And a second study indicates that exposure to sunlight may reduce the risk of getting cancer of the lymph glands.
Researchers stress that their findings do not mean people should rush out and start basking in the sun. As for what people should do to gain sunlight's benefits without its downsides, an editorial accompanying the studies said more research is needed.
"Sunlight, particularly ultraviolet radiation, is a very well established human carcinogen. Nothing in these papers should in any way detract from this message," said Kathleen M. Egan of Vanderbilt University Medical Center.
But the new reports, being published this week in the Journal of the National Cancer Institute, do provide important clues to the development of these cancers and some factors that may slow or stop them.
Melanoma has been increasing over the past half-century in developed countries with Caucasian populations, and studies have consistently found exposure to the sun a major risk factor.
However, a new look at 528 melanoma victims over five years also found that increased sun exposure led to increased survivability, according to the study led by Marianne Berwick of the department of internal medicine at the University of New Mexico.
"It's totally counterintuitive, and we're trying to investigate it," said Berwick, noting that she is now doing a similar study of 3,700 melanoma patients worldwide.
"It's really strange, because sunburn seems to be one of the factors associated with improved survival, and that doesn't make much sense, so we think sunburn's a proxy for the kind of sun exposure that leads to melanoma. But there's so much we need to know," Berwick said in a telephone interview.
She said Vitamin D, which the skin makes in response to sunlight, might be a factor. Vitamin D can help regulate cell growth and help cells stop unneeded growth through a process called apoptosis.
Another possibility is solar elastosis, a response to sunlight that breaks down collages in the skin — the same process that causes sun-related wrinkling.
"It may be something in solar elastosis itself ... it may be that some physical barrier created by this breakdown of collagen keeps the melanoma from getting into the blood and lymph system," Berwick said.
In the second study, a research team led by Karin Ekstrom Smedby of the Karolinska Institute in Stockholm, Sweden, studied 3,000 lymph cancer patients and a similar number of people without lymph cancer in Denmark and Sweden.
They found that increased exposure to ultraviolet radiation through sunbathing and sunburns resulted in a reduced incidence of non-Hodgkin's lymphoma.
Vanderbilt's Egan, who was not involved in either research team, said it's unlikely to be sunlight itself that is an explanation of these findings.
The scientific community is converging on the idea that Vitamin D is likely to be a protective agent in cancer, she said in a telephone interview.
"It's long been known that Vitamin D is a critically important agent in bone health," she noted. "More recently it has become increasingly obvious that Vitamin D has important regulatory functions in the cell, in terms of cell division," she said.
In an accompanying commentary in the journal, Egan and co-researchers at Vanderbilt say the two findings are of particular interest because non-Hodgkin's lymphoma is suspected of being caused in a way similar to skin cancer.
More than a million cases of skin cancer are attributed to sun exposure annually in the United States, with about 54,000 cases of melanoma diagnosed each year, noted the Vanderbilt researchers, which also included Jeffrey A. Sosman and William J. Blot.
By Randolph E. Schmid