A system for radiation therapy including a radiation planning system, wherein the radiation planning system comprises a parallel processor adapted to receive input information concerning a body having an intended radiation treatment region and to output information for providing radiation treatment to the intended radiation treatment region of the body, wherein the parallel processor is adapted to perform a plurality of reverse ray tracing calculations based on the input information concerning the body in determining the output information for providing radiation treatment, each of the plurality of reverse ray tracing calculations comprising: calculating a first physical property corresponding to a first sub-region of the intended radiation treatment region of the body that is intersected by a ray traced between a source position and the intended radiation treatment region; and calculating, subsequent to the first-mentioned calculating, a second physical property corresponding to a second sub-region of the intended radiation treatment region that is intersected by the ray at a location closer to the source position than is the first sub-region.