Associations between breast cancer risk factors and mammographic breast density in a large cross-section of Korean women

Background: We investigated the association between established risk factors for breast cancer and mammographic breast density in Korean women. Methods This large cross-sectional study included 8 460 928 women aged >40 years, who were screened for breast cancer between 2009 and 2018. Breast density was assessed using the Breast Imaging Reporting and Data System. This study used multiple logistic regression analyses of age, BMI, age at menarche, menopausal status, menopausal age, parity, breastfeeding status, oral contraceptive use, family history of breast cancer, physical activity, smoking, drinking and hormone replacement therapy use to investigate their associations with mammographic breast density. Analyses were performed using SAS software. Results Of 8 460 928 women, 4 139 869 (48.9%) had nondense breasts and 4 321 059 (51.1%) had dense breasts. Factors associated with dense breasts were: earlier age at menarche [<15 vs. ≥15; adjusted odds ratio (aOR), 1.18; 95% confidence interval (CI), 1.17–1.18], premenopausal status (aOR, 2.01; 95% CI, 2.00–2.02), later age at menopause (≥52 vs. <52; aOR, 1.23; 95% CI, 1.22–1.23), nulliparity (aOR, 1.64; 95% CI, 1.63–1.65), never breastfed (aOR, 1.23; 95% CI, 1.23–1.24) and use of hormone replacement therapy (aOR, 1.29; 95% CI, 1.28–1.29). Women with a higher BMI and the use of oral contraceptives were more likely to have nondense breasts. Conclusion Lower BMI, reproductive health and behavioral factors were associated with dense breasts in Korean women. Additional research should investigate the relationship between mammographic breast density, breast cancer risk factors and breast cancer risk.