Efficacy of Trastuzumab Deruxtecan in Metastatic Breast Cancer With Different HER2 Expression Patterns

Inclusion Criteria: * Patients with advanced or locally unresectable breast cancer; * Treated with T-DXd (DS-8201, trastuzumab deruxtecan) regardless of line of therapy, HR, and HER2 expression level; * HER2 immunohistochemistry staining information of the primary lesion or metastatic/recurrent lesion; * Measurable lesions with treatment response results that can be evaluated through imaging examinations; * Able to follow up with the latest progression-free survival or overall survival. Exclusion Criteria: * Patients with missing basic clinical information or HER2 immunohistochemistry staining information; * Patients with missing pathological results for both primary and metastatic/recurrent lesions; * Patients with no measurable lesions and unable to evaluate T-DXd treatment response; * Discontinue T-DXd therapy for unacceptable adverse events or other reasons; * Patients lost to follow-up after T-DXd treatment.