The Role of Adjuvant Chemotherapy in High-Risk Stage II Colon Cancer with Microsatellite Instability-High or DNA Mismatch Repair Deficiencies: A Multicenter Pooled Analysis (KCSG-CO24-03)

PurposeThe benefit of adjuvant chemotherapy (CTx) for high-risk stage II microsatellite instabilityhigh (MSI-H)/mismatch repair deficient (dMMR) colon cancer remains controversial with discordant international guidelines.This study evaluated outcomes comparing adjuvant chemotherapy group versus surgery only group. Materials and MethodsThis multicenter retrospective analysis included 192 patients with high-risk stage II MSI-H/dMMR colon cancer who underwent curative resection between 2010 and 2020.High-risk features included pT4, grade 3-4, lymphovascular invasion, bowel obstruction/perforation, perineural invasion (PNI), inadequate lymph node sampling (<12), or close margins.The primary endpoint was 5-year relapse-free survival (RFS), excluding second primary malignancies to isolate antitumor efficacy. ResultsOf 192 patients, 128 (66.7%) received adjuvant CTx and 64 (33.3%) underwent observation (median follow-up 60.5 months).Five-year RFS (94.0% vs. 83.7%;p=0.001) and OS (98.3% vs. 89.0%;p=0.001) were significantly higher with adjuvant CTx.Multivariate analysis confirmed adjuvant CTx as an independent predictor for RFS (HR 0.249, 95% CI 0.109-0.571;p=0.001) and OS (HR 0.155, 95% CI 0.051-0.470;p=0.001).PNI was also identified as a significant independent predictor of RFS (HR, 3.581; 95% CI, 1.529 to 8.387; p=0.003) and OS (HR, 4.968; 95% CI, 1.654 to 14.930; p=0.004).CTx-related toxicities were clinically manageable, and no treatment-related deaths were reported. ConclusionAdjuvant CTx significantly improves RFS and OS in high-risk stage II MSI-H/dMMR colon