Photoexposure region-weighting factors (PRWFs) for photosensitive disease severity assessment: development and preliminary reliability of PRWF-based scoring

BACKGROUND: Dermatological severity indices commonly use body surface area-based regional weightings to combine regional disease scores into an overall score. In photosensitive disease, however, disease activity often concentrates at highly photoexposed anatomical sites that constitute a limited proportion of total body surface area. This may introduce systematic bias by constraining the contribution of clinically important photoexposed regions to total severity scores. OBJECTIVES: To develop photoexposure region-weighting factors (PRWFs) as a standardised, photoexposure-informed approach to addressing surface area-related bias in photosensitive disease severity assessment. METHODS: Anatomical regions characteristically affected in photosensitive disease were defined through expert consensus and mapped onto a body surface model. Regional ultraviolet exposure was calculated using a computational model across walking, standing and sitting postures. Exposure calculations were combined across postures, integrated by regional surface area, normalised within the photoexposed assessment domain and rounded to derive clinically applicable PRWFs. Preliminary clinical reliability was assessed using an earlier PRWF-based modification of the Eczema Area and Severity Index in six image-based atopic dermatitis cases scored by eight assessors across four institutions. RESULTS: Five anatomical regions were included: the face, V-area of the neck and upper chest, dorsal neck, dorsal forearms and dorsal hands. Collectively, these regions accounted for 10.7% of total body surface area. Clinically rounded PRWFs were 0.3× for the face, 0.1× for the V-area, 0.1× for the dorsal neck, 0.3× for the dorsal forearms and 0.2× for the dorsal hands. The earlier PRWF-based modification of the Eczema Area and Severity Index showed good inter-rater reliability in the first assessment round and moderate reliability in the second (ICC 0.81 and 0.70, respectively), with good median intra-rater reliability (ICC 0.84). CONCLUSIONS: PRWFs provide a standardised framework for addressing surface area-related bias in photosensitive disease severity assessment by weighting anatomical regions according to ultraviolet exposure rather than surface area alone. Preliminary reliability findings support the reproducibility of PRWF-based clinical scoring, but prospective validation of the final PRWF framework is required before routine clinical or trial use.