Brachytherapy usually refers to a category of procedures where radiation sources are implanted into the tumor or into its close vicinity to deliver high radiation doses into the tumor for a therapy. A feedback about the dose rate distribution during the surgery is desired to guarantee the dose coverage and the success of a treatment. Calculation speed is crucial in this attempt and traditional Monte Carlo methods are not suitable for this purpose.;We developed a deterministic algorithm for computing three dimensional brachytherapy dose distributions. The deterministic algorithm has been based on the integral transport equation. The integral transport equation is solved by the Neumann Series and spatial, angular and energy discretizations. Source scheme and transport scheme are the two steps to calculate the scattered photon flux. The algorithm provided us a capability of computing dose distributions for multiple isotropic point and/or volumetric sources in a homogenous/heterogeneous medium. Two seed models, model 2301 and model 6711 were studied. The algorithm results for these two seeds have been benchmarked against the results from literatures and MCNP calculation. We have included fluorescence radiation physics to handle the silver fluorescence in seed model 6711. We designed and implemented a parallel algorithm to speed up the calculations. The introduction of parallel computing capability provided a means to compute the dose distribution for multiple seeds in a simultaneous manner.;We can compute dose distributions for a large set of seeds without resorting to the superposition methods, which is current technique to calculate overall dose distribution from multiple seeds. This provided a way to study strong heterogeneity and shadow effects induced by the presence of multiple seeds in an interstitial brachytherapy implant. By studying a 81 seeds problem, we observed that superposition of single seeds dose distribution can lead to at least 5~10% overestimation around implanted seeds. This provided more evidence that the shadow effect has significant impacts on the dose distribution and should be considered in the brachytherapy dosimetry.