Ultraviolet radiation exposure during daylight Photodynamic Therapy

Background: Daylight photodynamic therapy (dPDT) is an effective treatment for field-change actinic keratoses (AK), with similar efficacy to conventional PDT but lower patient pain scores. Whilst AK occur consequent to chronic solar ultraviolet (UV) exposure, paradoxically solar visible radiation is used during PDT. Objectives: To investigate the nature and levels of UV exposure, both erythemal UV and UVA, occurring during dPDT. Methods: Four years of solar erythemally effective UV (UVE) irradiance, UVA irradiance and illuminance data were obtained from Pubic Health England for 12 locations. For a standard 2 h treatment period, the data were converted into standard erythemal doses (SEDs), UVA dose and protoporphyrin-IX (PpIX)-weighted dose from UVE irradiance, UVA irradiance and illuminance respectively. These three parameters were compared ascertaining the UV exposure received during dPDT. Results: Analysis of UV exposure during dPDT showed a UK maximum average UVE exposure of 8.2 SED at Camborne (PpIX dose 23.4 J cm⁻²). Treatment earlier in the day reduces average UV exposure (Camborne 5.2 SED, PpIX dose 18.2 J cm⁻²), whilst PpIX dose achieves threshold during winter months (Camborne, November, 0.8 SED, PpIX dose 7.1 J cm⁻²). Cyprus and Gibraltar (with high UV exposure during dPDT) experience a maximum of 14.3 SED and 12.9 SED, with respective PpIX doses of 36.1 J cm⁻² and 35.1 J cm⁻², in June. UVA exposure is also presented for comparison. Conclusion: Therapeutically effective dPDT doses can be achieved at times of the day and year when UV exposure is minimal.