Breast cancer risk factors and mammographic density among 12518 average-risk women in rural China

Background Mammographic density (MD) is a strong risk factor for breast cancer. We aimed to evaluate the association between MD and breast cancer related risk factors among average-risk women in rural China. Methods This is a population-based screening study. 12518 women aged 45–64 years with complete MD data from three maternal and childcare hospitals in China were included in the final analysis. ORs and 95%CIs were estimated using generalized logit model by comparing each higher MD (BI-RADS b, c, d) to the lowest group (BI-RADS a). The cumulative logistic regression model was used to estimate the OR trend (95%CI) and P trend by treating MD as an ordinal variable. Results Older age (OR trend = 0.81, 95%CI: 0.79–0.81, per 2-year increase), higher BMI (OR trend = 0.73, 95%CI: 0.71–0.75, per 2 kg/m2), more births (OR trend = 0.47, 95%CI: 0.41–0.54, 3 + vs. 0–1), postmenopausal status (OR trend = 0.42, 95%CI: 0.38–0.46) were associated with lower MD. For parous women, longer duration of breastfeeding was found to be associated with higher MD when adjusting for study site, age, BMI, and age of first full-term birth (OR trend = 1.53, 95%CI: 1.27–1.85, 25 + months vs. no breastfeeding; OR trend = 1.45, 95%CI: 1.20–1.75, 19–24 months vs. no breastfeeding), however, the association became non-significant when adjusting all covariates. Associations between examined risk factors and MD were similar in premenopausal and postmenopausal women except for level of education and oral hormone drug usage. Higher education was only found to be associated with an increased proportion of dense breasts in postmenopausal women (OR trend = 1.08, 95%CI: 1.02–1.15). Premenopausal women who ever used oral hormone drug were less likely to have dense breasts, though the difference was marginally significant (OR = 0.54, P = 0.045). In postmenopausal women, we also found the proportion of dense breasts increased with age at menopause (OR trend = 1.31, 95%CI: 1.21–1.43). Conclusions In Chinese women with average risk for breast cancer, we found MD was associated with age, BMI, menopausal status, lactation, and age at menopausal. This finding may help to understand the etiology of breast cancer and have implications for breast cancer prevention in China.