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Understanding the distribution of light in tissue is necessary for dosimetry in photodynamic therapy of malignant tumors. Attenuation measurements from implanted fiber optic sources in R3327-AT (rat prostate) tumors show that light penetration is greatest in the direction of the beam itself and least in the backward direction. This anisotropy of the overall optical distribution persists to distances of at least 2 cm from the source and is a result of strong anisotropy in the scattering phase function. The inclusion of scattering anisotropy in the theoretical model represents an improvement over the usual diffusion description. Predictions of space irradiance utilizing the transport approximation show good general agreement with experimental space irradiance results. The specific optical constants used in the model are consistent with actual measured values of scattering, absorption, and attenuation coefficients. The theory can be applied to other optical sources.