This paper describes microscopic observations on porphyrin fluorescence localization and histologic changes in head and neck cancer xenografts in nude mice after photodynamic therapy (PDT). At three and twenty-four hours after porphyrin administration, fluorescence is visible in fibroblasts and not in malignant epithelial cells. Six hours after PDT, fluorescent fibroblasts showed early bleaching, and no vascular changes were visible. At twenty-four hours after PDT, fibroblast fluorescence was completely bleached and cytologic evidence of fibroblast necrosis was apparent. In addition, vascular ectasia and engorgement was obvious, grossly and microscopically. Implications of these observations for the basic mechanism of PDT are discussed.

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