Studies on intraoperative assessment of sentinel nodes by frozen section in breast cancer have reported low sensitivity rates. Recently, a molecular-based procedure to detect lymph node metastases, one-step nucleic acid amplification (OSNA) assay, has been developed. OSNA assay can assess a whole lymph node and yields definitive semi quantitative results. The aim of this study was to evaluate the ability of intraoperative assessment comparing sentinel lymph node (SLN) metastases with OSNA assay versus routine H&E frozen section (FS) histology. METHODS: A cohort of 478 consecutive patients diagnosed in 2010 with early breast cancer in two centers was analyzed. Frozen section was performed intraoperatively in 228 patients and OSNA assay in 250 patients. Patient characteristics were evaluated in both groups and rates of metastasis detected by both methods were compared. RESULTS: Patients and tumor characteristic are summarized in the table. A median of 1,95 SLNs were assessed by FS and a median of 2,06 SLNs by OSNA (p=0.15). Intraoperative histopathological assessment detected 67 SLNs metastasis by FS and 107 SLNs metastasis by OSNA assay (p=0.07). There were no differences in rates of macrometastasis (54 by FS and 74 by OSNA) and micrometastasis (13 by FS and 33 by OSNA) (p=0.09). When compared the OSNA assay with permanent section, 92 SLNs had metastasis by permanent section (63 macrometastasis and 29 micrometastasis) with no statistically significant differences (p=0.5). SLNs metastasis were found in 71 patients (31,1 %) by permanent section and in 83 (33,2 %) by OSNA assay (p=0.69). Axillary lymph node dissection (ALND) for metastatic SLN was performed in 148 patients, 79 (31,6 %) in OSNA group and 69 (30,2 %) in the permanent section group (p=0.68). All patients diagnosed by OSNA had a complete ALND during the initial surgical procedure. On the other hand, ALND was performed in 51 patients (73,9 %) in the permanent section in the initial surgery, and ALND was performed in a second surgical procedure in18 patients (26 %), due to false negative results of the FS (p<0.001). CONCLUSIONS: The OSNA assay can detect SLN metastasis as accurately as conventional pathology, with no increased detection of positive SLNs. Given the definitive pathology of the SLN intraoperatively, the use of OSNA can reduce the need for a second surgery in 26 % of patients with breast cancer and a positive SLN. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-07-11.