The main goal of this dissertation was to establish the feasibility of basing megavoltage external photon beam absorbed dose calculations in voxelized phantoms on S_N deterministic calculations and pre-calculated electron absorbed dose kernels derived from full-physics Monte Carlo. The S_N derived electron absorbed dose kernel method EDK-S_N, developed as part of this research, achieves total execution times that are on the order of several times to orders of magnitude faster than conventional full-physics Monte Carlo electron transport methods considering equivalently detailed models and data fidelity.

With the rapid movement toward intensity modulated radiation therapy (IMRT), radiation beam intensities have increased dramatically over the past decade, thus heightening the need for further characterization of out-of-field organ absorbed doses, along with their associated biological risks. Assessment of these tissue absorbed doses is complicated by two fundamental limitations. First, anatomic information on the patient is generally restricted to a partial body CT image acquired for treatment planning; consequently, whole-body computational phantoms must be employed to provide the out-of-field anatomy model structure for absorbed dose evaluation. Second, existing methods based on Monte Carlo radiation transport, even with the application significant variance reduction, are quite computationally inefficient at large distances from the primary beam, and point-kernel methods do not properly handle tissue inhomogeneities. Moreover, since absorbed dose are generally tracked in all major organs in the body, variance reduction schemes for Monte Carlo are not all effective in this regard.

The outcome of this dissertation is to demonstrate that absorbed dose from high-energy external beams radiation can be accurately computed for whole body and organ-specific absorbed doses. The EDK-S_N method implements voxelized phantoms with discrete ordinates (SN) transport computations coupled with directional influences and (pre-computed) full-physics Monte Carlo based electron absorbed dose kernels to yield total body absorbed dose information. This research shows that the deterministic techniques coupled with Monte Carlo based electron absorbed dose-kernels has significant potential for organ absorbed dose evaluation in the clinical management of radiation therapy patients.