Effect of breast density reporting on supplemental MRI and ultrasound use.

10568 Background: Mammography has lower sensitivity in women with dense breasts due to masking, as both dense tissue and tumor tissue appears white. Supplemental breast imaging, such as MRI or ultrasound, can increase cancer detection among women with dense breasts, but its use is limited by screening capacity, cost, and potential overdiagnosis. The purpose of this study is to understand the impact of changing breast density on supplemental screening use in a longitudinal cohort of women undergoing screening mammography. Methods: Participants are from the Boston Mammography Cohort Study (BMCS), a cohort of women undergoing routine screening mammography within the Mass General Brigham health system in Boston, MA enrolled from 2008-2016, with follow up collected through 2022. Participants completed a questionnaire at enrollment which provided demographic, reproductive, lifestyle and behavioral factors. Electronic medical record review provided dates and results of all breast imaging received within the health system. We performed a longitudinal analysis using generalized estimating equations to evaluate the association between breast density and supplemental MRI and/or ultrasound. Women with a history of breast cancer and those without at least one mammogram with reported BIRADS breast density were excluded from the analysis. Results: Of the 2,696 women enrolled in BMCS, 87 were excluded for breast cancer prior to enrollment and 16 were excluded for lack of reported BIRADS breast density, leaving 2,593 in the analytic cohort. Of these women, 37.6% were aged 40-49 at the time of enrollment, 71.5% were non-Hispanic white, 46.5% had a body mass index (BMI) of 18.5-24.9, 51.9% were postmenopausal, and 78.0% had no first-degree relatives with breast cancer. Overall, 1.5% (n = 40) were diagnosed with breast cancer, 3.5% (n = 98) received a breast MRI, and 25.8% (n = 669) received a breast ultrasound. In unadjusted analyses, increased breast density was associated with higher rates of supplemental imaging. Compared with women who never had dense breasts, those who always had dense breasts had higher use of MRI (RR 3.7, p < 0.001), while ultrasound use was higher among women who sometimes (RR 1.5, p = 0.0002) and always (RR 1.7, p < 0.0001) had dense breasts. After adjusting for age, race, BMI, menopausal status, family history, and mammogram results, the association between breast density and supplemental imaging was attenuated. Women who sometimes had dense breasts remained more likely to undergo ultrasound and any supplemental imaging compared with women who never had dense breasts, but the associations were smaller and did not reach significant statistical significance. Abnormal mammogram results were the strongest predictor across all models. Conclusions: These findings suggest that breast density alone may be insufficient to guide supplemental screening guidelines and support more targeted, risk-informed approaches.