Patient population

This prospective single-centre clinical trial protocol was approved by the medical ethics committee of the University Medical Centre Groningen (UMCG; ClinicalTrials.gov identifier {"type":"clinical-trial","attrs":{"text":"NCT01832051","term_id":"NCT01832051"}}NCT01832051). All patients provided written informed consent.

Patients with suspected metastatic disease or local recurrence of HER2-positive breast cancer with a clinical dilemma defined as failure of standard work-up to evaluate the HER2 status were eligible. HER2-positivity, reported in the patient’s history, was defined positive with an IHC of score 3+ or IHC of score 2+ followed by ISH showing HER2 amplification according to the American Society of Clinical Oncology guidelines [22]. Standard imaging work-up preferably consisted of a computed tomography (CT) of the chest and abdomen, a bone scintigraphy and a fluorine-18-fluorodeoxyglucose (¹⁸F-FDG) PET scan, accompanied by a metastasis biopsy, if feasible. Other eligibility criteria included age ≥ 18 years and Eastern Cooperative Oncology Group performance status of 0–2. Patients with a history of allergic reactions to immunoglobulins and pregnant or lactating women, as well as patients with any inabilities not allowing compliance with the study procedures, were excluded.