• cT1-3N1M0 breast cancer at diagnosis (prior to NACT) by American Joint Committee on Cancer (AJCC) staging 8th edition

• Patients with occult primary breast cancer (no identifiable invasive cancer in the breast) with FNA or core biopsy proven nodal metastases are also eligible for the study.

• Fine-needle aspiration (FNA) or core biopsy confirmed axillary nodal metastases at presentation

• Oestrogen receptor and HER2 status evaluated on primary tumour

• Received standard NACT as per local guidelines (Patients undergoing neoadjuvant endocrine therapy as part of another clinical trial are eligible)

• Imaging of the axilla, as required, to assess response to NACT (per local guidelines)

• Undergo a dual tracer sentinel node biopsy (SNB) after NACT with at least 3 nodes removed in total (sentinel nodes and marked node).

• If a single tracer SNB is performed, the patient is eligible only if the involved node is marked before or during NACT, and at least 3 nodes (including the marked node) are removed during sentinel node biopsy.

• If the node is not marked, or the marked node is not removed, the patient is eligible only if the histology report shows evidence of down-staging with complete pathological response e.g. fibrosis or scarring in at least one node and at least 3 nodes removed.

• If fewer than 3 nodes are found on histology, the patient is eligible only if BOTH points a) and b) below, are met:

‣ involved node was marked and removed during SNB; and

⁃ removed marked node shows evidence of downstaging on histology e.g. fibrosis or scarring.

• If the sentinel node(s) cannot be localised on SNB: axillary node sampling should be performed, the patient will be eligible if at least 3 nodes are removed (including the marked node).

• No evidence of nodal metastases post NACT (isolated tumour cells, micro or macro metastasis)

• Patients with complete pathological response in the axilla but residual disease in the breast post NACT are eligible for the study.