The paper considers rapid calculations of light distributions in a tissue for photodynamic therapy. The delivery system considered is a single optical fiber, although other configurations are easily modeled. The influence of optical heterogeneities in the tissue is considered. The optical distributions surrounding an optical fiber within a tissue can be well approximated by a point source of light placed one "reduced mean free path" (mfp') in front of the fiber tip. One mfp' equals 1/(μa + μs(1-g)). Then the simple diffusion theory expression for 3D diffusion from a point source predicts the light distribution in the tissue. The influence of optical heterogeneities in the tissue can also be well approximated. A method is described for representing regions of increased absorption (such as a region with increased vascularity) as virtual sources of "negative radiant power" such that linear superposition of the true primary source and the virtual sources yields the net light distribution in the heterogeneous tissue. The method is rapid, flexible, and generally accurate to within about 15 % error. The method yields light distributions in optically complex tissues.