To investigate the correlation between microsatellite status (MS) and poorly differentiated clusters (PDC) in stage Ⅱ colorectal cancer and their influences on prognosis, and to provide reference for postoperative adjuvant therapy.

The clinical, pathological and survival data of 311 patients with stage colorectal cancer who underwent radical surgery in the Second Affiliated Hospital of Jiaxing College from December 2014 to December 2017 were collected retrospectively. The correlation among MS, PDC and other clinicopathologic parameters was analyzed. After follow-up, the survival curves were plotted with Kaplan-Meier method to compare disease free survival (DFS) between different MS and PDC status. The prognostic factors were analyzed by multivariate Cox regression.

Among the 311 patients, there were 252 microsatellite stability (MSS) and 59 microsatellite instability (MSI) cases. According to two classification method, the number of PDC positive (PDC+) and PDC negative (PDC-)were 171 and 140 cases, respectively. According to three grade method, the number of PDC G1, G2 and G3 were 210, 57 and 44 cases, respectively. There was no clear correlation between MS and PDC. There was no significant difference in median DFS between MSS and MSI patients in overall comparison or PDC-subgroup (P=0.232, 0.589), but with a statistically significant difference in PDC+ subgroup (P=0.027). There were significant differences in median DFS between PDC+ and PDC-patients in overall comparison and MSS subgroup (P<0.01), but without significant difference in MSI subgroup (P=0.935). There was no significant difference in the median DFS of G1, G2 and G3 patients, either in overall comparison or in MSS subgroup and MSI subgroup (P=0.246, 0.371, 0.498). Joint grouping based on PDC and MS, there was significant difference in median DFS of PDC+MSI, PDC-MSI, PDC+MSS and PDC-MSS groups (P=0.001), and PDC+MSS group had a worse survival than the other groups. Multivariate Cox regression analysis showed that PDC+MSS was an independent prognostic factor for DFS (HR=2.670, 95% CI: 1.537-4.637, P<0.001).

There is no definite correlation between MS and PDC in stage Ⅱ CRC, but both of them affect the prognosis of patients. Survival difference between MSS and MSI is mainly found in PDC+ patients, but not in PDC-patients. Survival difference between PDC+ and PDC-patients is mainly found in MSS patients, but not in MSI patients. For patients with MSS and PDC+, active adjuvant therapy should be given. For stage Ⅱ CRC, two classification method of PDC is more clinically valuable than three grade method, and PDC+ may be considered as a risk factor for recurrence.