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Sunscreens and melanoma: an on-going controversy

Sober, A.J.

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doi: 10.1097/01.cmr.0000382752.55377.ed
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Not all melanomas are caused by solar exposure. The fraction of UV attributable melanoma varies from perhaps 90% in Australia to near zero in individuals with type VI skin in Africa. This area is further confounded by the lack of clear data determining which wavelengths participate in the generation of melanomas. The topic is further confounded by the apparent paradox of studies from Australia and elsewhere that individuals using sun block had higher risk of melanoma development than non-users. A meta-analysis of articles published from 1966 to 2003 failed to demonstrate an association between melanoma and sunscreen use (Dennis et al.Ann Intern Med 2003; 139:966–978). Numerous lines of evidence support a role for solar exposure and melanoma including association of lentigo maligna melanoma with chronic cumulative exposure, increase risk for melanoma in PUVA patients and in those individuals using tanning equipment. Recent studies have also suggested an effect of UV exposure on DNA producing many somatic mutations found within the melanoma cells’ genome (Pleasance et al.Nature 2010; 463:191–196). Sunscreen use has been shown to reduce the number of actinic keratoses. How to explain this apparent paradox? Effective sunscreens have only been available since the 1980's. Adequate UVA protection has been added much more recently. Widespread use of sunscreens is a relatively recent phenomenon. Many applications of sunscreen are not uniformly applied nor sufficiently thick to provide the anticipated degree of photo protection. Since the use of sunscreen allows longer periods of solar exposure without burn, this effect on sun exposure behavior may outweigh any benefit derived from physical protection. If solar (UV) exposures before adulthood are the most important for melanoma generation, then protection early in life will be critical and future cohorts will need to be examined to discern any benefit from UV reduction strategies. Such a finding is being observed in Australian males where younger cohorts appear to be demonstrating falling rates of melanoma development. Melanoma risk has been related to the number of nevi present. Sun exposure early in life increases nevus number. Sunscreen use early in life has been shown to reduce the number of nevi. Use of modern sunscreen enhances immune protection. On the other hand, an additional concern has been raised recently about whether nanoparticle formulations of sunscreen result in penetration of nanoparticles through the skin. At the present time evidence that sunscreen use would reduce melanoma risk is at best inferential.

© 2010 Lippincott Williams & Wilkins, Inc.