Microsatellite instability screening should be done for right-sided colon cancer patients less than 60 years of age

Background and aims: Microsatellite analysis is a screening tool used for the identification of Lynch syndrome. We evaluated the occurrence of high-frequency microsatellite instability (MSI-H) in 160 patients with colorectal cancer ≤60 years old to determine if these individuals should be routinely tested for microsatellite instability. Materials and methods: From January 2004 and December 2006, we tested specimens of colorectal cancer from 160 patients under 60 years of age for microsatellite instability. The relationships between clinicopathological parameters and MSI-H status were analyzed. Results: MSI-H occurred in 11.3% (18/160) of the tumors assayed, and colorectal tumors with MSI-H status were located predominantly to the right side (56%, P < 0.001) and had a lower pathological stage (72%, P = 0.011). Of the 18 MSI-H tumors, six displayed characteristic MSI histology. Furthermore, of the 18 MSI-H tumors, instability in BAT-26 was 100%, BAT-25 was 94%, D17S250 was 72%, D2S123 was 68%, and D5S346 was 68%. Of the patients with MSI-H tumors, 55.6% were more than 50 years of age, and about 70% of MSI-H tumors did not display characteristic MSI histology. Importantly, up to 40% of the MSI-H patients in this study would have been overlooked using the revised Bethesda guidelines. The revised Bethesda guidelines, broadened to include patients with right-sided colon cancer, could have identified 94% of the MSI-H tumors in this study. Conclusions: Colorectal cancers with MSI-H were predominantly located on the right side and had an early pathological stage. The results of this study suggest that microsatellite instability test should be used for all patients under 60 years of age with right-sided colon cancer.