Ultrastaging of sentinel lymph nodes (SLNs) compared to non-SLNs in colorectal (CRca) cancer: Do we need both?

4055 Background: SLN mapping (M) accurately stages many solid tumors including CRca. SLNs are 3–5 times more likely to have metastases (mets) when ultrastaged by microsections and IHC as compared to non-SLNs examined by standard pathological methods. It is unknown whether ultrastaging of initially -ve non-SLNs would lead to higher incidence of +ve nodes. Hence, we retrospectively analyzed all initially -ve non-SLNs by microsections and IHC similar to SLNs in CRca patients (pts) undergoing SLNM to determine its impact on final nodal staging. Methods: All CRca pts underwent SLNM by circumferential subserosal injection of 1% lymphazurin. First 1–4 blue nodes were marked as SLNs and ultrastaged by 4 section with H&E and 1 with IHC. All non-SLNs were initially examined by single H& E section and initial staging was made as per AJCC criteria. We re-examined all initially -ve non-SLNs similar to the SLNs by a senior pathologist blinded to prior results. Results: There were 156 pts with Cca and 44 pts with Rca. SLNM was successful in 100% pts with 94% accuracy rate. A total of 2,755 nodes (13.78/pt) were identified, of which 494 were SLNs and 2,261 were non-SLNs. Nodal positivity was 46% and 16% for Cca and Rca pts respectively. Mets were detected in 20.9% of SLNs vs. 8.6% of non-SLNs (p< 0.0001). The exclusive site of nodal mets was detected in 6.5% of SLNs vs.0.8% of non-SLNs (p< 0.0001). Skip mets were found in 6% of pts. After ultrastaging all initially -ve non-SLNs (n=2,065), only 0.58% (12/2065) nonSLNs became +ve in 12 pts. Of these, 10 pts already had +ve SLNs, hence no change of staging occurred. Only 2/200 pts (1%) with initially -ve non-SLNs were found to have a cluster of tumor cells. Thus, ultrastaging of 2065 initially -ve non-SLNs in 200 pts changed the staging from II to III only in 1% of pts. Conclusions: SLNM is highly accurate in staging CRca. The chance of finding additional mets by ultrastaging of all non-SLNs is extremely low (< 1%), hence of little benefit. Therefore, ultrastaging restricted to SLNs alone will assure accurate staging of CRca. [Table: see text] No significant financial relationships to disclose.