Trastuzumab Emtansine(T-DM1) After Trastuzumab Deruxtecan(T-DXd) in HER2-Positive Breast Cancer in Korea

Inclusion Criteria: * HER2-positive, advanced (unresectable locally advanced or metastatic) breast cancer * Prior treatment with trastuzumab deruxtecan(T-DXd) in the advanced setting * Intention to treat with Trastuzumab Emtansine (T-DM1) as first therapy upon T-DXd progression Exclusion Criteria: * Any systemic treatment, other than T-DM1, for breast cancer after trastuzumab deruxtecan (As an exception, patients treated with endocrine agents, not in combination with cytotoxics or antibody-drug conjugates, will be permitted in the study) * Has been previously treated with T-DM1 for advanced breast cancer * Has been previously treated with 4 or more lines of therapy for advanced breast cancer * Half (50%) or more patients will be those who were treated with second-line trastuzumab deruxtecan (i.e. patients at the later line will be capped).