Sun Can Show a Cancer Benefit

TUESDAY, Feb. 1, 2005 (HealthDayNews) -- In a perplexing finding, a new study suggests that exposure to sunlight may help people with melanoma live longer.

And a second study found sunshine confers yet another cancer benefit: It may reduce the risk of developing non-Hodgkins lymphoma.

The results of both pieces of research raise obvious questions.

"Sunlight is one of only 60 agents designated by the World Health Organization as an established human carcinogen," said Kathleen Egan, an associate professor of medicine at Vanderbilt University School of Medicine and lead author of an editorial accompanying the studies in the Feb. 2 issue of the Journal of the National Cancer Institute. "How is it that this human carcinogen can be associated with a beneficial impact with regards to cancer?"

The answer is that it might not be the sunlight per se that is responsible, but some other capacity of sunlight, specifically its role in vitamin D synthesis, Egan explained.

Scientists have long noted that while the incidence of melanoma has been rising in developed nations, so, too, has survival.

To try to understand this apparent anomaly, the authors of the first study followed 528 individuals with melanoma for five years. People who scored high on three measures of sun exposure -- sunburn, high intermittent sun exposure and solar elastosis (a measure of skin damage due to the sun) -- and had high skin awareness were less likely to die.

It's not clear what biological mechanisms are at work, said study author Marianne Berwick, a professor of internal medicine at the University of New Mexico Health Sciences Center, but she also pointed to vitamin D as the logical path of investigation. "We're realizing more and more that vitamin D controls cell proliferation," she said.

"It's possible that people who have genetic factors in the vitamin D receptor gene or other genes in that pathway are less able to metabolize vitamin D through sunlight or milk, and therefore have a much higher susceptibility to developing melanoma under low levels of sun exposure," she said.

Another explanation may be that sun exposure is linked with less aggressive cancers because it increases the ability of DNA to repair itself.

Or it may be that there are differences between melanomas.

"There may be a variant of melanoma that is not linked to sun exposure that tends to be more lethal than the normal variety," said Dr. Sumayah Jamal, an assistant professor of dermatology and microbiology at New York University School of Medicine.

Another expert seemed equally puzzled by the findings.

"These studies are interesting, and raise many questions that have to be answered by further study," said Dr. Martin Weinstock, chairman of the Advisory Committee on Skin Cancer for the American Cancer Society.

"In the melanoma study, sun exposure caused more melanomas but there were indications that people with higher sun exposure had a slightly better prognosis. It is not clear whether that was the result of sun exposure before or after the melanoma was diagnosed," He said. "It may be that the type of melanomas that develop in people who have lots of sun exposure are not as aggressive as those that arise in people with less sun exposure."

"Alternately, it might be that something about sun exposure affects the prognosis of melanoma," Weinstock said, "and the researchers suggest vitamin D may act as a protective factor, but at this point it is too early to say."

Regardless, the findings should not be seen as an invitation to bake in the sun. "It's clear and undeniable that the incidence of melanoma overall is higher in those with a history of childhood sunburn, of intermittent sun exposure, and of sun damage," Jamal said.

The second study found that exposure to ultraviolet (UV) radiation reduced risk for non-Hodgkins lymphoma. Like melanoma, this type of cancer has also been increasing worldwide.

The study authors, based in Sweden, looked at the history of UV exposure and other risk factors for lymphoma in 3,000 lymphoma patients and a similar number of controls.

People with a higher exposure to UV radiation through sunbathing and sunburns had a decreased risk of developing non-Hodgkins lymphoma. Specifically, a lot of sunbathing and sunburns at around the age of 20, and five to 10 years before the interview for the study, were associated with a 30 percent to 40 percent reduced risk.

It's too early, however, to determine if the link is a causal one, the study authors cautioned.

While the link between sunlight and vitamin D is an established one, the idea of a connection between vitamin D and cancer is relatively new, Egan said.

And there are other ways to boost vitamin D. "If people are concerned about their vitamin D production, the answer is to supplement their diets with vitamin D [fortified milk] as opposed to going out and getting more sun," Jamal said.

More information

For more on melanoma detection, visit the American Academy of Dermatology.