Fluorescence Diagnosis and Photodynamic Therapy in Dermatology

Summary In several countries throughout the world the photosensitiser Photofrin® has been approved for systemic photodynamic therapy (PDT) for different oncological indications. However, owing to the prolonged cutaneous photosensitization entailed, the use of this porphyrin derivative is restricted. Currently, the most promising sensitizers in dermatology that can be applied topically are 5-aminolevulinic acid (ALA) or ester derivatives that are precursors of heme biosynthesis. ALA has shown good clinical and excellent cosmetic results in superficial skin cancer and precancerous conditions, e.g. superficial basal cell carcinoma (BCC), or actinic keratoses (AK). ALA-PDT for AK was approved by the FDA in late 1999 and the corresponding registration process for ALA-methyl ester in Europe led to approval in 2001. Besides its usefulness in PDT, ALA also has a unique feature that can be exploited for diagnostic purposes: after topical or systemic application protoporphyrin IX is induced rather selectively in epithelial tumours, with a high tumour-to-surrounding tissue ratio. Upon irradiation with light the tumour becomes visible and can be delineated from the surrounding tissue. This procedure called fluorescence diagnosis (FD) will enable the dermatologist to perform either a directed biopsy or very likely a controlled and complete resection of the tumour sparing vital tissue. By using a CCD camera system together with digital imaging, the contrast of the acquired fluorescence images can be significantly enhanced and allows the determination of a threshold, which can be utilized either for a directed biopsy or for preoperative planning when Moh's surgical technique is scheduled. Moreover, FD is a helpful tool to prove the efficacy of PDT. At present, the routine employment of such systems is being assessed in prospective studies.