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.