Abstract P4-03-10: BREAST DENSITY AND RISK OF BREAST CANCER

Abstract Background: Early studies reported a 4-6-fold risk of breast cancer between women with extremely dense and fatty breasts. As most early studies were case-control studies, we took advantage of a population-based screening program to study density and breast cancer incidence in a cohort design. Methods: In the Capital Region, Denmark, woman aged 50-69 are invited to screening biennially. Women screened November 2012 - December 2017 were included, and classified by BI-RADS density code, version 4, at first screen after recruitment. Women were followed up for incident breast cancer, including ductal carcinoma in situ (DCIS), to 2020 in nationwide pathology data. Rate ratios (RR) and 95% confidence intervals (CI) were compared across density groups using Poisson-regression. Results: We included 189,609 women; 1,067,293 person-years; and 4110 incident breast cancers/DCIS. Thirty-three percent of women had BI-RADS density code 1; 38% code 2; 24% code 3; 4.7% code 4; and missing 0.3%. Using women with BI-RADS density code 1 as baseline; women with code 2 had RR 1.69 (95% CI 1.56-1.84); women with code 3, RR 2.06 (95% CI 1.89-2.25); and women with code 4, RR 2.37 (95% CI 1.05-2.74). Results differed between observations accumulated during screening and above screening age. Conclusions: This cohort study showed a 2.37-fold difference in breast cancer risk between women with highest and lowest breast density. Translated into absolute risk of breast cancer after age 50, this was a 6.2% risk for the one-third of women with lowest density, and 14.7% for the five percent of women highest density. Citation Format: Elsebeth Lynge, Ilse Vejborg, Martin Lillholm, Mads Nielsen, George Napolitano, My von Euler-Chelpin. BREAST DENSITY AND RISK OF BREAST CANCER [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-03-10.