The Role of Adjuvant Chemotherapy in Microsatellite Instability–High Colorectal Cancer

Background The optimal adjuvant therapy for colorectal cancer (CRC) with microsatellite instability–high (MSI-H), and deficient mismatch repair (dMMR) continues to be evolved, particularly with the integration of immunotherapy. Objectives In this study, we evaluated oxaliplatin-based adjuvant chemotherapy in stage II and III MSI-H colorectal cancer. Design We analyzed MSI-H CRC patients who received adjuvant chemotherapy at Samsung Medical Center between 2017 and 2021. Methods Clinicopathologic characteristics and survival outcomes were assessed. The survival outcomes were estimated using the Kaplan–Meier method, with subgroup analyses by tumor features. Results Among 1,842 patients who received adjuvant chemotherapy for CRC, 116 (6.3%) had MSI-H tumors. The location of MSI-H tumors was predominantly in the proximal colon, with the ascending colon (39.7%) and transverse colon (19.8%) most frequently involved, followed by the hepatic flexure (12.9%) and cecum (8.6%). For pathologic stage, the majority had stage III disease (62.9%), including IIIB (38.8%), IIIA (15.5%), and IIIC (8.6%). Stage II disease accounted for 37.1% of cases. The estimated 3- and 5-year disease-free survival (DFS) rates were 93.8% and 90.9%, respectively. The 3- and 5-year overall survival (OS) rates were 93.8% and 91.8%. In stage III disease, 3- and 5-year DFS rates were 90.3% and 87.1%. In high-risk stage II disease, 3- and 5-year DFS rates were 100.0% and 92.3%. There was no significant difference for DFS according to tumor sidedness, pathologic stage (stage II vs III), pathological high-risk features, or regimen (FOLFOX vs XELOX). Conclusions Oxaliplatin-based adjuvant chemotherapy showed the useful outcomes in CRC with MSI-H. The survival outcome was consistent irrespective of tumor sidedness, pathologic stage (stage II vs III), pathological high-risk features, or regimen (FOLFOX vs XELOX).