Abstract LB381: Estimating trends in early-onset breast cancer incidence in the United States by race and molecular subtype, 2011-2019

Abstract While breast cancer mortality continues to trend downward due to screening, education and improved treatment, disparities in incidence persist and are more pronounced when we consider molecular subtypes of this heterogeneous disease. Black women and younger women experience a higher frequency of adverse histological features, and incident cases have increased among women under age 50 at a greater rate than those aged 50 years or older. Estimating and addressing the racial gap between women is crucial to achieving equity. To date, epidemiologic studies are largely based on smaller observational studies or are geographically limited to SEER registry data. Using data from the National Program of Cancer Registries (NPCR) which covers 100% of the population, this study estimated annual trends in early-onset age-adjusted female breast cancer rates by molecular subtype and race across the United States. Early-onset was defined as women with first malignant, invasive breast cancer diagnosis between ages 20 and 49. From 2011-2019, 436, 591 women were diagnosed with early-onset breast cancer, excluding cases with unknown race. HR+/HER2- incidence rates increased in all women with significant increases occurring for white (APC from 2017-2019 = 2. 80%, 95% CI = 1. 63, 3. 52), Black (APC from 2017-2019 = 4. 39%, 95% CI = 1. 13, 6. 79) and American Indian/Alaskan Native (AI/AN) (APC from 2015-2019 = 8. 18%, 95% CI =5. 17, 15. 05) women in the latter years of the time period, while rates for Asian/Pacific Islander (A/PI) women increased across the entire time period (APC from 2011-2019 = 5. 55%, 95% CI = 4. 57, 6. 67). HR+/HER2+ incidence rates increased for white women before 2015 then decreased from 2015-2019 (APC from 2011-2015 = 3. 99%, 95% CI = 4. 57, 6. 67, APC from 2015-2019 = -2. 27%, 95% CI = -6. 34, -0. 46). Similar trends were observed in Black women (APC from 2011-2015 = 5. 49%, 95% CI = 3. 85, 7. 79; APC from 2015-2019 = -1. 72%; 95% CI = -3. 71, -0. 18), while no significant trends were observed for AI/AN women. Rates increased at a high rate from A/PI women from 2011-2013 and slowed from 2013-2019 (APC from 2011-2013 = 16. 30%, 95% CI = 8. 26, 22. 10; APC from 2013-2019 = 2. 80%; 95% CI 0. 25, 4. 15). Incidence rates for HR-/HER2+ increased during the early period for all women, then decreased at varying rates. Trends were not significant for white women, significantly increased then decreased for Black women (APC from 2011-2015 = 7. 27%, 95% CI = 4. 21, 11. 77; APC from 2015-2019 = -5. 53%, 95% CI = -9. 20, -2. 82), significantly decreased for AI/AN women (APC from 2016-2019 = -18. 27%, 95% CI = -46. 76, -1. 27), and significantly increased for A/PI women (APC from 2011-2015 = 6. 21%, 95% CI = 3. 24, 16. 11). Unlike the other molecular subtypes, HR-/HER2- decreased across all years among all races, with no significant findings among AI/AN women. Incidence rates among white (APC from 2011-2016 = -3. 16%, 95% CI = -5. 05, -2. 42) and A/PI women (APC from 2011-2017 = -2. 02%, 95% CI = -6. 16, -0. 25) decreased in the early period, while rates among Black women decreased across the entire time period (APC from 2011-2013 = -5. 35%, 95% CI = -6. 67, -3. 11; APC from 2013-2019 = -1. 33%, 95% CI = -1. 80, -0. 20). While breast cancer rates overall are increasing among younger women, trends differ by molecular subtype and by race with no clear pattern of a singular racial groups consistently experiencing the greatest rate increase or decrease. Citation Format: Lia C. Scott, Karen E. Nielsen. Estimating trends in early-onset breast cancer incidence in the United States by race and molecular subtype, 2011-2019 abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (8Suppl): Abstract nr LB381.