Geographic differences in breast cancer incidence and trends in young women ages <40 in the United States

PURPOSE: Incidence of breast cancer in young women is on the rise. This study examined breast cancer incidence and trends in young women in the U.S. by tumor characteristics and geography. METHODS: Using the U.S. Cancer Statistics Public Use Research Database, age-adjusted incidence rates (AAIRs) of breast cancer in women ages < 40 for the U.S. Census Bureau's nine geographic divisions were calculated by tumor stage and subtype for 2011-2022. Incidence rate ratios (IRRs) and 95% confidence intervals (CIs) were calculated comparing each division's AAIR to the Pacific division's AAIR as a comparison group. Average annual percent changes (AAPCs) were determined using joinpoint regression models. RESULTS: Incidence of distant and triple negative breast cancer was highest in the South Atlantic, with AAIRs that were 20% and 25% higher than the Pacific (IRR=1.20, 95% CI 1.13-1.28 and IRR=1.25, 95% CI 1.20-1.30, respectively). The largest increasing trend was observed in the Middle Atlantic for distant breast cancer (AAPC=4.28, 95 CI 2.40-7.53). CONCLUSION: There are important geographic differences in the burden of distant stage and triple negative breast cancer in young women that should be further studied to help clarify the etiology of young breast cancer.