Our goal is to investigate the use of 89Zr-trastuzumab as a HER2 imaging agent to determine which patients are likely to respond to targeted HER2 agents as single agent therapy. We are proposing to perform a pilot study with goals of demonstrating the feasibility of imaging breast cancer patients with 89Zr-trastuzumab-PET/MRI, evaluating the relationship between tumor 89Zr-trastuzumab uptake and in vitro positivity of HER2, assessing the relationship between 89Zr-trastuzumab uptake and response to HER2 therapy.

Inclusion Criteria:

• Be at least 18 years of age.
• Diagnosis of HER2 positive breast cancer as defined by to American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) 2018 guidelines: Immunohistochemistry (IHC) 3+ OR Single prove In situ hybridization (ISH) with average HER2 copy number >= 6 OR dual probe ISH with both average HER2 copy number >= 4 AND HER2 to CEP17 ratio >=2
• Patients eligible for radiation therapy or systemic therapy using a regimen containing at least one anti-HER2 agent
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 and 1
• Ejection fraction greater than 50% by echocardiogram or multiple-gated acquisition (MUGA) scans

Exclusion Criteria:

• Inability to provide informed consent
• Pregnancy
• Inability to lie still for the imaging study
• Weight over 350 lbs., due to the scanner bore size
• Contraindication for MRI study (e.g. non-removable metal implants or certain tattoos)