Melanoma experts grapple with the sun question: one hypothesis is that vitamin D may help protect against cancer, but epidemiologic evidence is scant.
That was a major topic of discussion by the experts at the recent Sixth World Congress on Melanoma.
The specific subjects ranged from evidence that vitamin D might have a role in protecting against cancer, to evidence that genetic susceptibility might play a previously underappreciated role in melanoma risk, to the fact that messages about sunburn avoidance and use of sunscreen are having little impact on behavior and so could be the wrong strategy for reducing mortality.
New evidence strongly suggests that people who have had more sun exposure have better survival from melanoma than those who have had less, said Marianne Berwick, Ph.D., an epidemiologist at the University of New Mexico Cancer Research and Treatment Center in Albuquerque.
She and her colleagues recently found that patients who reported ever having been sunburned had half the mortality of those who had never been sunburned. The study looked at 528 individuals with melanomas who were followed until death or to 5 years. They also found similar increases in survival likelihood related to overall sun exposure and to the presence of marked solar elastosis (J. Natl. Cancer Inst. 2005;97:195-9).
The same observations have been noted in previous research, conducted by others as well as by herself, Dr. Berwick said.
More recently, Dr. Berwick and her colleagues examined data they collected from the multinational Genes, Environment and Melanoma Study. They compared melanoma cases from Australia, where melanoma incidence is high, with those from Canada, where sun exposure is low.
Most of the cases (60%) were thin melanomas (0.76 mm or less Breslow thickness).
Although there was no difference in median thickness between cases from the two countries, there was a difference in the presence of solar elastosis and the percentage of tumors that showed evidence of being in a vertical growth phase, the investigation found.
Solar elastosis was present in 60% of the Canadian patients, and 64% of the melanomas were in a vertical growth phase when they were removed. Solar elastosis was present in 75% of the Australians, but only 45% of the melanomas were in a vertical growth phase, Dr. Berwick said.
Although this evidence may help explain why survival might be better in areas where melanoma incidence is highest and why incidence is rising but mortality is not, the question of mechanism remains, Dr. Berwick said.
The popular hypothesis is that this might be due to the effects of vitamin D, she noted. But the epidemiologic evidence that vitamin D is chemoprotective in people who might otherwise be at risk of melanoma is slim. No one knows what the optimal level of vitamin D is, and there are different ways to measure levels. Moreover, studies that have tried to correlate vitamin D level with cancer cases have had conflicting results.
Alternative possible explanations for reduced mortality include the idea that the more aggressive types of melanoma are not the types associated with UV light damage, or that the collagen deposits and other skin changes that come with solar elastosis form a barrier to invasiveness and/or metastasis.
So is there a safe level of sun exposure, one that perhaps should be recommended to the public?
Some experts are beginning to think there is, said Bruce Armstrong, M.D., head of the school of public health at the University of Sydney, Australia, at the meeting. Reviewing the history of the evidence. Dr. Armstrong said it is fairly clear that sun exposure attenuates cancer risks, and that melanoma is not the sole malignancy to have a relationship to lack of sun.
The observation that sunlight may protect against cancer was first made in 1941 by investigators exploring the idea that people who got skin cancer did not get other cancers; since then, low sun exposure has been correlated with a high risk of prostate, breast, ovarian, and colon cancers and, most recently, non-Hodgkin's lymphoma.
As with melanoma, data have suggested that sun exposure might attenuate the mortality of some of these cancers, Dr. Armstrong said.
In 1983 in Australia, his group documented an association between increased melanoma survival and sun exposure. More recently, another group has shown, using a large database, that in Europe, melanoma diagnosis and mortality has a seasonal variation that may relate to sun exposure (Eur. J. Cancer 2005;41:126-32).
Although it is not proven that vitamin D is the mechanism of the cancer effects seen, there is some evidence that even in sunny climates vitamin D deficiency may be common, Dr. Armstrong said.
One recent survey of Australian populations found that even at the end of summer, 30% of Australian boys were marginally vitamin D deficient, as was a much higher percentage of the elderly, particularly those with dark skin.
It also must be noted that sun exposure has a number of other health benefits, including promoting bone health and even just creating a sense of well-being, Dr. Armstrong added.
It has been suggested that receiving the equivalent of one minimal erythema dose to the hands and face per week is sufficient for vitamin D synthesis, Dr. Armstrong said. That amount of exposure, Dr. Berwick said, would translate into about 5-10 minutes per day spent in sunshine, two to three times per week.
However, no melanoma experts appear to be recommending that campaigns to warn about the dangers of too much sun exposure be abandoned.
Addressing that question directly in two separate lectures, Martin A. Weinstock, M.D., noted that sun education campaigns appear to have made little difference in behavior. He said regular self-performed skin examination appears to be the more important melanoma strategy because studies show that it can be encouraged and that it significantly impacts mortality.
But both approaches are still needed, said Dr. Weinstock, a professor of dermatology at Brown University, Providence, R.I. Evidence still implicates sun exposure in the genesis of melanoma, and primary prevention will always be important, he said. "Obviously there are new hypotheses out there, but a lot remains to be elucidated," he said. "People still need to take measures for both prevention and early detection."
"We don't know how sun exposure really relates to risk, so I would suggest that caution is warranted on both sides of the issue," Dr. Berwick added.
BY TIMOTHY F. KIRN
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|Author:||Kirn, Timothy F.|
|Publication:||Internal Medicine News|
|Date:||Nov 15, 2005|
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