On the roles of solar UV irradiance and smoking on the diagnosis of second cancers after diagnosis of melanoma

Several recent papers have reported standardized incidence ratios (SIRs) for second cancers after diagnosis of cutaneous malignant melanoma. This review divides the types of cancer into five types: (1) those for which UV-B (UVB) irradiance and vitamin D reduces risk; (2) those for which UVB/vitamin D reduces risk and smoking increases risk; (3) smoking related; (4) unknown UVB/vitamin D and smoking sensitivity and (5) those for which UV irradiance increases risk. For those in category 1, SIRs were either significantly elevated or not significantly different from 1.0. For those in category 2, the SIR for kidney cancer was significantly elevated, whereas the SIRs for cervical, laryngeal and rectal cancer were significantly reduced. For those in category 3, all SIRs were significantly reduced. For those in categories 4 and 5, SIRs for all types except lip cancer were significantly elevated. A registry linkage study found significantly reduced SIRs for second cancers after diagnosis of nonmelanoma skin cancer in sunny countries but found increased SIRs in less sunny countries. The SIRs for second cancer for melanoma were elevated in both sunny and less sunny countries. This review concludes that sun exposure without sufficient vitamin D production may explain the elevated SIRs for vitamin D-sensitive cancers, whereas smoking-through production of skin elastosis, thereby reducing the risk of melanoma-probably explains the findings for smoking-related cancers. Thus, guidelines on UV irradiance should emphasize regular moderate UVB irradiance rather than avoidance for those who can tan.