"After examining the available epidemiological data and conducting our own large case- control population-based study, we have found no relationship between sunscreen use at any age and the development of melanoma skin cancer," said Dr. Berwick at the Annual Meeting of the American Association for the Advancement of Science (AAAS) in Philadelphia, Pennsylvania.
The logic behind using sunscreen stems from many studies in scientific journals and articles in the lay press that have linked a specific number of sunburns to an increased risk for developing melanoma skin cancer. But when Dr. Berwick re-examined these results in a large population-based study, she found that people's reports of their sunburn histories were unreliable.
"When asked the same question at different times, people often gave inconsistent answers about their sunburn history," said Dr. Berwick.
Examining the relationship between sunscreen use and the development of skin cancer is further complicated by evidence that people who are sensitive to the sun engage in fewer activities in the bright sun and wear sunscreen when they do. But Dr. Berwick reports that evidence suggests that when these people develop melanoma, it is because they are genetically susceptible and likely to develop skin cancer regardless of the amount of sunlight exposure or protection from sunscreen. Also, it is likely that those people who are normally sun sensitive may use sunscreens to stay out in the sun longer -- thereby eliminating sunburn, which would have otherwise signaled them to get out of the sun. Subsequently, these people expose themselves to more sun than they should.
"Based on the evidence, we conclude that sunburn itself probably does not cause melanoma, but that it is an important sign of excessive sun exposure particularly among those who are genetically susceptible because of their skin-type," said Dr. Berwick.
More powerful determinants for the risk of melanoma appear to be genetically determined characteristics such as the number of moles a person has and their pigmentary phenotype, or the combination of skin, eye, and hair color. In data gathered from Dr. Berwick's large population-based study, the risk for melanoma was estimated by looking at these genetic characteristics, sun exposure in early life, as well as sun exposure 10 years prior to the development of melanoma. Dr. Berwick found that the melanoma risk for people with numerous moles was six times higher than that of someone with only a few moles. The risk for melanoma with the most sensitive pigmentary phenotype (those with red or blond hair and lighter colored eyes) was almost six times that of someone with the least sensitive phenotype.
Increased amounts of sun exposure have also been linked to the alarming rise in melanoma incidence -- the highest rising cancer in men and the second highest in women, after lung cancer. But Dr. Berwick reports that epidemiologic studies show that to the contrary, people are spending less constant time outdoors and thus engaging in more intermittent sun exposure, which may explain the increase in melanoma rates.
"The evidence indicates that chronic sun exposure may be protective for the development of melanoma because the skin has adapted to the sun, having become thicker as it has tanned. On the other hand, intermittent sun exposure appears to increase risk, making it much less protective," added Dr. Berwick.
Given the data, blanket advice to the public to wear sunscreens as protection against melanoma skin cancer at any time outdoors is not warranted.
"People need to focus on their individual risk characteristics, such as their pigmentary phenotype, their family history, and the type and number of moles they have. I recommend that people avoid the sun when they are clearly at high risk and that they should enjoy a reasonable amount of outdoor activities with less anxiety when they are clearly at reduced risk," advised Dr. Berwick.
Memorial Sloan-Kettering Cancer Center is the world's oldest and largest private institution devoted to prevention, patient care, research, and education in cancer. Throughout its long, distinguished history, the Center has played a leadership role in defining the standard of care for patients with cancer. In 1997, Memorial Sloan-Kettering was named the nation's best cancer center for the fifth consecutive year by U.S. News and World Report.