Protoporphyrin IX photobleaching during the light irradiation phase of standard dermatological methyl-aminolevulinate photodynamic therapy

SummaryBackgroundMethyl-aminolevulinate photodynamic therapy (MAL-PDT) is a successful treatment for non-melanoma skin cancers in the UK. Monitoring the photobleaching of the photosensitiser, protoporphyrin IX (PpIX) during treatment has been demonstrated to indicate the efficacy of the treatment. This study investigated photobleaching during light irradiation.MethodsA validated non-invasive fluorescence imaging system was utilised to monitor changes in PpIX fluorescence during light irradiation. Fifty patients were recruited to this study, with patients monitored before, during (forty patients at the half way stage and ten at regular intervals in the initial phase of treatment) and after light irradiation.ResultsPhased PpIX photobleaching was observed during light irradiation with a significantly greater change (P < 0.001) in PpIX photobleaching during the first half of light treatment. Within the ten patients monitored periodically the phased photobleaching observed fitted a double exponential decay curve (r2 = 0.99, P < 0.005) suggesting a rapid initial phase of reaction when the light treatment was commenced.ConclusionsPhotobleaching was observed to be maximal in the initial phases of treatment, however photobleaching of PpIX continued until the completion of light treatment indicating that current clinical protocols for MAL-PDT do not over-treat the lesion with light.