Real-world Study of Trastuzumab Deruxtecan in Patients With Unresectable or Metastatic HER2-low Breast Cancer

This study is collecting real-world data on how a targeted drug called trastuzumab deruxtecan (T-DXd) works in patients with HER2-low metastatic breast cancer — a type where the HER2 protein is present at low levels. The goal is to understand how the drug performs outside of clinical trials. **You may be eligible if...** - You are 18 or older with confirmed advanced or metastatic breast cancer that cannot be surgically removed - Your tumor has been tested and found to be HER2-low (specific lab result levels: IHC1+ or IHC2+/ISH-) - You have previously received chemotherapy for metastatic disease, OR your cancer came back within 6 months of finishing chemotherapy after early-stage treatment - Your doctor recently decided (within the last 30 days) to start you on T-DXd **You may NOT be eligible if...** - You are pregnant or breastfeeding - You had a different type of cancer within the past 2 years - You are in a blinded clinical trial - Your tumor was previously classified as HER2-positive Talk to your doctor to see if this trial is right for you.