Real-World Treatment Patterns and Outcomes in HER2-Altered Metastatic Breast Cancer Patients in the United States

Inclusion criteria: Overall metastatic breast cancer (mBC) cohort (total population): * Histologically or cytologically confirmed diagnosis of breast cancer (BC) * Patient has at least 2 documented clinical visits in the Flatiron network, on different days, during the study period from 1 January 2011 through 31 March 2025 * Initial diagnosis of de novo or recurrent mBC established during the case selection window from 1 January 2018 through 31 March 2024 (1 year prior to the data cutoff date, 31 March 2025) \-- The date of initial mBC diagnosis will define the study index date * Aged ≥ 18 years at the study index date Human epidermal growth factor receptor 2 (HER2)-positive subcohort (mBC HER2+): * Patients meeting eligibility for the mBC cohort * Patients can be HER2-mutant or HER2 nonmutant * Evidence of HER2+ as defined by immunohistochemistry (IHC)/in situ hybridization (ISH) based on a test performed within 180 days before or 180 days after the index date. If a patient does not have any IHC/ISH test during the 180-day periods before or after the index date, then tests performed at any time in the pre-index date period will be examined and any evidence of HER2+ status will classify the patient as HER2+. * IHC 3+ or IHC 2+ and evidence of HER2 amplification by ISH or ISH+ (without further information on IHC) according to American Society of Clinical Oncology/College of American Pathologists guidelines. * The sample date will be used to define biomarker status. If the sample date is missing, the report date will be used. HER2-mutant subcohort (mBC HER2-mutant): * Patients meeting eligibility for the mBC cohort * HER2-mutant: Patients are classified as having an HER2 mutation if there is evidence of NGS short variant alterations at any time in the database, regardless of the functional type, which includes missense, nonsense, frameshift, nonframeshift, or splice variants. Exclusion criteria mBC cohort (total population): -To ensure adequate treatment and outcome data, any patient without a visit or medication record (i.e., medication order/administration or LOT) within 90 days of mBC diagnosis (i.e., the time window from 90 days before to 90 days after the mBC diagnosis). HER2+ subcohort (mBC HER2+): \- None (other than those applicable for the mBC cohort). HER2-mutant subcohort (mBC HER2-mutant): \- Patients who are included in the HER2+ subcohort.