Association Between a High Number of Isolated Lymph Nodes in T1 to T4 N0M0 Colorectal Cancer and the Microsatellite Instability Phenotype

HYPOTHESIS: Stage I or II colorectal carcinomas with microsatellite instability (MSI) are characterized by more isolated lymph nodes in the resected specimen than their counterparts with microsatellite stability (MSS). DESIGN: Prospective study. SETTING: Academic research. PATIENTS: Using a pentaplex polymerase chain reaction assay, MSI status was determined prospectively for 135 operative patients. MAIN OUTCOME MEASURES: Mismatch repair defects were investigated by immunohistochemistry on tumors demonstrating MSI. RESULTS: Among 82 stage I or II colorectal carcinomas, 11 had MSI, and 71 had MSS, with a mean (SD) number of 23.6 (3.1) and 13.7 (1.0) negative lymph nodes, respectively (P = .001). The mean number of lymph nodes for all resected stage I or II colorectal carcinomas analyzed at our hospital was 15. The prevalence of MSI among tumors with more than 15 lymph nodes in the specimen was 25% (9 of 36), and 82% (9 of 11) of MSI tumors belonged to this group. CONCLUSIONS: A high number of isolated lymph nodes in stage I or II colorectal carcinomas was associated with the MSI phenotype. Good prognosis that is usually associated with tumors having a high number of uninvolved lymph nodes might reflect the high prevalence of MSI among these tumors. The number of examined lymph nodes as a quality criterion should be used with caution. For stage I or stage II colorectal carcinomas, restricting MSI phenotyping to tumors with more than the mean number of lymph nodes identifies almost all MSI tumors.