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

Inclusion Criteria: * Adult patient (age ≥18 years) with histological or cytological confirmed diagnosis of unresectable or mBC. * Documented HER2-low status (IHC1+, IHC2+/ISH-) in patients who have received prior chemotherapy in the metastatic setting or documented HER2-low status (IHC 1+, IHC 2+/ISH-) in patients who have developed disease recurrence during or within 6 months of completing adjuvant chemotherapy. * Recent prior decision to initiate therapy of T-DXd per SmPC (up to 30 days). Documentation confirming this decision will be required and collected. * Able and willing to provide informed consent. Exclusion Criteria: * Pregnancy or breastfeeding. * History of other primary malignancies in 2 years prior to unresectable or mBC diagnosis. * Patients who at time of data collection for this study are participating in or have participated in an interventional study that remains blinded. * HER2-low status patients who have previously documented HER2+ status in the same tumor.