Sunscreen may not help prevent melanoma

By slathering sunscreen on yourself and your children this summer, you are fending off sunburn and cutting the odds of at least one kind of skin cancer.

But does sunscreen help prevent melanoma, the deadliest of skin cancers? Remarkably, scientists still aren't sure.

About 54,000 cases of Americans with melanoma are diagnosed each year — triple the incidence of 30 years ago — and 7,600 die of it, according to the National Cancer Institute. And the numbers keep climbing despite decades of widespread sunscreen use.

Some researchers, concerned about this apparent paradox, wonder whether sun lotions may have contributed to the problem. Older sunscreens absorbed ultraviolet B radiation, which is most responsible for sunburn, but did not block deeply penetrating ultraviolet A rays.

"The old sunscreens that were UVB blockers only may have been a bad idea," says Dr. James M. Spencer, vice chairman of dermatology at Mount Sinai School of Medicine in New York. "You didn't burn, so you stayed on the beach all day, getting massive doses of UVA."

Others disagree. Dr. Martin A. Weinstock, professor of dermatology at Brown University, says he believes sunscreens — even those that block only ultraviolet B — do help prevent melanoma. There is probably a lag time of at least 15 or 20 years between sun exposure and the cancer it causes, he says, and he thinks the melanoma numbers are beginning to turn around.

But Weinstock, too, believes sunscreen can offer false security — because so many people fail to use it properly.

"They put it on too late, they put it on too thinly, and they don't reapply it as often as they should," says Weinstock, chairman of the American Cancer Society's skin cancer advisory group. In a 2002 survey, he says, 39 percent of teenagers claimed they were using a broad-spectrum sunscreen when they got their worst sunburns.

In the sunscreen-melanoma conundrum, as in medical controversies involving diet strategies and hormone therapy, the desire of millions of people for practical advice collides with the ambiguity of scientific evidence.

"Physicians want to have something to tell their patients," says Marianne Berwick, an epidemiologist at Memorial Sloan-Kettering Cancer Center in New York. "But the question really is wide open."

Berwick should know. She's moderating a debate today in Washington at the annual meeting of the American Association for Cancer Research with the title: "Does sunscreen prevent melanoma?" And she's helping conduct an international study of 4,000 melanoma patients that may help answer the question.

Like other researchers, Berwick is quick to say it's a good idea to use sunscreen anyway. It prevents sunburn, slows the wrinkling of aging skin and clearly protects against one common kind of skin cancer, squamous cell carcinoma, which usually can be easily treated. Whether sunscreen helps prevent basal cell carcinoma, the most common skin cancer, is less certain, she says.

Yet it is melanoma that accounts for more than 75 percent of skin cancer deaths, so health experts are eager to find better ways to prevent it. Part of that quest has been the push for "broad-spectrum" sunscreens with "sun protection factor" ratings over SPF 15, because they contain chemicals that absorb longer-wave ultraviolet A as well as shorter-wave ultraviolet B. But experts are divided on whether even the most advanced formulas will defeat melanoma.

The complex interaction of human skin and sunlight began tens of thousands of years ago with the evolution of varying skin colors. Scientists believe early humans who migrated out of Africa and settled in Europe developed lighter skin to absorb enough of the weaker northern sunlight to produce enough vitamin D for normal development of bones, muscles and nerves.

But the same fair skin that protected northerners against such crippling disorders as rickets also put them at high risk of melanoma. Dark skin protects against melanoma; white Americans have melanoma rates about 20 times higher than black Americans, according to the National Cancer Institute.

If evolution made light-skinned people vulnerable to skin cancers, 20th-century social trends hugely increased their risk. Traditionally, pale skin was considered desirable among the well-to-do, because a tan was usually a sign that a person worked outdoors.

But beginning in the 1920s, partly through the influence of French fashion designer Gabrielle "Coco" Chanel, tan skin gradually became fashionable. People threw away their hats and parasols as a tan came to symbolize not the harsh life of the farm but the luxury of swimming pools and beach vacations.

That trend, reflected more recently in the boom in tanning salons, has translated into an epidemic of skin cancers, including melanoma.

Thomas M. Nordlund, a biophysicist at the University of Alabama, was prompted by his own susceptibility to sunburn to seek a better understanding of what he calls "sunscreen failure" — its ability to block sunburn but not cancer. His experiments are designed to see whether the energy absorbed by sunscreen as it soaks up ultraviolet light is transferred to the DNA in skin cells rather than turning into harmless heat.

"The longer you let energy hang around in a cell, the more trouble it can cause," he said.

With no certainty that sunscreen prevents melanoma, experts are hedging their advice.

"Twenty years ago, sunscreen was the first thing dermatologists would recommend," said Frank P. Gasparro, past president of the American Society for Photobiology and a leading expert on sun dangers. "Now it's usually third on the list, after avoiding intense sunlight in the middle of the day and wearing sun-protective clothing. There are still a lot of unanswered questions."