Monte Carlo volume integration of dose point kernels was used for calculating voxel S values of beta emissions of radionuclides of interest for internal radiotherapy. The method was verified by comparing our results with others derived from Monte Carlo radiation transport simulation. The algorithm has been implemented in a C++ program that can be used by any laboratory to calculate voxel S values of beta emissions from tabulated dose point kernels and for any combination of pixel edges and the thickness of SPECT and PET images without the complexity and expertise needed for direct Monte Carlo radiation transport simulations.