Background/Objectives: Breast density, as defined by the Breast Imaging Reporting and Data System (BI-RADS), reduces mammographic sensitivity and is a risk factor for breast cancer. However, its association with cancer risk and stage at diagnosis remains debated, with limited large-scale evidence. Methods: We conducted a nationwide retrospective cohort study of 952,755 Korean women who underwent screening mammography between 2013 and 2014, with breast cancer diagnoses identified over a 5-year follow-up period. Breast density was categorized by BI-RADS criteria (A–D). Breast cancer risk was evaluated using logistic regression, with odds ratios representing relative odds of developing breast cancer during the 5-year interval. Stage at diagnosis was classified as localized versus regional/distant disease according to national cancer registry records. Results: During 5 years of follow-up, 11,286 women (1.2%) were diagnosed with breast cancer. Higher breast density was significantly associated with increased breast cancer risk: multivariable-adjusted odds ratios (ORs) were 1.174 (95% CI, 1.093–1.260), 1.268 (95% CI, 1.186–1.356), and 1.287 (95% CI, 1.196–1.385) for BI-RADS B–D, respectively, compared with BI-RADS A (allp< 0.001). However, the risk of advanced stage (regional/distant vs. localized) disease at diagnosis did not significantly differ across breast density categories except for a modest association in BI-RADS B (OR 1.16, 95% CI, 1.01–1.33;p= 0.035). Conclusions: Higher breast density was independently associated with increased breast cancer risk but not with advanced-stage disease at diagnosis. These findings underscore the importance of individualized screening strategies for women with dense breasts.
背景/目的：根据乳腺影像报告和数据系统（BI-RADS）定义的乳腺密度会降低乳腺X线摄影的敏感性，并且是乳腺癌的危险因素。然而，其与癌症风险及诊断分期的关联仍存在争议，且缺乏大规模证据支持。方法：我们对2013年至2014年间接受乳腺X线筛查的952,755名韩国女性进行了全国性回顾性队列研究，并在5年随访期内追踪乳腺癌诊断情况。乳腺密度按BI-RADS标准（A-D类）分类。采用逻辑回归评估乳腺癌风险，比值比代表5年随访期内发生乳腺癌的相对几率。根据国家癌症登记记录，诊断分期分为局部病变与区域/远处转移病变。结果：在5年随访期间，共有11,286名女性（1.2%）被诊断为乳腺癌。较高的乳腺密度与乳腺癌风险增加显著相关：与BI-RADS A类相比，B-D类的多变量调整比值比分别为1.174（95% CI，1.093-1.260）、1.268（95% CI，1.186-1.356）和1.287（95% CI，1.196-1.385）（所有p<0.001）。然而，除BI-RADS B类与晚期诊断风险存在微弱关联外（OR 1.16，95% CI，1.01-1.33；p=0.035），不同乳腺密度类别在诊断时晚期（区域/远处转移 vs. 局部）病变的风险并无显著差异。结论：较高的乳腺密度与乳腺癌风险增加独立相关，但与诊断时的晚期病变无关。这些发现强调了针对致密型乳腺女性制定个体化筛查策略的重要性。
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