A New Concept for the Role of Ex vivo Sentinel Lymph Nodes in Node-Negative Colorectal Cancer

Background. We recently introduced ex vivo, intra-arterial methylene blue injection as a simple method to improve the lymph node (LN) harvest in gastrointestinal cancer. We now combined it with a novel ex vivo sentinel lymph node (evSLN) mapping technique. Methods. evSLN mapping was performed by subserosal (n = 20) or submucosal (n = 30) India ink injection. Subsequently, methylene blue was injected intra-arterially to enhance visibility of all LNs to improve the overall LN harvest. Manual LN dissection was carried out after fixing overnight. evSLNs nodes were identified by detecting carbon particles during histological examination. In primary node-negative cases, all detected LNs were step sectioned and immunohistochemically stained for pan-cytokeratin. Results. India ink injection was easy to perform. Methylene blue injection failed in I case. The mean lymph node harvest was 42 +/- 18 LNs, and the SLN detection rate was 78%. The sensitivity for detecting metastases was 75%. The mean SLN number was 3 +/- 1. LN metastases were found in 20 of 47 malignant cases (43%). Skip metastases occurred in 4 cases. Of these cases, 3 showed involvement of at least 1 entire LN. True upstaging (NO -> N1mi) was found in 1 of 23 cases (4%) within a SLN after advanced evaluation. Conclusions. Combination of methylene blue technique and ex vivo sentinel mapping is feasible, easy to perform, and cost effective. It guarantees an optimal LN harvest and has the potential to heighten the sensitivity of metastasis detection.