ERBB2 and neoplasm: In HR+/HER2 - patients, metachronous MBC patients who received prior (neo)adjuvant systemic treatment had worse OS than de novo cases (prior chemotherapy: aHR 1.52, 95%CI 1.29-1.78); prior hormonal therapy only: aHR 1.33, 95%CI 1.10-1.61), whereas those without prior systemic treatment had similar outcomes.<h4>Conclusion</h4>De novo and metachronous MBC have different tumor biology, treatment patterns, and survival.