Abstract: Objective To analyze the clinical characteristics and microsatellite instability（MSI）of multiple primary cancer（MPC）with colorectal cancer（CRC）. Methods Clinical data of 224 CRC patients who had been diagnosed with pathology in the Fourth Hospital of Wuhan on the Gutian Road were retrospectively analyzed from January 1，2015 to December 31，2018，and the MSI of each CRC patient was detected by immunohistochemical method. Results Of all patients with CRC，18. 3% （41/224） were MPC. The difference was not statistically significant in the sex，age，colorectal cancer site，and TNM stage of MPC patients with CRC and single CRC patients. Most patients were more than 50 years old in both MPC patients with CRC and single CRC patients，each took 87. 8%（36/41）and 87. 9% （161/183）. The most common location was the rectum in both of them，each took 46. 3%（19/41）and 42. 6%（78/183）. I ~ II stage was more common in MPC patients with CRC，which took 65. 9%（27/41）. The MSI-H took more percentage in MPC patients with CRC than patients with single CRC，the difference was statistically significant （P < 0. 05）. Among 41 MPC patients with CRC，the ratio of men to women is approximately 1∶1，and they were 28 to 86 years old，with an average age of 61. In extracolonic organ，digestive system［29. 3%（12/41）］was the most commonly occurring system and stomach［22. 0% （9/41）］ was the most common lesion. There were 8 cases with synchronous carcinoma（SC）and 33 cases with metachronous carcinoma（MC）. The majority of SC was the right colon，and the rectum was the majority of MC. Conclusion The rectum is the most common lesion site in single CRC patients. The stomach is the most common extracolonic organ of MPC patients with CRC. MSI-H colorectal cancer patients have a higher risk of developing into MPC patients.

Key words: colorectal cancer, multiple primary cancer, microsatellite instability, synchronous carcinoma, metachronous carcinoma