Mammographic density as a predictor of invasive breast cancer and ductal carcinoma in situ in over six million South Korean women.

Purpose: Evidence on the associations between categorical mammographic density and breast cancer risk by tumor invasiveness remains limited in Asian women. This large, population-based cohort study investigated the distribution of mammographic density by age and menopausal status, as well as its association with the risks of invasive breast cancer and ductal carcinoma in situ (DCIS) in South Korean women.

Methods: Mammographic screening was performed on 6,365,522 women between 2009 and 2014 through the Korean National Cancer Screening Program. Mammographic parenchymal composition was classified using the fourth edition of Breast Imaging Reporting and Data System. We computed multivariable-adjusted hazard ratios and 95% confidence intervals (CIs) using Cox proportional hazards regression models for the association between mammographic parenchymal composition and the risk of invasive cancer and DCIS.

Results: Overall, 40.6% of women had dense breasts, with the proportion decreasing with increasing age. A total of 44,468 incident breast cancer cases (0.7%) were documented. Compared with almost entirely fatty breasts, increasing mammographic density was associated with a higher risk of breast cancer (HR, 1.55; 95% CI 1.51-1.60 for scattered fibroglandular densities; HR, 2.14; 95% CI 2.08-2.21 for heterogeneously dense breasts; and HR, 2.59; 95% CI 2.50-2.69 for extremely dense breasts). Associations between mammographic density and breast cancer risk were similar for invasive cancer and DCIS, and did not vary significantly by menopausal status.

Conclusions: Mammographic density may be a significant risk factor for both invasive cancer and DCIS, regardless of menopausal status. It should be incorporated into breast cancer risk stratification and screening strategies.