Study to see if PET/MRI can predict breast cancer treatment success before It's visible

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Patients must be ≥ 18 years old and ≤ 75 years old * Patients with HER2+ metastatic breast cancer * HER2-positive breast cancer prospectively determined on the primary tumor by a local pathology laboratory and defined as immunohistochemistry (IHC) score of 3+ and/or positive by in situ hybridization (ISH) (defined by ISH ratio of ≥ 2.0 for the number of HER2 gene copies to the number of chromosome 17 copies). Only one positive result is required for eligibility * Estrogen/progesterone receptor positive OR negative disease allowed * Patients must have measurable disease in one metastatic lesion per RECIST v 1.1 * Stage IV HER2+ breast cancer patients eligible for a new therapeutic regimen that includes HER2-targeted treatment who are naïve to that regimen * Estimated life expectancy of greater than six months Exclusion Criteria: * Children, less than 18 years of age, will be excluded from this study * Metastatic breast cancer patients who are HER2 positive and have already started their current HER2-targeted therapy regimen for metastatic disease * Patients who have not recovered from grade 2 or higher toxicities of prior therapy to the point that they would be appropriate for re-dosing will be ineligible for study treatment * Patient is pregnant or breastfeeding, or expecting to conceive children within the projected duration of the study, starting with the screening visit through 7 months after the last dose of study treatment * Patient is considered a poor medical risk due to a serious, uncontrolled medical disorder, nonmalignant systemic disease or active infection that requires systemic therapy. Specific examples include, but are not limited to, active, non-infectious pneumonitis; uncontrolled major seizure disorder; unstable spinal cord compression; superior vena cava syndrome; or any psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the study (including obtaining informed consent). * Patients found to have constitutionally present non-magnetic resonance (MR) compatible ferromagnetic materials * Unable to lie still on the imaging table for one (1) hour * Inability to receive gadolinium-based contrast agent * Patients for whom a PET/MRI is technically not feasible (e.g. breast volume, obesity, \> body mass index (BMI) 36)