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.