The problem of computing dosimetric quantities of interest such as dose, exposure, or fluence for a given arrangement of x-ray sources and absorbing media is central to all branches of radiological physics. Unfortunately, all but the very simplest geometries defy rigorous solution either by analytical or numerical techniques. The main source of mathematical complexity is that scattering processes such as Compton effect and coherent scattering, as well as production of characteristic, annihilation, and bremsstrahlung photons, change both the energies and trajectories of the primary photons. The progressive degradation of photon energy and dispersion of photon trajectories due to multiple scattering makes the solution of a transport problem highly dependent upon its boundary conditions.