Daylight photodynamic therapy (dPDT) is widely used in Europe for field-change actinic keratosis (AK). Within 30 minutes of topical photosensitiser prodrug application to AK, solar illumination commences, inducing a photochemical reaction. dPDT is as effective as conventional PDT but allows large field treatment, with reduced pain; resulting in clinician and patient preference. As AK result from chronic UV exposure, it is important to limit ultraviolet (UV) radiation exposure during dPDT.
Historical illuminance, irradiance and erythema-weighted irradiance data from 2013 to 2017, covering 12 locations in the UK and Europe were obtained from Public Health England. Data were converted into protoporphyrin-IX (PpIX) weighted dose, UVA exposure and erythema-effective dose assuming a 2 hour dPDT treatment. Results were subsequently averaged for each time period during a calendar month. Analysis was performed evaluating the UV exposures during viable dPDT periods, assuming a minimum threshold dose of 4 Jcm-2.
The maximum average UV exposure for the UK was 8.2 standard erythemal dose (SED) in July between 1200 and 1400 in Camborne (UVA dose 25.4 Jcm-2, PpIX dose of 23.4 Jcm-2). For the same location and month between 0900 and 1100, a reduced exposure of 5.2 SED, with PpIX dose of 18.2Jcm-2 occurred. In November, at the same location between 1200 and 1400, average UV exposure was 0.8 SED and PpIX dose of 7.1 Jcm-2.
Thus, visible light levels can still be high enough for effective dPDT during periods of relatively low UV exposure, which is important, particularly when treating patients at high risk of skin cancer.