On-treatment core needle biopsy identifies early responders for axillary surgery de-escalation: results from a prospective cohort.

BACKGROUND: Increasing axillary pathologic complete response (pCR) rates after neoadjuvant chemotherapy (NACT) challenge the need for routine axillary surgery. On-treatment core needle biopsy (CNB) can assess early response, but its predictive and prognostic value for axillary surgery de-escalation remains unclear. METHODS: This prospective cohort included breast cancer patients receiving on-treatment CNB (after 2-4 cycles) of the primary tumor during NACT (2013-2021). Early responders were defined as having no residual disease of on-treatment CNB. These results were compared with axillary pCR and survival outcomes. RESULTS: The overall axillary pCR rate of 1693 patients was 54.3%. Of 500 early responders, 83.4% achieved axillary pCR. In clinically node-negative (cN0) patients, 98.0% (99/101) of those early responders presented with no nodal disease. While 79.7% of cN+ early responders achieved axillary pCR, this rate increased to 89.6% when combined with post-treatment MRI complete response (92.6% in cN1 and 90.0% in cN2 patients). Survival analysis revealed early responders with superior five-year overall and event-free survival with HR-negative tumors (p < 0.001). CONCLUSIONS: On-treatment CNB reliably identifies patients likely to achieve axillary pCR, offering an approach to guide de-escalation strategies and potentially identifying patients who may benefit from tailored systemic and surgical treatments.