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