THE USE OF PHOTODYNAMIC THERAPY FOR SKIN-CANCER

A photodynamic reaction is the excitation of a photosensitizer, mainly porphyrin derivatives, by visible light emitted by lamps or lasers in the presence of oxygen, resulting in the generation of reactive oxyge n species (ROS), in particular singlet oxygen. These ROS mediate cellu lar, e.g. lipid peroxidation, and vascular effects, e.g. thrombosis, d epending on the tissue localization of the photosensitizer. The result is a direct or indirect cytotoxic effect on the target cells. Photody namic therapy (PDT) is a treatment modality which has been mainly show n to be effective for precancerous and cancerous conditions such as ac tinic keratosis, Bowen's disease, initial squamous cell carcinoma and basal cell carcinoma. The main research topic over the last decade has been the synthesis and experimental evaluation of chemically pure pho tosensitizers which generate a high quantum yield of singlet oxygen, s how a high selectivity for the diseased tissue and exhibit limited pho tosensitivity when administered systemically. However, for dermatologi c indications the use of photosensitizers which can be applied topical ly is of particular interest, because systemic application of porphyri ns results in a prolonged cutaneous photosensitization for weeks. More over, to facilitate the introduction into dermatologic routine therapy , there is also a need for cheap and reliable light sources. Therefore , recent work has been focused on the development and evaluation of to pical photosensitizers (like the heme precursor 5-aminolevulinic acid inducing photosensitizing porphyrins) and incoherent light sources (la mps). Cure rates reported for very superficial lesions (tumor thicknes s < 3 mm) are similar to that achieved by other therapeutic modalities The benefit of PDT as compared with established therapies is the exce llent cosmetic result after treatment. However, PDT is still experimen tal, and there is a need for comparative phase III studies and standar dized therapeutic protocols to define the place of PDT in the treatmen t of skin cancer.