A radiation planning system includes a dose volume kernel determiner (122) and an expected absorbed dose determiner (124). The dose volume kernel determiner (122) generates a dose volume kernel for each of a plurality of voxels in a dose calculation grid. Each of the dose volume kernels is based on a radial dose distribution and a template function for a particular voxel size. The expected absorbed dose determiner (124) determines an expected absorbed dose distribution for each of the plurality of voxels based on the dose volume kernel.
