Laird Harrison

March 23, 2015

SAN FRANCISCO — Exposure to the sun during adulthood might cause more nonmelanoma skin cancer than exposure during childhood, a new study suggests.

In fact, women who lived in the southern latitudes of the United States as adults but not as children were 39% more likely to get nonmelanoma skin cancer than women who lived in northern latitudes.

This finding contradicts previous reports, said Katherine Ransohoff, a medical student at Stanford University in California. It is commonly thought that "it's childhood exposure that's important," she told Medscape Medical News here at the American Academy of Dermatology 73rd Annual Meeting.

Her team evaluated longitudinal data from the Women's Health Initiative, which involved 161,808 generally healthy postmenopausal women in the United States.

Among the data collected was residential history, which the team used as an indicator of sun exposure. They hypothesized that exposure during childhood "would have a greater impact on skin cancer risk than adulthood exposure," Ransohoff said.

In a subset of 56,000 white women during a median follow-up period of 11.9 years, 518 (0.9%) developed melanoma, as determined by medical records, and 9195 (16.3%) developed nonmelanoma skin cancer, as determined by the women's own annual reports.

Women's Health Initiative Data

The investigators assumed that women who lived south of the 37th parallel during the first 15 years of their life were exposed to more ultraviolet light than those who lived north of that.

The team also looked at data collected on behavior such as using sunscreen and wearing a hat, and the amount of time spent outdoors as a child and as an adult.

"This was important because we needed to adjust for behavioral variables, which may vary depending on location," Ransohoff explained.

They also adjusted for the women's age, education, body mass index, physical activity, history of melanoma and nonmelanoma skin cancer, skin reaction to the sun, vitamin D intake, alcohol use, and smoking status.

Women who lived in northern latitudes their entire lives, and were therefore considered to have low sun exposure during childhood and adulthood, were used as a reference group.

Not surprisingly, the risk for nonmelanoma skin cancer was significantly higher in women with high sun exposure during childhood and adulthood than in the reference group (OR, 1.12). However, the fact that the risk was even higher for women with low exposure during childhood and high exposure during adulthood than in the reference group (odds ratio [OR], 1.39) was a surprise, said Ransohoff.

"The lesson from this is not to retire to Florida," she joked.

The risk for melanoma was not significantly different between the groups. It might be that there were not enough melanoma cases for the statistical analysis to uncover differences in risk, Ransohoff reported.

She acknowledged that using geography to look at exposure to ultraviolet light has its drawbacks. For example, the team could not fully adjust for all variables, such as diet, that might go along with geography.

In addition, "we are not able to account for cloud cover and altitude that affected the amount of ultraviolet light that reached the surface," said Ransohoff.

However, geography is a more reliable indicator of ultraviolet light than recall, and the size of the sample gave the study an unusually good statistical power, she said.

"Our next step is to convert to more nuanced measures of ultraviolet exposure," she added.

After the presentation, a member of the audience asked whether the investigators had separated statistics on basal cell carcinoma from those on squamous cell carcinoma.

"Unfortunately, the Women's Health Initiative does not separate those outcomes, which is a limitation of this study," said Ransohoff.

Session moderator Sewon Kang, MD, from Johns Hopkins University in Baltimore, asked if the data suggest a protective effect of exposure to ultraviolet light in childhood.

"There are some data on the protective effects of early-life ultraviolet exposure," said Ransohoff. "I think work with more cases could shed light on whether that's true."

The study is "intriguing," Dr Kang told Medscape Medical News. But, he added, it is important to keep the study's limitations in mind.

"More work needs to be done. I don't think we should somehow lower the protections in childhood based on this work," he said.

Dr Kang and Ms Ransohoff have disclosed no relevant financial relationships.

American Academy of Dermatology (AAD) 73rd Annual Meeting: Abstract F025. Presented March 20, 2015.


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