HER-TEMPO - Real World Treatment-related Outcomes on T-DXd in Canada

Inclusion Criteria: Enrollment criteria for HER2-positive PSP: * Adults, 18 years of age or older * Unresectable or metastatic HER2-positive breast cancer who have received at least one prior anti-HER2-based regimen either in the metastatic setting, or in the neoadjuvant or adjuvant setting and developed disease recurrence during or within 6 months of completing neoadjuvant or adjuvant therapy * Patients must not have received a prior anti-HER2 ADC, such as trastuzumab emtansine, in the metastatic setting * Patients who received a prior anti-HER2 ADC in the adjuvant setting must have progressed \>12 months following the completion of therapy Enrollment criteria for HER2-low PSP: * Adults, 18 years of age or older * Unresectable or metastatic HER2-low (IHC 1+ or IHC 2+/ISH-) breast cancer who have received at least one prior line of chemotherapy in the metastatic setting or developed disease recurrence during or within 6 months of completing adjuvant chemotherapy * Patients with HR+ breast cancer should have received at least one or be considered ineligible for endocrine therapy. Patients in the SPP must additionally meet the following criteria: * Patients must have provided consent to be contacted for future research and provided consent to participate in the current observational study Exclusion Criteria (both cohorts): * Patients who enrolled in the PSP, but did not initiate therapy with T-DXd by the end of the PSP program * Patients with medical history of Interstitial Lung Disease (ILD) / pneumonitis that required steroids or current ILD /pneumonitis * Patients who do not have adequate renal or hepatic function, defined as: * Inadequate renal function is defined as Creatinine clearance \<30 mL/min, as calculated using the Cockcroft-Gault equation . * Inadequate hepatic function is aspartate aminotransferase (AST)/alanine aminotransferase (ALT) \>5 × upper limit of normal (ULN). Total bilirubin \>1.5 × ULN if no liver metastases or ≥ 3 x ULN in the presence of documented Gilbert's syndrome (unconjugated hyperbilirubinemia) or liver metastases at baseline.