Inclusion Criteria:

• Age ≥ 18

• Male or female

• cT1-3N1M0 breast cancer at diagnosis (prior to NACT) as per AJCC 8th edition (see Section 6.4.1)

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

• 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.

  • 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:

    1. involved node was marked and removed during SNB; and
    2. 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.

• 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

• Patients who are cN0 (node negative) at initial presentation, with a negative sentinel node post NACT showing evidence of treatment response or downstaging will be eligible, provided at least 3 nodes are removed.

5.2. Exclusion Criteria

Participants will be excluded if they have any one of the following:

• Bilateral synchronous invasive breast cancer

• Sentinel node biopsy prior to NACT

• Previous axillary nodal surgery on the same body side as the scheduled targeted sampling

• Any previous cancer within last 5 years or concomitant malignancy except

  • basal or squamous cell carcinoma of the skin
  • in situ carcinoma of the cervix
  • in situ or stage 1 melanoma
  • contra- or ipsilateral in situ breast cancer
  • chronic lymphocytic leukaemia not on treatment