An Investigational Scan (64Cu-DOTA-Trastuzumab PET/MRI) in Imaging Patients With HER2+ Breast Cancer With Brain Metastasis

Inclusion Criteria: * Documented informed consent of the participant and/or legally authorized representative * Women with documented metastatic HER2 positive breast cancer (American Society of Clinical Oncology \[ASCO\] College of American Pathologist \[CAP\] guidelines) who have brain metastases * Age \> 18 years * Eastern Cooperative Oncology Group (ECOG) 0-2 * Patients with leptomeningeal disease will be considered eligible * Planned therapy with fam-trastuzumab deruxtecan * Left ventricular ejection fraction (LVEF) \> 50% * Absolute neutrophil count (ANC) \> 1.5 x 10\^9/L * Platelets \> 100 x 10\^9/L * Hemoglobin \> 9 g/dL * Total (T.) bilirubin \< 3 x upper limit of normal (ULN) * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< 5 x ULN * Creatinine clearance \> 30 ml/min (by Cockcroft-Gault formula) * Activated partial thromboplastin time (aPTT) \< 1.5 x ULN * Prior therapy for central nervous system (CNS) disease is allowed, but at least 1 lesion \> 1.5 cm is evident on MRI Exclusion Criteria: * Need for immediate local intervention for brain metastases * Noninfectious interstitial lung disease or pneumonitis requiring glucocorticoids * Clinically significant corneal disease * Myocardial infarction \< 6 months before, congestive heart failure (CHF), unstable angina, or serious cardiac arrhythmia