surgery and PDT. Anesthesia was induced with intravenous (i.v.) thiopental and maintained with 2% isoflurane inhalation via an endotracheal tube. Each dog was placed in dorsal recumbency, and the skin was prepared for surgery using standard aseptic technique. A midline incision was made through the skin extending from the umbilicus to the pubis symphysis and through the linea alba. The connective tissue and fat were dissected to expose the prostate gland. The prostate was visually assessed for any abnormalities and the dimensions, craniocaudal, left–right and dorsal–ventral, were measured. Diffusion theory and optimization algorithm. la and l0 s values characterize the scattering and absorption properties of tissues. With the diffusion approximation, the light fluence rate u at a distance, r, from a point source of source strength, S, can be expressed as (14)