Materials with visually important heterogeneous subsurface scattering, including marble, skin, leaves, and minerals, are common in the real world. However, general, accurate and efficient rendering of these materials is an open problem. In this short report, we describe the heterogeneous diffusion equation (DE) formulation that solves this problem. This formulation has two key results: an accurate model of the reduced intensity (RI) source, the diffusive source boundary condition (DSBC), and its associated render query function. Using there results, we can render subsurface scattering nearly as accurately as Monte Carlo (MC) algorithms. At the end of this report, we demonstrate this accuracy by comparing our new formulation to other methods used in previous work.