Use of illuminance as a guide to effective light delivery during daylight photodynamic therapy in the U.K.

Background: Daylight PDT (dPDT) is an effective and nearly painless treatment for field-change actinic keratosis. Measuring the protoporphyrin-IX (PpIX)-weighted exposure dose can give an indication of when conditions are most viable for effective dPDT. It would be advantageous for practitioners if more detailed information of exposure dose and appropriate treatment conditions were available. Where sophisticated measurement equipment is unavailable, simpler and more cost-effective methods of dose measurement are desirable.

Objectives: To devise a model whereby illuminance data can be converted into PpIX-weighted exposure dose, and to use this model to estimate appropriate times for dPDT across the U.K. and Ireland.

Methods: Spectral irradiance data were analysed to obtain a conversion model for illuminance to PpIX-weighted dose. This model was applied to historic illuminance data from nine sites to obtain PpIX-weighted dose across the U.K. and Ireland. Temperature data and an analysis of conservatory-based dPDT were also considered.

Results: Distribution of the expected PpIX-weighted dose across the nine locations is presented. Temperature data showed that it could be too cold for dPDT, even when there is sufficient light exposure. Conservatory-based dPDT could extend the times in the year for possible treatment.

Conclusions: This proposed conversion model provides a means of using an illuminance reading to calculate the PpIX-weighted exposure dose. Dosimetry of dPDT may be carried out simply and at low cost using the presented method; however, the results presented may be used as a guide for those considering dPDT, without the need to conduct measurements themselves.