OPTIMIZATION OF MULTIFIBER LIGHT DELIVERY FOR THE PHOTODYNAMIC THERAPY OF LOCALIZED PROSTATE-CANCER

The understanding of light distribution within the target organ is ess ential in ensuring efficacy and safety in photodynamic therapy (PDT). A computer simulator of light distribution in prostatic tissue was emp loyed for optimizing dosimetry for PDT in localized prostatic cancer. The program was based on empirically determined light distributions an d optical constants and an assumed fluence rate differential from fibe r source to necrosis periphery. The diffusion theory approximation to the Boltzmann transport equation was the applicable formulation releva nt to prostatic tissue, which has a high albedo with forward-scatterin g characteristics. Solving this equation of diffusive transfer for the appropriate fiber geometry yielded the energy fluence distributions f or cleaved fiber and cylindrical diffuser light delivery. These distri butions, confirmed by our measurements, show a 1/r and 1/square-root r dependency (r = distance from light source) of the fluence phi(r) for the cleaved fiber and diffuser, respectively. This manifests itself b y the tighter spacing of energy fluence isodoses in the case of the cl eaved fiber. It was predicted that for a typical PDT regime a single i nterstitially placed cleaved fiber would treat 0.05-0.72 cm3. Four par allel fibers improve the uniformity of light distribution and treatmen t volume, and an interfiber separation of 12 mm would be necessary to provide optimal overlap of PDT necrosis, treating 0.26-3.6 cm3. The cy lindrical diffuser, however, could treat larger volumes, and it was pr edicted that four 3 cm long diffusers at an optimal separation of 25 m m would treat 25-88 cm3 of prostatic tissue.