Microscopic distribution of protoporphyrin (PpIX) fluorescence in superficial basal cell carcinoma during light-fractionated aminolaevulinic acid photodynamic therapy

Light fractionation in aminolaevulinic acid photodynamic therapy (PDT) of superficial basal cell carcinoma has been shown to enhance the therapeutic efficacy significantly. We have shown previously that this increase in efficacy is not simply due to an increase in the amount of protoporphyrin utilized during therapy. The present study investigated the spatial distribution of protoporphyrin in 32 superficial basal cell carcinomas undergoing light-fractionated PDT. Superficial fluorescence imaging performed during therapy was compared with the microscopic analysis of protoporphyrin fluorescence in biopsies acquired immediately before the second of two light fractions. The microscopic distribution of fluorescence was also compared with tumour sections immunohistochemically stained for Ki-67. Large variations in superficial and microscopic protoporphyrin fluorescence were found in both control and treated lesions. Despite these variations there was a reasonable correlation between the two techniques (R2=0.86). The mean fluorescence intensity in control biopsies was greater than in illuminated lesions before the second light fraction, but there was no significant difference in the variation within and between regions of interest in each of these sets of lesions. There was no clear trend in depth of protoporphyrin reappearance during the dark interval between light fractions. The general distribution of cells stained positive for Ki-67 followed the protoporphyrin fluorescence that was associated with islands of basal cell carcinoma. In conclusion, this study confirms that the mean relative re-synthesis of protoporphyrin after PDT is consistent with that found previously in pre-clinical models. There are large variations in fluorescence within superficial basal cell carcinoma that include regions of tumour cells that do not synthesize protoporphyrin.

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