However effective sunny days may be at warding off the blues, spending a lot of time in the sun, especially when it results in sunburns, is known to increase a persons risk of developing skin cancer. Several studies, both completed and ongoing, have looked at whether the use of sunscreen might prevent some of these cancers, especially melanoma, the most deadly form of skin cancer. Thus far, however, the evidence is mixedand clear answers are not in the immediate forecast.
There are several kinds of skin cancer, all of which seem linked to sun exposure, fair skin, and family history of the disease. Basal cell carcinoma is the most common cancer in people, yet is rarely deadly. Squamous cell carcinoma often springs from small scaly lesions called solar keratoses, or actinic keratoses. Both types of cancer are typically found on the head, neck, and arms, areas of the body most frequently exposed to sunlight. A number of studiesincluding a recent report in the Archives of Dermatologysuggest that regular sunscreen use reduces a persons risk of developing actinic keratoses, and thus, presumably, a persons risk of squamous cell carcinoma.
The data are considerably less clear when it comes to melanoma, said Marianne Berwick, Ph.D., of Memorial Sloan-Kettering Cancer Center in New York. (Berwick is scheduled to chair a symposium on this issue at the July meeting of the American Association of Cancer Research.) About 54,000 people in the United States each year are diagnosed with melanoma and about 7,600 people die of the disease. Melanoma grows from pigment cells called melanocytes in the outer layer of the skin. It can take more than a decade to develop, and thus is most common in older adults. Risk factors include a large number of moles, irregularly shaped or pigmented moles, and a history of sunburn, especially during childhood.
Dozens of studies looking at the relationship between sunscreen use and melanoma incidence have shown no clear association. In some studies, in fact, sunscreen use is associated with an increase in melanoma.
Several meta-analyses over the last few years have been unable to tease out any clear relationship between sunscreen use and melanoma incidence, said Antony R. Young, Ph.D., of Kings College London. Because of this variability, a report released last year by the International Agency for Research on Cancer in Lyon, France, called for more studies to elucidate the relationship between sun exposure, sunscreen use, and melanoma.
Ultraviolet Radiation
One of the most important areas of current research is teasing out the effects of the suns burning rays, or ultraviolet B radiation, from the effects of the deeper-penetrating ultraviolet A wavelengths. Most sunscreens marketed in the United States protect against the suns UVB much more strongly than against UVA rays. UVA, however, is increasingly being recognized as an immune suppressant that may set the stage for cancer development.
A team led by Philippe Autier, M.D., of the European Institute of Oncology in Milan, Italy, will soon publish a study demonstrating that the kind of solar radiation implicated in the formation of new moles in children is not the same as the UVB radiation linked to the development of large moles from smaller ones. The findings suggest that the genesis of melanoma is at least a two-step process and, if further research implicates UVA radiation as increasing the number of moles a child develops, may have implications for preventing melanoma, Autier said. He added that better animal models of skin cancers, especially melanoma, would further advance understanding of the mechanisms of melanoma and help get around some of the difficulties of epidemiologic studies.
Confounding Factors
There are several factors that confound conclusions from the epidemiologic studies and may contribute to their varied findings, said Marty Weinstock, M.D., Ph.D., a dermatologist at Brown University in Providence, R.I. For example, fair-skinned people who are out in the sun are both more likely to use sunscreen and most at risk of developing melanomas. In addition, numerous studies have demonstrated that those using sunscreen fail to apply enough to get the sun protection marked on the bottle, and thus ineffective use might counteract any protective effect, he said.
But there is some evidence that suggests that sunscreen may have an effect on melanoma risk, said Weinstock, who is head of the American Cancer Societys skin cancer advisory group. He pointed to a randomized controlled study showing that sunscreen can reduce the numbers of moles that children develop. Further, he said, the long lag time between initiation of melanoma and detection of the disease means that studies have not yet enrolled people who used high-protection sunscreen throughout their lives. "As sunscreens are increasingly used effectively and more powerful (broad-spectrum) sunscreens are available for longer periods of time, I think that a benefit [i.e., a reduction in melanoma incidence] eventually will become clear from epidemiological studies," he concluded.
Autier is not so sanguine about that eventual conclusion. Over the last decade, he said, it has become clear that sunburn is probably not part of the mechanism of cancer, but simply an indicator of at-risk people who have gotten too much sun. "That does not mean that if you avoid sunburn you will avoid melanoma," he said. This is in part because the relationship between sun exposure and cancer is complex. Squamous cell carcinoma seems to be linked to cumulative time in the sun, so that people who work in the sun are most likely to develop this kind of cancer. Melanoma seems to be linked to intermittent sun exposure and sunburnin other words, recreational sun exposure, especially sunbathing. Basal cell carcinoma is influenced by both long-term and intermittent sun exposure.
"Its clear that when sun exposure is not intentional," he said, such as when people are skiing or gardening or working outside, "using sunscreen can prevent skin cancer." But he has shown that sunscreen use tends to prolong the amount of time people spend in the sun while they are on vacationand that only sunburn modifies the behavior of sun-seekers.
"When people are going to the beach and trying to get a tan, they want to stay in the sun as long as possibleand thats where the problem arises with sunscreens," he said. Staying out in the sun longer and sunning more during mid-day, when the suns UV rays are most direct, may end up increasing a persons exposure to ultraviolet radiation beyond what it would have been without sunscreen, he said. It is this kind of intense, intentional time in the sun that is most closely linked to melanoma risk, he said.
Evidence-Based Approach
"Both sides have good arguments for their point of view," said Olaf Gefeller, Ph.D., of the University of Erlangen-Nuremberg in Germany. "The eloquent controversy shows that the case is still open." However, he said, given the failure of studies to date to demonstrate a protective effect of sunscreen against melanoma, the rigorous tests of evidence-based medicine suggest that "as a pharmaceutical product marketed for melanoma protection (instead of the prevention of sunburns), sunscreens would have failed the tests of efficacy during the approval procedure."
The conflicting results might stem from inadequate control of complex confounding mechanisms, issues of exposure measurement (people are notoriously inconsistent in reporting prior sunburns, sun exposure, and sunscreen use), or simply a lack of statistical power, Gefeller said. Regardless, he added, public health messages should emphasize minimizing exposure during peak UV hours, wearing protective clothing, hats, and sunglasses, and seeking shade over wearing sunscreen.
Although most agencies, such as the American Cancer Society and the American Academy of Dermatology, recommend all of these measures, this message may not be reaching the public, said Julia Verne, M.B.B.S., Ph.D., director of the South West Public Health Observatory and acting director of the South West Cancer Intelligence Service in Bristol, England.
Given that people do not use sunscreen properly and may use it to extend the amount of time they spend in the sun, "weve put undue emphasis on the use of sunscreen," she said. In the United Kingdom, for example, surveys have shown that people regard sunscreen as the most important sun protection measure. "Messages about prevention," she said, "may need to shift the emphasis still further toward covering up and staying out of the sun if the trend of [increasing melanoma] incidence is to be reversed."
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