Solar Radiation's Role in Melanoma Etiology Debated

LOS ANGELES, June 2 ? Although there is little argument that the sun plays a powerful role in basal-cell and squamous-cell carcinoma, the influence of solar radiation in malignant melanoma etiology is less clear cut.

Indeed, some researchers have contended that a person's genetic makeup overshadows sun exposure as the most important factor in the genesis of melanoma, said dermatologist Elisabeth K. Shim, M.D., and colleagues, at the University of California Keck School of Medicine here.

In the April issue of Dermatology Surgery, Dr. Shim and colleagues reviewed 100 studies on melanoma and the sun published over the last two decades.

Key evidence against the sun and favoring a genetic explanation included:

• Epidemiologic studies have not shown a dose-response curve between sunlight and melanoma. In fact, some showed that people who are chronically exposed to the sun have less risk of melanoma than people who are intermittently exposed (i.e. on vacation). However, these studies may not have taken into account that sun exposure produces melanin, which protects cells from ultraviolet radiation, the California team said.
• Scientists can not point to a clear-cut relationship between sunburns and melanoma. It was once common wisdom that childhood sunburns increase the risk of melanoma, but a review of 16 case-control studies did not bear this out. Furthermore, there is no clear relationship between number of sunburns and the melanoma risk.
• People with the genetic disorder xeroderma pigmentosum, whose cells can not repair certain types of UV radiation damage to DNA, have 1,000 times the melanoma risk of others.
• Mutations in the melanocortin-1 receptor have been found in some melanomas. This receptor helps determine what type of melanin a person makes. One type of melanin, pheomelanin, which is found in people with red hair, is less protective against UV radiation damage and may explain why redheads have more melanoma.
• Growing evidence is linking melanoma to mutations in other oncogenes, including N-ras, cyclic-dependent kinase N2A, and p53.Key evidence that sunlight is the main culprit in melanoma include:
• Several studies have shown that melanoma occurs primarily in lighter-skinned people. In the United States, older white men have the most sun exposure and the highest rate of melanoma.
• Many studies have shown that fair-skinned people living closer to the equator have higher rates of melanoma. For example, Australians have five to eight times more melanoma cancer than the British, according to a study from The Lancet.
• Studies from both Australia and New Zealand demonstrated that melanoma tends to appear on areas of the body with the most sun exposure, such as the head and neck. One such study showed that men, whose short haircuts tend to leave their ears more exposed to the sun that women, had six times the rate of melanoma on their ears compared with women.
• A study published in the journal Nature found that Americans get twice the annual dose of ultraviolet radiation as the Dutch and also have twice the annual incidence of melanoma.
• Migration studies in which people move from an area of low sun exposure to high sun exposure especially before age 15 to 30 have an increased risk of melanoma.Both average annual intensity of ultraviolet B radiation exposure and the number of hours spent outdoors were significantly associated with melanoma risk, according to a study from the National Cancer Institute.

?Likely, there is a complex interaction among genetic factors, the environmental factor of sun exposure, and other possible factors, which lead to melanoma development,? the California team said.

?Nonetheless, overall, there is a large amount of evidence from epidemiologic, genetic, and animal studies suggesting that sun exposure plays a large contributing role in the development of most cutaneous melanomas,? they added.

?It seems prudent to continue to recommend lifelong sun-protective measures to patients,? they advised.

While agreeing with the California researchers, Martin A. Weinstock, M.D., Ph.D., a professor of dermatology at Brown University in Providence, R.I., noted that there are some arguments against blocking the sun out of one?s life to too great a degree. Dr. Weinstock chairs the American Cancer Society?s skin cancer advisory group.

First, persons who spend more time outdoors tend to be physically active, Dr. Weinstock said. Even though many of us work out in indoor gyms, outdoor activities remain a significant part of our overall physical activity, which is important for overall health, he said.

Second, people need sunlight on their skin in order to produce vitamin D, Dr. Weinstock said. This vitamin is believed to help maintain bones health by regulating serum levels of calcium and phosphorous. However, for those with limited sunlight exposure, such as those living at higher latitudes, vitamin D can also be obtained from dietary supplements, he said.

When it comes to sun protection, the cancer society promotes the ?slip, slop, slap, and wrap? approach, Dr. Weinstock said, as in slip on a shirt, slop on sunscreen (with a sun protection factor of at least 15), slap on a hat, and wrap on sunglasses to protect the eyes from ultraviolet light.