Background: Breast cancer survival outcomes vary across different ethnic groups. We clarified the differences in clinicopathological and survival characteristics of breast cancer among Japanese, US residents with Japanese origin (USJ), and US residents with other origins (USO). Method: Using Surveillance, Epidemiology, and End Results (SEER) 18 dataset and Japanese Breast Cancer Society (JBCS) registry, we included patients first diagnosed with breast cancer between 2004 and 2015. We categorized the patients into three groups based on the database and the recorded ethnicity: Japanese (all those from the JBCS registry), USJ (those from SEER with ethnicity: Japanese), and USO (those from SEER with ethnicity other than Japanese). Excluding patients diagnosed after 2012, stage 0, and 4 patients, we examined the overall survival (OS) and breast cancer-specific survival (BCSS) using the Kaplan–Meier method and Cox proportional hazards models, adjusting for age, sex, cancer stage, and hormone receptor (HR) status. Results: We identified 7362 USJ, 701,751 USO, and 503,013 Japanese breast cancer patients. The proportion of HR-positive breast cancer was the highest among USJ (71%). OS was significantly longer among Japanese and USJ than USO (Hazard ratio 0.46; 95% Confidence Interval [CI] 0.45–0.47 for Japanese and 0.66 [95% CI 0.59–0.74] for USJ) after adjusting for baseline covariates. BCSS was also significantly higher in the two groups (HR 0.53 [95% CI 0.51–0.55] for Japanese and 0.53 [95% CI 0.52–0.74] for USJ). Conclusions: In stage I–III breast cancer, Japanese and US residents with Japanese origin experienced significantly longer survival than US residents with non-Japanese origins.
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