Photodynamic therapy for actinic keratosis of the forehead and scalp with the Aktilite CL 128: Is there a cut-off value for PpIX-weighted irradiance for effective treatment?

Background/purpose: Photodynamic therapy (PDT) is an established treatment for actinic keratosis (AK). Among the approved protocols in Europe, the most widely used involves irradiation with the Aktilite CL 128 (C-PDT). We aimed to assess the heterogeneity of irradiance over the treatment area when using C-PDT. We also investigated whether there is a cut-off value for protoporphyrin IX (PpIX)-weighted irradiance that may predict the treatment outcome of C-PDT.

Methods: An Ophir PD300 photodiode sensor connected to an Ophir Laser Star power meter was used to measure the irradiance delivered to 114 AKs of the scalp and forehead of 19 patients treated with C-PDT. The PpIX-weighted irradiances were deduced and cross-referenced with the complete responses at 3 months.

Results: From the measured irradiances ranging from 0.25 to 60 mW/cm (average: 31.94 mW/cm ), a standard deviation of 17.17 mW/cm was computed. Irradiance heterogeneity over the treatment area during C-PDT was demonstrated. The 66/114 AKs with a complete response at 3 months (57.89%) received a mean PpIX-weighted irradiance of 0.52 mW/cm vs 0.56 mW/cm for the resistant 48/114 AKs (42.11%). No significant effect of PpIX-weighted irradiance on the complete response at 3 months was found (odds ratio for a 0.1-unit change, 0.96; 95% confidence interval, 0.83 to 1.10; P = 0.53). Therefore, no cut-off value for PpIX-weighted irradiance that predicts treatment outcome could be identified.

Conclusions: A device enabling homogeneous irradiation at a lower irradiance than the Aktilite CL 128 may therefore be suitable. This lower irradiance may further increase the treatment tolerance by patients.