Background: Recent trials have demonstrated the feasibility of SLN biopsy in cN1 patients who become cN0 after NAC. We sought to evaluate success of SLN mapping and accuracy of intraop frozen section (FS) by residual nodal disease burden. Methods: cT1-3 cN1 patients receiving NAC and surgery (1/2016 to 5/2017) were identified from a prospective database. Pts who converted to cN0 and had SLN biopsy with dual-tracer were included. Adequate mapping (defined as ≥3 SLN) and false negative rate (FNR) of intraop FS were assessed by residual nodal disease burden (ypN0, ypNmi+ITC, ypN1-3). Results: Among 137 cT1-3 cN1 pts, 76 met inclusion criteria. Median age 45 yrs [27-82]; median tumor size 4.3cm [0.8-15.0]. 32 (42%) pts were ER+HER2-, 24 (32%) HER2+ and 20 (26%) ER-HER2-. Adequate mapping was achieved in 50 (66%) pts; 14 (18%) failed to map and 12 (16%) had < 3 SLN identified. Adequate mapping was not associated with residual node burden (table, p = 0.21). Among 48 pts with adequate mapping and FS, 16 were ypN+ on FS and 28 were ypN+ on final pathology; FNR of 12/28 (43%). Smaller residual node burden was associated with false negative FS (table, p = 0.005). 28/76 (37%) pts achieved axillary pCR, of whom 20 (71%) had ≥3 negative SLN and were spared ALND. Of 36 pts with successful mapping and positive SLN, 22 (61%) underwent ALND, of whom 8 (36%) had additional nodal disease; the remaining 14 (39%) had axillary radiation. Conclusions: Among pre-NAC cN1 pts, SLN biopsy was technically adequate in 66%. Of these, 40% achieved axillary pCR and avoided ALND. The FNR of intraop FS was 43%. Residual nodal disease burden was not associated with adequate mapping; micrometastases and ITCs were associated with higher likelihood of false negative FS. Preoperative counseling for SLN biopsy should include realistic assessment of the limitations of SLN mapping and intraop FS and the potential need for ALND.

*≥3 SLN