[Photodynamic therapy in dermatology: basic principles]

Photodynamic therapy involves the administration of a photosensitizing drug and its subsequent activation by light at wavelengths matching the absorption spectrum of the photosensitizer. Currently, topical photodynamic therapy has received approval for the treatment of cutaneous oncologic conditions such as actinic keratoses, Bowen's disease and superficial basal cell carcinoma in many countries in the world. Multicenter randomized controlled studies have demonstrated high efficacy and superior cosmetic outcome over standard therapies. For many non-oncologic dermatological diseases such as acne vulgaris, viral warts and localized scleroderma, case reports and small series have confirmed the potential of photodynamic therapy. After the development of topical photosensitizers 5-aminolevulinic acid (ALA) or its methyl ester (MAL), photodynamic therapy has gained worldwide popularity in dermatology, as these drugs do not induce prolonged phototoxicity as the systemic photosensitizing hematoporphyrin derivatives do. The production of reactive oxygen intermediates such as singlet oxygen depends on the concentration and localization of the photosensitizer in the diseased tissue as well as the applied light dose. Either incoherent lamps or LED arrays are suitable for the cytotoxic effects resulting in tumor destruction or immunomodulatory effects improving inflammatory condition.