Real-World Study of Post-Resistance Treatment Strategies in Advanced Breast Cancer Following CDK4/6i, PIK3CA Inhibitors, or T-DXd

Inclusion Criteria: 1. Adults (≥18 years old) with histologically or cytologically confirmed advanced or metastatic breast cancer 2. Received prior treatment with at least one of the following: CDK4/6 inhibitors, PIK3CA inhibitors, trastuzumab deruxtecan (T-DXd), or other targeted therapies 3. Documented disease progression following prior targeted therapy 4. Initiated a subsequent line of systemic therapy (chemotherapy, endocrine therapy, targeted therapy, or combination) after resistance 5. Available clinical data including baseline characteristics and treatment details 6. At least one follow-up evaluation after initiation of post-resistance therapy Exclusion Criteria: 1. Incomplete medical records or missing key clinical follow-up data 2. Concurrent diagnosis of other active malignancies (except non-melanoma skin cancer or in situ cervical cancer) 3. Known central nervous system disease requiring immediate local treatment (unless clinically stable) 4. Poor general condition with an Eastern Cooperative Oncology Group (ECOG) performance status ≥2 5. Life expectancy estimated to be less than 6 months based on clinical judgment