Fausto Petrelli, Antonio Ghidini, Karen Borgonovo, Mara Ghilardi, Maria Chiara Parati, Loredana Burgoa, Lorenzo Dottorini, Mauro Rossitto, Giuseppina Dognini, Daniela Petrò, Veronica Lonati, Michele Ghidini, Gianluca Tomasello
Sunlight exposure may influence breast cancer risk through cutaneous vitamin D synthesis, circadian regulation, and immune modulation, yet prior meta-analyses were limited and did not examine hormone-receptor subtypes. In this systematic review and random-effects meta-analysis of 31 epidemiologic studies, higher sunlight exposure was associated with lower breast cancer incidence (pooled odds ratio 0.80; 95% confidence interval 0.74-0.86), with the strongest associations in case-control studies using personal exposure measures and weaker associations in cohorts relying on ambient-ultraviolet proxies. Protection was confined to estrogen-receptor-negative (0.71; 0.58-0.87) and triple-negative tumours (0.55; 0.37-0.81), with no association for estrogen-receptor-positive disease, and survival was improved for diagnoses in sunnier seasons (hazard ratio 0.84; 0.80-0.88), while a dose-response analysis indicated an 8% lower risk per 10 relative exposure units. Because the evidence is observational and subject to confounding and exposure misclassification, these hypothesis-generating findings nonetheless support further study of the light environment in breast cancer aetiology and prevention, balanced against skin-cancer risk.