The purpose of this study is to learn more about how a new study agent works inside the body. The study agent is a protein called 64Cu-trastuzumab. This is a radioactive tracer that was developed at MSKCC to target HER2 protein on cancer cells. A radioactive tracer is a small amount of radioactive dye that follows something else that is not radioactive In this study the study agent, 64Cu-trastuzumab, is the tracer and what's being followed is trastuzumab (Herceptin™). By giving you this tracer after you have treatment with trastuzumab (Herceptin™), we will be able to use PET scans to show us which parts of your body and tumor sites the Herceptin goes to. This will help us to understand better how Herceptin works in the body to fight cancer.

Inclusion Criteria:

• Registered patient at MSKCC
• Age ≥18 years
• Patients with HER2+ metastatic breast cancer (either 3+ by immunohistochemistry or with evidence of gene amplification (>2.0) by fluorescence in situ hybridization (FISH))
• Measurable or evaluable disease
• Currently taking or about to commence treatment with trastuzumab at a dosing schedule of 2 mg/kg weekly or 6 mg/kg every three weeks, as per standard of care
• Karnofsky Performance Score ≥ 60
• Signed informed consent

Exclusion Criteria:

• Previous Grade 3 or higher allergic reaction to trastuzumab that resulted in discontinuation of trastuzumab therapy
• Claustrophobia/pain leading to inability to lie still for the duration of the scanning procedure.
• Pregnancy Test to be performed on female patients of childbearing potential within 24hrs before administration of radioactive material.
• Inability to provide written informed consent.
• Patients with liver metastases as the only site of distant disease
• Patients with known sensitivity or contraindication to Herceptin.