Early UVA light exposure is unlikely cause for high melanoma incidence

Absorbing a higher does of UVA does not increase the risk of melanoma, say researchers from The University of Texas M.D. Anderson Cancer Center.

According to background information provided by the authors, “UVB exposure damages DNA directly, while UVA is thought to inflict its damage indirectly by inducing melanin free radicals that react with DNA to form oxidative damage that leads to melanoma.”

In a study led by David Mitchell, PhD, professor in M.D. Anderson's Department of Carcinogenesis, researchers tested the effects of UVA and UVBexposure in melanoma-prone fish hybrids that develop the disease spontaneously 15% to 20% of the time without exposure to UV light. The fish were exposed to either UVA or UVB daily between their fifth and tenth day of life, and the researchers scored the fish for melanoma 14 months after exposure.

“We found that UVB exposure induced melanoma in 43% of the 194 treated fish, a much higher rate than the 18.5 percent incidence in the control group that received no UV exposure,” Dr Mitchell said. “This was expected because UV exposure at an early age is a well-established cause of melanoma.” Specifically, only 12.4% of 282 fish exposed to UVA developed the disease, a proportion, the researchers explained, that is not statistically different from the control group.

“The thought was that people who used sunscreen stayed out in the sun longer, absorbing a higher dose of UVA, causing a higher risk for melanoma” said Dr. Mitchell. “Most sunscreens now protect against UVA. However, the increase in the incidence of melanoma has been thought to be partly attributable to childhood exposure to UVA back when sunscreens blocked only UVB. That's unlikely, given the new results.”

The findings were published in the Proceedings of the National Academy of Sciences 2010 May 3 [Epub ahead of print]).

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