BEYOND Study: A Multicentre, Prospective, Observational Study of Real-world Treatment Patterns and Outcomes of Trastuzumab Deruxtecan in Patients With HR Positive, HER2-low or Ultra-low Metastatic Breast Cancer Previously Treated With Endocrine Therapy

Inclusion Criteria:

* -Female patients aged ≥18 years old at the time of T-DXd initiation
* Patients with a confirmed histological or cytologically based diagnosis of HR-positive mBC.
* Patients who were classified as HER2-low or HER2-ultralow mBC confirmed by the closest locally obtained HER2 test prior to or on the index date, and who meet the following definitions:
* HER2-low status defined as IHC scores 1+ and 2+ without ISH gene amplification based on the pathology report.
* HER2-ultralow status defined as IHC 0 with membrane staining based on the pathology report.
* Patient who received at least one prior line of ET (± targeted therapy) in the metastatic setting.
* Patients who are chemotherapy-naïve in the metastatic setting.
* Patients who initiated T-DXd up to 30 days before the signature of the ICF. The decision to initiate T-DXd must be made independently by treating physicians as part of routine clinical practice.
* Patients are willing to sign the written ICF, indicating that they understand the purpose of the study and procedures required for participation.

Exclusion Criteria:

* -Patients with a history of other malignancies, other than basal cell carcinoma of the skin and squamous cell carcinoma of the skin.
* Patients who received prior chemotherapy in the metastatic setting.
* Patients with Eastern Cooperative Oncology Group (ECOG) status ≥2 or missing ECOG status at the time of initiation of T-DXd (index date).
* Patients with a history of participation in another clinical trial in the metastatic setting.
* Female patient with current or planned pregnancy, or breastfeeding.