Impact of residual nodal disease burden on sentinel node mapping and accuracy of intraoperative frozen section in node positive (cN1) breast cancer patients treated with neoadjuvant chemotherapy (NAC).

584Background: 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 ...