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The oxidation of photosensitizers during photodynamic therapy (PDT) has important implications for their therapeutic and diagnostic potential. The reduction in sensitizer concentration during illumination progressively reduces the effectiveness of therapy and, ultimately, limits the destruction of the host tissue. In the course of our studies of the effects of PDT upon superficial skin disorders, following topical application of 5- aminolaevulinic acid (ALA), we routinely record the surface fluorescence emission of protoporphyrin IX (PpIX) before, during, and after therapy, in order to monitor the sensitizer photo-oxidation. It is important, therefore, to establish that measurements made in this way are representative of the variation in sensitizer concentration throughout the illuminated volume. We have developed a time- dependent Monte-Carlo model to simulate PpIX photo-oxidation during either low intensity laser (488 nm) or white light irradiation of plaque psoriasis. We have assessed the effect of differences in the optical properties of tissue at sites on different patients prior to treatment, and the effect of these variations on the surface fluorescence signal detected during treatment, at sites within the same plaque. The results show that the PpIX fluorescence intensity recorded from plaque psoriasis is an accurate indicator of the relative concentration of the sensitizer and can be used as a direct comparison between different sites and different patients. Also, the reduction in fluorescence emission during PDT is an effective measure of the depletion in sensitizer concentration throughout the illuminated volume. These results illustrate that the light dose required to achieve significant PpIX photo-oxidation is significantly lower than that often adopted for the treatment of superficial skin conditions.
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