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Photodosimetry of interstitial light delivery to solid tumors.

Both anaplastic and well-differentiated Dunning prostate adenocarcinomas were illuminated in anesthetized Fischer X Copenhagen rats by single-fiber and multiple-fiber illuminators. Each illuminator consisted of a 2-cm laterally diffusing optical fiber placed within a plastic brachytherapy needle which was implanted into a tumor. Light attenuation coefficients for various wavelengths were obtained from measures of the radial falloff of intensity with distance from single fibers. These coefficients served as input to a 2-dimensional (2-D) photodosimetry computer code which calculated relative light intensities in planes perpendicular to single-fiber and various multiple-fiber configurations. These calculations assumed uniform optical property of tissue throughout each tumor, uniform and equal illuminance from diffusing fibers, and precise needle implantation. Relative light intensities along specific tumor tracks were measured and compared with those predicted by the 2-D photodosimetry code. Agreement within +/- 14% was observed for all configurations studied. Variations in relative intensity in tumor planes perpendicular to a standard seven-fiber illuminator were determined as a function of the distance between the implanted needles. Light wavelengths of 700 nm and greater produced relatively uniform light fields (approximately +/- 20%) in the R3327-AT tumor with needle spacings of at least 1 cm. The addition of two fibers at the periphery of this illuminator (a nine-fiber illuminator) improved the uniformity of light delivery to the encompassed tumor volumes. The importance of precision photodosimetry for interstitial applications of photodynamic therapy is discussed.