Scientists have long recognized that rates of melanoma are higher in areas that are closer to the equator or receive more sunlight, and that ultraviolet-B rays are the primary cause of sunburn and skin cancer. In this study, the authors developed a novel approach to measure an individual's sun exposure over a lifetime, taking into account where an individual has lived throughout his or her life. This information determines average annual UVB intensity the average amount of ultraviolet-B rays that a person could be exposed to per year over his or her lifetime. The data led the researchers to conclude that a 10 percent increase in the average annual intensity was associated with a 19 percent increase in the individual's risk for melanoma in men and a 16 percent increase in women, at any age.
"We're learning more about the kinds of exposures that cause melanoma," said Thomas Fears, Ph.D., the first author of the paper and a scientist at the National Cancer Institute in Bethesda, Md. "The risk of melanoma is greatest for people who develop little or no tan. However, we've learned that where people live as both kids and adults and how much UVB shines in those places are important factors regardless of tanning ability."
According to Fears, it is not unusual to see at least a 10 percent difference in intensity between two locations. New Orleans, for example, receives 20 percent more UVB each year than Atlanta.
In addition to estimating individual risk, the authors analyzed the number of hours that study participants spent outdoors. They found that the number of summer hours spent outside prior to age 20 was much larger than after age 20. In light of this distinction, the researchers hypothesized that differences in melanoma risk previously attributed to the "critical period" of childhood may, in fact, be due to the larger number of hours that children typically spend outdoors compared to adults.
"Studies such as this one serve as a reminder of the importance for adults and children alike to develop good, lifelong habits for protecting themselves from skin-damaging sun exposure," said Health and Human Services Secretary Tommy G. Thompson. "We are now seeing that not only length of sun exposure, but also the intensity of the sun's rays can affect one's risk of melanoma."
This study included 718 melanoma patients recruited from the Hospital of the University of Pennsylvania, in Philadelphia, and from the University of California in San Francisco. The comparison group included 945 non-melanoma patients from those areas. The researchers limited the analysis to non-Hispanic whites because the numbers of cases in other racial/ethnic groups were too few for analysis.
Each participant was interviewed in person to gather data including tendency to sunburn and ability to tan, along with medical, occupational, residential and outdoor exposure histories. Residential histories were constructed in six-month intervals, from date of birth to date of interview.
Robertson-Berger (RB) meters, which measure the amount of solar radiation received in a particular location, were used to estimate the UVB intensity. A person's cumulative intensity was estimated by adding up the RB counts for each residence location in six-month increments. Average annual intensity was determined by dividing the cumulative intensity by the person's age in years.
Future analyses will examine the effects of intermittent exposures on individual melanoma risk. For example, researchers will consider whether people who remain indoors for much of the week and then spend large amounts of time outdoors over the weekend or during a vacation are at higher risk of melanoma.
An estimated 53,600 people will be diagnosed with melanoma in the United States in 2002, and an estimated 7,400 people will die of the disease. Melanoma can be cured if detected and treated early.
For additional information about melanoma, please visit http://www.cancer.gov/cancer_information/cancer_type/melanoma
For more information about cancer in general, please go to the National Cancer Institute's
Web site at www.cancer.gov.