Carsten Philipp, MD, Consultant Lasermedicine, Berlin, Germany
Wednesday, March 6th 2024, 2:00 – 3:30 pm
Skin is the largest organ of the human body. It is easy accessible for topical application of drugs and light dosimetry, due to the rather flat appearance in a first approximation. Furthermore, a larger number of skin diseases cover larger areas or show multifocal appearance, which favors regional or large field therapies and impedes surgical interventions. As scar formation is another unwanted result of surgery but less frequently seen in PDT, the widespread use of PDT in dermatology is a necessary consequence.
Since the introduction of ALA or MAL as precursor drugs topical PDT with PPIX has been developed into a reliable and effective tool for the treatment of superficial basal cell carcinoma (BCC) or Bowen’s disease (MB) and actinic keratosis. This is even more important as the incidence of those lesions shows a significant increase during the last decade and an even stronger increase must be expected in the future. The typical fluorescence of PPIX may also be used for fluorescence diagnostic and control of therapy and is another advantage. Overtreatment is unlikely, as PPIX shows a “bleaching” with subsequent decay of ROS-generation during the light exposure. A combination with OCT for tumor demarcation and depth measurement has shown to be applicable and helpful.
Pain during light exposure is one of the limiting factors for topical PDT. In a prospective control study, a dosimetry concept for pain reduction employing an escalating light fluence rate, first described by Nathalie Zeitouni et al.(1) was confirmed. Since then, we use this protocol with good patient acceptance and clinical results.
Nevertheless, topical PDT is only effective in some tumor types and thickness of the tumors is crucial as penetration of the drug (ALA or its derivatives) and conversion into PPIX is the limiting factor. In larger volumes and other tumor entities systemic PDT with intravenously applied photosensitizers (PS) offer advantages. As photosenzitation of skin in general is one of the typical features that come along with systemic PS, a differentiated light dosimetry becomes much more important. Shielding of non-diseased sites, interval between PS administration and light application, light dose and way of application (surface application or interstitial), single or repeated exposures are examples for the numerous variables in light application that may be applied.MAL-PDT, systemic PDT, actinic keratosis, NMSC, M. Bowen, BCC, pain, dosimetry, light fluence rate, local anesthesia
1) Zeitouni N C, Sunar U, Rohrbach D J, Paquette A D, Bellnier D A, Shi Y, Wilding G, Foster T H and Henderson B W, 2014, A prospective study of pain control by a 2-step irradiance schedule during topical photodynamic therapy of nonmelanoma skin cancer,Dermatol Surg. 2014 Dec; 40(12): 1390–1394