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.