Mammographic density and urbanization: a population-based screening study

OBJECTIVES: The high incidence of female breast cancer that has been consistently reported in urban areas could be mediated by breast density, which is considered to reflect the cumulative exposure of breast tissues to hormones. The aim of this study was to assess how mammographic density varies by the degree of urbanization. SETTING: The population consisted of 55,597 cancer-free women, aged 50-59 years, who participated in a French breast cancer screening programme (Franche-Comté region) between 2005 and 2009. METHODS: Ordered logistic regression was run with mammographic density as the outcome, and degree of urbanization as the independent variable, while adjusting for some known confounding factors. Multiple imputation was used to deal with missing data. RESULTS: A significant positive linear trend with urbanization was found in a univariate approach (P trend <10(-3)), and after adjusting for risk factors (P trend = 10(-3)). A negative and highly significant association with mammographic density was highlighted both for age at the time of mammography (odds ratio (OR) 0.41, 95% confidence interval (CI) 0.39-0.43, per 10 years), and for low socioeconomic status (OR 0.71, 95% CI 0.67-0.75). The OR for hormone replacement therapy use was 1.51 (95% CI 1.43-1.58). CONCLUSIONS: Knowledge of this urbanization gradient in density (whatever its mechanism) may help to identify women who may require full-field digital mammography for the early detection of breast cancer, and could assist primary care providers in recommending the best screening strategy in a risk factor-based approach.