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Background: Randomized trials have shown that axillary clearance (AC) can safely be omitted in patients with sentinel lymph node-positive breast cancer. At the same time, de-escalation of chemotherapy in postmenopausal patients with ER+HER2− breast cancer may depend on detailed axillary nodal stage. The aim of this pre-specified secondary analysis of the SENOMAC trial was to investigate whether the choice of axillary staging affected the proportion of patients receiving adjuvant chemotherapy, and recurrence-free survival (RFS).

Methods: Proportion receiving adjuvant chemotherapy was calculated according to AC or sentinel lymph node biopsy (SLNB) only, menopausal status, and region of inclusion, for 2168 patients with clinically node-negative ER+HER2− breast cancer and 1–2 sentinel lymph node macrometastases included in the SENOMAC trial.

Findings: In premenopausal patients, 514 out of 615 patients (83.6%) received adjuvant chemotherapy with no significant difference between randomization arms. In postmenopausal patients, the proportion receiving chemotherapy varied considerably by region and country (36.0–82.4%). In Denmark, where 194 out of 539 postmenopausal patients (36.0%) received adjuvant chemotherapy, rates differed significantly between the AC and the SLNB only arm (41.3% vs 31.4%, p = 0.019). After a median follow-up of 44.88 months for Danish postmenopausal patients, no significant difference was seen in 5-year RFS, which was 91% (85.6%–96.6%) for the SLNB only and 90.9% (86.3%–95.6%) for the AC arm (p = 0.42).

Interpretation: When omitting axillary clearance, and thus reducing the risk of long-term arm morbidity, potential under-treatment of postmenopausal patients with ER+HER2− breast cancer may require the development of new predictive and imaging tools.