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Six radiation dosimetry and medical physics problems were analyzed using a beta version of MCNP 5 as part of an international intercomparison of radiation dosimetry computer codes, sponsored by the European Commission committee on the quality assurance of computational tools in radiation dosimetry. Results have been submitted to the committee, which will perform the inter-code comparison and publish the results independently. A comparison of the beta version of MCNP 5 with MCNP 4C2 is made, as well as a comparison of the new Doppler broadening feature. Comparisons are also made between the *F8 and F6 tallies, neutron tally results with and without the use of the S(a,b) cross sections, and analytically derived peak positions with pulse height distributions of a Ge detector obtained using the beta version of MCNP 5. The following problems from the study were examined: Problem 1 was modeled to determine the near-field angular anisotropy and dose distribution from a high dose rate 192Ir brachytherapy source in a surrounding spherical water phantom. Problem 2 was modeled to find radial and axial dose in an artery wall from an intravascular brachytherapy 32P source. Problem 4 was modeled to investigate the response of a four-element TLD-albedo personal dosimeter from neutrons and/or photons. Significant differences in neutron response with S(a,b) cross sections compared to results without these cross sections were found. Problem 5 was modeled to obtain air kerma backscatter profiles for 150 and 200 kVp X-rays upon a water phantom. Air kerma backscatter profiles were determined along the apothem and diagonal of the front face of the phantom. A comparison of experimental results is also made. Problem 6 was modeled to determine indirect spectral and energy fluences upon two neutron detectors within a calibration bunker. The largest indirect contribution was found to come from low energy neutrons with an average angle of 47o where 0o is a plane parallel to the floor. Problem 7 was modeled to obtain pulse height distributions for a germanium detector. Comparison of analytically derived peaks with peak positions in the spectra are made. An examination of the Doppler broadening feature is also included.

Monte Carlo (MC) methods for radiotherapy dose calculation are widely accepted as capable of achieving high accuracy. In particular, MC calculations have been demonstrated to successfully reproduce measured dose distributions in complex situations where alternative dose calculation algorithms failed (for example, regions of charged particle disequilibrium). For this reason, MC methods are likely to play a central role in radiotherapy dose calculations and dose verification in the future. However, clinical implementations of MC calculations have typically been limited due to the high computational demands. In order to improve the feasibility of using MC simulations clinically, the simulation techniques must be made more efficient. This dissertation presents a number of approaches to improve the efficiency of MC dose calculations. One of the most time consuming parts of source modeling is the simulation of the secondary collimators, which absorb particles to define the rectangular boundaries of radiation fields. The approximation of assuming negligible transmission through and scatter from the secondary collimators was evaluated for accuracy and efficiency using both graphics processing unit (GPU)-based and central processing unit (CPU)-based MC approaches. The new dose calculation engine, gDPM, that utilizes GPUs to perform MC simulations was developed to a state where accuracy comparable to conventional MC algorithms was attained. However, in GPU- based dose calculation, source modeling was found to be an efficiency bottleneck. To address this, a sorted phase-space source model was implemented (the phase-space- let, or PSL model), as well as a hybrid source model where a phase-space source was used only for extra-focal radiation and a point source modeled focal source photons. All of these methods produced results comparable with standard CPU-based MC simulations in minutes, rather than hours, of calculation time. While maintaining reasonable accuracy, the hybrid source model increased source generation time by a factor of ~2-5 when compared with the PSL source model. A variance reduction technique known as photon splitting was also implemented into gDPM, to evaluate its effectiveness at reducing simulation times in GPU calculations. Finally, an alternative CPU-based MC dose calculation technique was presented for specific applications in pre-treatment dose verification. The method avoids the requirement of plan-specific MC simulations. Using measurements from an electronic portal imaging device (EPID), pre-calculated MC beamlets in a spherical water phantom were modulated to obtain a dose reconstruction. / Graduate