DESTINY Breast Respond HER2-(Ultra)Low Europe

Trastuzumab deruxtecan (T-DXd) as monotherapy is indicated for the treatment of adult patients with unresectable or metastatic HER2-low breast cancer who have received prior chemotherapy in the metastatic setting or developed disease recurrence during or within 6 months of completing adjuvant chemotherapy. Based on the extended therapeutic indication of Trastuzumab deruxtecan (Enhertu®), a new patient population will be enrolled, comprising adult patients with unresectable or metastatic HR-positive, HER2-low, or HER2-(ultra)low breast cancer who have received at least one endocrine therapy in the metastatic setting and are not considered suitable for endocrine therapy as the next line of treatment.

Inclusion Criteria: Study Group 1 and Study Group 2 * Adult patient (age ≥ 18 years) with histological or cytological confirmed diagnosis of unresectable and/or mBC * Decision to newly initiate therapy of T-DXd or conventional chemotherapy according to the physicians choice per SmPC * Written and signed Informed Consent to participate in the study Study Group 1 * Documented HER2-low status (IHC1+, IHC2+/ISH-) * Patients who have received prior chemotherapy in the metastatic setting or * Patients who have developed disease recurrence during or within 6 months of completing adjuvant chemotherapy Study Group 2: * Documented HR+ status * Documented HER2-low status (IHC1+ or IHC2+/ISH-) or HER2-ultralow (defined as IHC 0 with membrane staining \[IHC \> 0 to \