1. A radiation treatment planning system comprising: a dose volume kernel determiner that determines a dose volume kernel of a targeted radionuclide therapy for each of a plurality of voxels in a dose calculation grid in image data generated by one of a SPECT scanner or a PET scanner, and the dose volume kernel includes a multi-dimensional array that describes a dose absorbed in neighboring voxels due to an amount of radioactivity of a radionuclide in a source voxel; wherein the dose volume kernel determiner determines the dose volume kernel for a voxel of the plurality of voxels by sampling a single pre-calculated and stored radial dose distribution, which is pre-calculated around a point source and represents an absorbed dose due to the point source of a therapy isotope as a function of distance from the point source to a predetermined voxel size using a template function for the predetermined voxel size corresponding to a size of the voxel; an expected absorbed dose determiner that determines an expected absorbed dose distribution for each of the plurality of voxels based on the dose volume kernel; and a user interface that displays an expected absorbed dose map with a total expected absorbed dose determined for each voxel using the determined expected absorbed dose distribution.