carlo calculated surface doses for megavoltage photon beams

Abstract

Recent work has shown that there is significant uncertainty in measuring build‐up doses in megavoltage photon beams especially at high energies. In this present investigation we used a phantom‐embedded extrapolation chamber (PEEC) made of Solid Water™ to validate Monte Carlo (MC)‐calculated doses in the dose build‐up region for 6 and x‐ray beams. The study showed that the percentage depth ionizations (PDIs) obtained from measurements are higher than the percentage depth doses (PDDs) obtained with Monte Carlo techniques. To validate the MC‐calculated PDDs, the design of the PEEC was incorporated into the simulations. While the MC‐calculated and measured PDIs in the dose build‐up region agree with one another for the beam, a non‐negligible difference is observed for the x‐ray beam. A number of experiments and theoretical studies of various possible effects that could be the source of this discrepancy were performed. The contribution of contaminating neutrons and protons to the build‐up dose region in the x‐ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross‐section database and the NIST bremsstrahlung differential cross section do not explain the discrepancy between the MC calculations and measurement in the dose build‐up region for the . A simple incorporation of triplet production events into the MC dose calculation increases the calculated doses in the build‐up region but does not fully account for the discrepancy between measurement and calculations for the x‐ray beam.