Comparison of MCNPX and measured doses at interfaces for photon and electron beams

The Monte Carlo method calculates the dose distributions within a patient by simulating the transport of particles through media. This research will perform a comparison of dose distributions in heterogeneous phantoms interfaces in clinical photon and electron beams with Monte Carlo methods, specifically the code MCNPX, and measured values. The primary focus of this project is to calculate dose delivered at interfaces to layers of different patient specific media by photons and electrons beams commonly used in radiation therapy. The central hypothesis of this research is that Monte Carlo methods, specifically the code MCNPX can be used to accurately calculate the dose distribution at tissue inhomogeneities interfaces. The specific aims of this work are to: (i) validate the accuracy of MCNPX for calculating the dose between interfaces of various media; (ii) compare these calculated values with measured values obtained from TLD-100 (LiF) and GAFCHROMICRTM EBT film measurements. The long-term objective is to evaluate the effectiveness of using Monte Carlo codes, specifically MCNPX, for calculating prescribed radiotherapy dose distributions within a patient.; A muscle/bone and fat/muscle/bone phantom with a SAD setup was used to measure interface doses for a 10x10cm2 6 MV, and 15 MV photon beams in addition to a 10x10cm2 8 MeV, and 15MeV electron beams. TLDs and GAFCHROMICRTM EBT film were placed directly in the interface to measure the interface doses. The Monte Carlo model was first validated by comparing measured PDD curves with PDD curves within 1% generated with MCNPX. In the MCNPX simulations TLD's and film were modeled in detail, taking into account their specific atomic composition. Comparing Monte Carlo calculation simulations with TLD and film measurements at the interface reveals that MCNP is able to predict the absorbed dose accurately for photon and electron beams. The average deviation of 2.4% is found between the TLD and MCNPX. Excellent agreement was found between the EBT film and MCNP averaging 1.02% for all beam energies. The MCNPX code has been shown to successfully model interface doses at a 2% error uncertainty level.