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