Two Studies Find Evidence That Sunlight May Have Beneficial Influence On Cancer

Solar radiation is a major risk factor for melanoma. The incidence of and mortality from melanoma have been increasing over the last 50 years in all developed countries with large Caucasian populations. But as the incidence of melanoma increases, so does survival, suggesting the possibility that increasing sun exposure increases melanoma survival in addition to melanoma incidence. However, increased early detection of melanoma might also explain the increased survival.

To examine the relationship between sun exposure, early detection, and melanoma survival, Marianne Berwick, Ph.D., M.P.H., of the University of New Mexico in Albuquerque, and colleagues conducted a population-based, case–control study of more than 500 patients from the Connecticut Tumor Registry who had been diagnosed with melanoma in the late 1980s.

Three measures of sun exposure--sunburn, high intermittent sun exposure, and solar elastosis (an indicator of the skin's sun damage)--and a personal history of skin awareness (a measure of early detection) were all inversely associated with death from melanoma. Melanoma patients with higher levels of sun exposure or skin awareness were less likely to die. In addition, both solar elastosis and skin awareness were independently associated with increased survival from melanoma, even after adjusting for certain melanoma characteristics, such as lesion thickness and location. The authors conclude that sun exposure is associated with increased survival from melanoma.

"It would be reasonable to speculate … that the apparently beneficial relationship between sun exposure and survival from melanoma could be mediated by vitamin D," Berwick and colleagues write. "However, an alternative hypothesis is that sun exposure induces less aggressive melanomas by inducing melanization and increasing DNA repair capacity, both of which might reduce further mutational changes in a melanoma. Which, if either, hypothesis is more plausible remains to be determined."

Non-Hodgkin lymphoma has also been on the rise worldwide, and it has been suggested that increasing ultraviolet (UV) radiation and sun exposure may be partly responsible. To investigate this hypothesis, Karin Ekström Smedby, of the Karolinska Institute in Stockholm, and colleagues conducted a population-based, case–control study in Denmark and Sweden in which they obtained detailed information on history of UV exposure and other risk factors for lymphoma from more than 3,000 lymphoma patients and a similar number of control subjects.

They found that increased exposure to UV radiation through sunbathing and sunburns was associated with a decrease, rather than an increase, in the risk of non-Hodgkin lymphoma. Increased UV exposure was also associated, although more weakly, with a decreased risk of Hodgkin lymphoma.

"[These] results suggest an inverse association between UV light exposure and non-Hodgkin lymphoma risk," Smedby and colleagues write. "However, before this association can be considered causal we need further confirmatory data from other epidemiologic studies and, ideally, a better understanding of possible biologic mechanisms," including UV-induced systemic immune modulation and the photo-initiation of vitamin D production.

In an editorial, William J. Blot, Ph.D., of the International Epidemiology Institute in Rockville, Md., and colleagues discuss how the results of these two studies provide new evidence that sunlight may have a beneficial influence on both cancer incidence and outcome and hypothesize that vitamin D may be a critical mediator in the relationship between sunlight and cancer. "In view of the major potential public health consequence of these results, further studies of sunlight and the vitamin D connection to cancer are certainly warranted," they conclude.

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Citations:# Article: Berwick M, Armstrong BK, Ben-Porat L, Fine J, Kricker A, Eberle C, et al. Sun Exposure and Mortality From Melanoma. J Natl Cancer Inst 2005;97:195–99.# Article: Smedby KE, Hjalgrim H, Melbye M, Torrång A, Rostgaard K, Munksgaard L, et al. Ultraviolet Radiation Exposure and Risk of Malignant Lymphomas. J Natl Cancer Inst 2005;97:199–209.# Editorial: Egan KM, Sosman JA, Blot WJ. Sunlight and Reduced Risk of Cancer: Is The Real Story Vitamin D? J Natl Cancer Inst 2005;97:161–63.

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