Irradiance and light dose influence histological localization of photodamage induced by photodynamic therapy with aminolaevulinic acid.

Abstract

Photodynamic therapy (PDT) with aminolaevulinic acid (ALA) is used in many countries for the treatment of actinic keratoses. The objectives of the current study were to evaluate the influence of irradiance and light dose on the localization and extent of photodamage to mouse skin following ALA-PDT. In this study we evaluated the influence of irradiance and light dose on epidermal photodamage following ALA-PDT. Groups of hairless mice received an intraperitoneal injection of ALA followed 2 h later by exposure to 12 J cm(-2), 24 J cm(-2) or 48 J cm(-2) of unfiltered light from a slide projector. For each of these fluences, groups of mice were exposed at the following irradiances: 5 mW cm(-2), 20 mW cm(-2) or 40 mW cm(-2). Skin biopsies were performed 24 h and 72 h later. Histological localization of photodamage was influenced by irradiance and light dose. At high irradiance and low fluence, PDT photodamaged cells were mostly located in the upper area of the epidermis whereas at lower irradiance and high fluence the complete epidermis was necrotic and often absent 24 h after light exposure. Protoporphyrin IX (PpIX) fluorescence intensity was similar in the upper and lower region of the epidermis 2 h after ALA injection. The decrease in PpIX fluorescence intensity immediately after light exposure was also similar in both regions of the epidermis. The localization of photodamage following ALA-PDT is influenced by irradiance and light dose. This phenomenon cannot be explained by differences in PpIX intensity in the epidermis either before or after light exposure.