Information about the presence of human epidermal growth factor receptor 2 (HER2) in tumor lesions in breast cancer patients is essential for diagnostic and therapeutic management of metastatic breast cancer. In daily practice however, obtaining a metastasis biopsy can be difficult or impracticable. Therefore, clinicians can be faced with a persistent clinical dilemma in some breast cancer patients, leading to suboptimal therapy decisions due to lack of HER2 receptor information. Circulating tumor cells (CTCs), which may provide additional information, have so far not been able to replace the biopsy. To solve this problem, non-invasive whole body visualization and quantification of HER2 expression by means of the HER2-PET may be a valuable tool.

In this prospective multicenter imaging study, eligible patients will receive one HER2-PET and CTC analysis in addition to standard work up for metastatic disease. Subsequent administration of anti-HER2 therapy will be evaluated. Referring physicians fill in three questionnaires, one before HER2-PET and two after HER2-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