Microsatellite instability of colorectal cancer and adenoma in synchronous multiple colorectal cancer patients with associated extracolonic malignancies.

We examined microsatellite instability (MSI) in nonfamilial multiple synchronous colorectal cancer (multiple CC) patients. We divided the patients into two groups, those with and without extracolonic primary malignancies, and compared the frequency of MSI between the two groups. A colectomy was performed in 52 multiple CC patients between 1985 and 1998. Of them, 10 patients had extracolonic malignancies, while the other 42 patients did not. The MSI frequency was higher in the patients with extracolonic malignancies than in those without extracolonic malignancies, although it was not statistically significant (40% vs 19%, P = 0.21). Regarding the lesions, MSI frequency of cancers was higher in the multiple CC with extracolonic malignancies than in those without extracolonic malignancies (33% vs 13%, P = 0.033). From our results, there was statistically no difference in the existence of extracolonic malignancies between the patients with at least one MSI-positive cancer and those patients without any MSI-positive cancers. On the other hand, there was a significant correlation between MSI-positivity and the existence of extracolonic malignancies.