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

This study follows Korean patients with HER2-positive advanced breast cancer who are switching to a drug called T-DM1 after their previous drug (T-DXd, also known as trastuzumab deruxtecan) stopped working. The goal is to understand how well T-DM1 works in this situation. **You may be eligible if...** - You have HER2-positive advanced (unresectable or metastatic) breast cancer - You previously received trastuzumab deruxtecan (T-DXd) and your disease progressed - You are planning to start T-DM1 as your next treatment **You may NOT be eligible if...** - You have already received T-DM1 for advanced breast cancer - You received any other systemic cancer treatment after T-DXd (besides hormone therapy alone) - You have already received 4 or more lines of therapy for advanced breast cancer Talk to your doctor to see if this trial is right for you.