Insights gained from regression analysis of PpIX fluorescence imaging undertaken during routine dermatological photodynamic therapy

Clinical protoporphyrin IX (PpIX) fluorescence imaging was conducted using a pre-validated, non-invasive imaging system (Dyaderm, Biocam, Germany) during routine methyl aminolevulinate (MAL)-PDT treatment of 172 patients with licensed dermatological indications (37.2% actinic keratosis, 27.3% superficial basal cell carcinoma and 35.5% Bowen’s disease). Linear and logistic regressions were employed to model any relationships between variables that may have affected PpIX accumulation and/or PpIX photobleaching during irradiation and thus clinical outcome at three months. Patient age was found to be associated with lower PpIX accumulation and photobleaching, however only a reduction in PpIX photobleaching appeared to consistently adversely affect treatment efficacy. Clinical clearance was reduced in lesions located on the limbs, hands and feet with lower PpIX accumulation and subsequent photobleaching adversely affecting the outcome achieved (OR: 0.5 (0.2, 0.9; p<0.05). If air cooling pain relief was employed during light irradiation, PpIX photobleaching was significantly reduced (p < 0.05) and this resulted in an approximate three-fold reduction in the likelihood of achieving clinical clearance (OR: 0.4 (0.2, 0.7; p<0.01). PpIX accumulation and photobleaching are therefore concluded to be important indicators of dermatological MAL-PDT treatment success and anything that adversely effects them has the potential to reduce treatment efficacy. PpIX photobleaching during the first treatment was found to be an excellent predictor of clinical outcome across all lesion types and non-invasive imaging of PpIX fluorescence during MAL-PDT continues to provide important treatment insights that can be utilised to improve treatment protocols and thus clinical outcomes.