Contribution of socioeconomic and environmental factors to geographic disparities in breast cancer risk in the Nurses' Health Study II

Background: Evidence of geographic disparities in breast cancer incidence within the United States and spatial analyses can provide insight into the potential contribution of environmental exposures or other geographically varying factors to these disparities. Methods: We applied generalized additive models to smooth geocoded residential coordinates while adjusting for covariates. Our analysis included 3,478 breast cancer cases among 24,519 control women from the Nurses' Health Study II. We first examined associations with residential location during adolescence (high school address) or early adulthood (address in 1991). We then assessed the contribution from known individual-level risk factors, measures of socioeconomic status, and occupational and environmental factors that vary spatially and have been linked to breast cancer. Secondary analyses by estrogen receptor and menopausal status were also conducted. Results: We identified geographic patterns of breast cancer risk associated with location during adolescence, with increased risk apparent in Michigan, the Northwest, and the New York City area, that shifted to southern New England when addresses during early adulthood were analyzed. Similar results were observed after adjustment for individual- and area-level factors, although spatial associations were no longer statistically significant. Conclusions: Breast cancer risk is not spatially uniform across the United States, and incidence patterns varied depending on the timing during life of the residence considered. Geographic disparities persisted even after accounting for established and suspected breast cancer risk factors, suggesting that unmeasured environmental or lifestyle risk factors may explain geographic variation in risk in different parts of the country.