Only 12.4 percent of 282 fish exposed to UVA developed the disease, which is not statistically different from the control group.
An influential 1993 study using the same hybrid fish connected UVA exposure to melanoma. Until that study, Mitchell said, sunscreens protected only against UVB exposure, which was of immediate public health concern because UVA makes up 90 percent of the ultraviolet light spectrum of sunlight.
"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" Mitchell said. 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, Mitchell said.
The 1993 experiment could not be replicated in mammalian models of melanoma, Mitchell said, and a statistical retrospective of the 1993 paper indicated problems with sample sizes that were too small to yield a definitive answer on UVA exposure.
So, Mitchell and colleagues conducted the experiment again, with much larger sample sizes that provided the statistical power to reach stronger conclusions.
They also stratified the melanomas found in each group by severity, with the control and UVB-exposed fish having a higher incidence of severe, stage IV disease, while those exposed to UVA had significantly more early stage melanomas.
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. Previous studies had shown a correlation between melanin radical formation and melanoma in the UVA range of the solar spectrum. Since
|Contact: Scott Merville|
University of Texas M. D. Anderson Cancer Center