Abstract: Brachytherapy dose attributable to a brachytherapy source is computed for portions of a patient treatment volume corresponding to a pathological target volume and critical structures. Patient image data is accessed to derive a material voxel array. Multiple computation grids are derived. Primary particle fluence is computed for each first grid element using a ray tracing process from which a primary dose and a first scattered particle source are derived. Scattered particle fluence of the first scattered particle source is derived for each second grid element from which a secondary dose is derived. Each first grid element corresponds to a plurality of second grid elements. Primary dose and scattered dose combine to provide total dose at specific volumes. Brachytherapy source models and non-anatomical body surface models may be applied as applicable.