Ultraviolet radiation and the development of non-melanoma and melanoma skin cancer: clinical and experimental evidence.

Clinical and experimental evidence explaining and supporting the role of UV radiation as a causal factor for the induction and promotion of nonmelanoma and malignant melanoma skin cancer are presented. While there is excellent animal experimental data and human epidemiologic evidence supporting the causal relationship of UVR (UVB, as well as UVA radiation) for basal and squamous cell carcinomas, the data establishing a direct causal relationship between melanoma and exposure to sunlight appear to be complex. They do, however, suggest a definite promotional role of sunlight in the causation of melanoma. Using a hairless pigmented mouse strain (Skh-hr2), experiments were initiated to examine the role of UVR in the induction of melanoma. A single application of DMBA as an initiator and subsequent thrice-weekly exposures to either UVB (290-320 nm) or UVA (320-400 nm) or the combined exposures of UVA and UVB resulted in the formation of blue nevus-like lesions. Repeated UVR exposures for over 30 weeks resulted in the development of melanoma (38%), as well as lymphoma and squamous cell carcinoma only in those mice that were pretreated with DMBA and had developed nevi. Mice receiving UVB, UVA, or the combination treatments of UVB plus UVA without DMBA pretreatment developed papillomas and squamous cell carcinoma but no melanoma. These studies indicate that some initiation event is essential to transform melanocytes to blue nevus-like lesions before UVR (UVB + UVA) can act as a promoter and accelerate the development of malignant melanoma, as as well as lymphoma.