Prognostic Impact of Tumor Size in Patients with Stage T3N1 Colon Cancer.

: Tumor size is not included in the TNM staging system for colon cancer, and its prognostic significance remains controversial. We aimed to evaluate the impact of tumor size on recurrence-free survival (RFS) and overall survival (OS) in patients with stage T3N1 colon cancer. : We retrospectively analyzed 336 patients with pathologically confirmed pT3N1 colon cancer who underwent curative resection between January 2015 and January 2025 at our tertiary institution. Clinicopathological features, adjuvant chemotherapy details, and survival outcomes were collected. Tumor size was measured pathologically, and a cutoff was determined by receiver operating characteristic (ROC) analysis. Kaplan-Meier and Cox regression analyses were performed to identify prognostic factors. : The optimal cutoff for tumor size predicting recurrence was 4 cm. Patients with tumors ≥ 4 cm had significantly lower 5-year RFS compared to those with smaller tumors (65.1% vs. 80.3%, = 0.007). In multivariate analysis, tumor size ≥ 4 cm (HR: 2.014, 95% CI: 1.093-3.714, = 0.025), ECOG performance status ≥ 2 ( = 0.005), positive resection margin ( = 0.011), and failure to complete adjuvant chemotherapy ( = 0.007) were identified as independent adverse prognostic factors for RFS. Tumor size was not independently associated with OS ( = 0.46). Adjuvant chemotherapy significantly improved both RFS ( < 0.001) and OS ( < 0.001). : In patients with stage T3N1 colon cancer, tumor size ≥ 4 cm is an independent adverse prognostic factor for RFS. Incorporating tumor size into risk stratification, alongside TNM staging and treatment completion status, may improve prognostic assessment and guide clinical decision-making.