SIGNIFICANCE OF MICROSATELLITE INSTABILITY IN DIFFERENT TYPES OF EARLY-STAGE NONFAMILIAL COLORECTAL CARCINOMAS

PURPOSE: The aim of this study was to investigate the genetic alterati ons of early-stage nonfamilial colorectal carcinomas regarding microsa tellite instability, with special reference to the shape of the tumors and the site of the lesions. METHODS: Formalin-fixed, paraffin-embedd ed specimens of 44 early-stage nonfamilial colorectal carcinomas were examined for microsatellite instability with use of polymerase chain r eaction. RESULTS: The 44 carcinomas consisted of 16 flat carcinomas an d 28 polypoid carcinomas. Nineteen carcinomas were located in the prox imal colon (9 flat type and 10 polypoid type), whereas 25 were in the distal colon and rectum (7 Bat type and 18 polypoid type). Ten (22.7 p ercent) of the 44 carcinomas had at least one positive locus, whereas five (11.4 percent) of them had two or more positive loci. In the prox imal colon the percentage of flat carcinomas with at least one positiv e locus was significantly greater than that of the polypoid carcinomas (4/9 (44 percent) vs. 0/10; P = 0.04). Six patients had synchronous o r metachronous colorectal carcinomas or bath. They harbored microsatel lite instability more frequently than patients with single colorectal carcinomas, and the differences were statistically significant (P < 0. 02). CONCLUSIONS: These data suggest that in nonfamilial carcinomas in the proximal colon, the genetic pathway in flat carcinomas may be dif ferent from that in polypoid carcinomas.