Clinical Value of 89Zr-trastuzumab PET

Inclusion Criteria: 1. Patients with a history of histological and/or cytological proven HER2-positive primary breast cancer. In the Netherlands HER2-positivity is defined as: * HER2 immunohistochemical score of 3+, or * HER2 immunohistochemical score of 2+ and positive FISH for HER2/c-erbB2 amplification. In Belgium HER2-positivity is defined as positive FISH for HER2/c-erbB2 amplification. 2. Patients with suspected metastatic disease or local recurrence of HER2-positive breast cancer and a clinical dilemma: * in whom standard work up with imaging has failed to solve the clinical dilemma (diagnostic/therapeutic), leaving issues with regard to HER2 status of lesions and * in whom a biopsy is desirable but cannot (easily) be performed due to technical or patient factors or otherwise. 3. Standard work-up with imaging is defined as CT chest and abdomen, bone scintigraphy, as well as FDG-PET. 4. Age \>18 years of age. 5. WHO performance status 0-2. 6. Signed written informed consent. 7. Able to comply with the protocol. Exclusion Criteria: 1. Pregnant or lactating women. 2. Prior allergic reaction to immunoglobulins or immunoglobulin allergy. 3. Inability to comply with study procedures