Axillary lymph node status is an important prognostic factor for patients with breast cancer. After breast cancer diagnosis, current nodal staging consists of axillary ultrasound (US) combined with tissue sampling when deemed necessary. In case of positive axillary lymph nodes, patients will undergo axillary lymph node dissection (ALND). In case of no suspicious axillary lymph nodes (i.e. clinically node negative patients), patients will undergo sentinel lymph node biopsy (SLNB). This surgical nodal staging is accompanied by co-morbidity. In theory, if non-invasive imaging can evaluate the lymph node status accurately, a node negative patient would no longer have to undergo axillary surgery. Since MRI is suitable for soft tissue imaging and PET has the advantage of showing increased metabolic uptake in lymph node metastases, a combination of these techniques in hybrid PET/MRI would be highly desirable. If dedicated axillary hybrid PET/MRI is equally accurate to SLNB for the detection of negative axillary lymph nodes, work-up could be more efficient by bypassing SLNB. However, the accuracy of dedicated axillary hybrid PET/MRI needs to be compared with the pathological outcome of SLNB (gold standard) first.

Inclusion Criteria:

1. Female patient with histologically confirmed breast cancer and clinically confirmed negative lymph nodes in the axilla, scheduled to undergo SLNB
2. Patients who are willing and able to undergo the study procedures
3. The patient has provided personally written informed consent

Exclusion Criteria:

1. Patients treated with neoadjuvant systemic therapy prior to axillary nodal staging
2. Patients with clinically positive axillary lymph nodes
3. Age < 18 years
4. Inability to provide informed consent
5. Pregnancy
6. Weight >100 kg (because of the format of the PET/MRI scanner)
7. General contraindications for MRI (such as pacemaker, aneurysm clips, metallic device in their body, severe claustrophobia) or PET (i.e. known allergy to 18F-FDG)
8. Hyperglycaemia (> 11 mmol/L) at the time of 18F-FDG injection