80 Background: In NSABP B-32 trial 5,611 women with invasive breast cancer were randomly assigned to sentinel lymph node biopsy (SLNB) plus axillary dissection (Group [Grp] 1) or SLNB alone (Grp 2). There was no significant difference in OS or DFS reported between the two groups. Initial permanent pathologic assessment of the SLNs at participating sites was designed to identify all macrometastases >2 mm and consisted of slicing the SLNs at approximately 2.0 mm intervals, embedding all slices in paraffin tissue blocks, and examining one hematoxylin and eosin stained slide from each block. Routine use of IHC or sectioning of deeper levels was prohibited. SLN paraffin tissue blocks from cases with pathologically negative SLNs were later evaluated centrally for occult metastases deeper in the blocks. This study evaluates group outcomes related to occult metastases. Methods: Routine and cytokeratin IHC stains were used at two widely spaced additional levels. Using this technique, occult metastases were detected in 15.9% of the patients (pts) with initially negative SLNs (616 / 3884 cases). Of the 616 cases, 431 (69.8%) had isolated tumor-cell clusters, 171 (27.9%) had micrometastases, and 14 (2.3%) had macrometastases (Weaver D, et al: N Engl J Med, 2011). Results: 316 (16.4%) of 1924 pts had occult metastases in Grp 1 and 300 (15.3%) of 1960 in Grp 2. Non-sentinel node status was available in 312/316 pts in Grp 1; 23 (7.4%) had positive non-sentinel nodes. In pts with occult metastases, there were no significant differences in OS or DFS between groups (Grp 2 vs. Grp 1 OS HR: 0.89, p=0.62; DFS HR: 0.79, p=0.16). There were no significant differences in OS or DFS between the groups in pts who were negative for occult metastases (Grp 2 vs. 1 OS HR:1.25, p=0.07; DFS HR: 1.11, p=0.22). Conclusions: A more detailed assessment of the SLNs with deeper sectioning and IHC staining detected a significant level of occult metastases in clinically node negative pts. However, no benefit was seen by the addition of axillary dissection in pts with occult SLN metastases. Supported by NCI: U10-CA-12027, U10-CA-37377, U10-CA-69974, U10-CA-69651, and ARRA ROI CA 74137
