Purpose: Data regarding risk factors for recurrence in stage I colorectal cancer patients are limited. The aim of this study was to clarify the existence of a high-recurrence-risk population among stage I colorectal cancer patients. Methods: This analysis included 7,539 stage I colorectal cancer patients treated between 1997 and 2012 at 24 leading hospitals in Japan. Risk factors for time to recurrence were evaluated using a Cox proportional hazards model, and a high-risk group for recurrence was identified. Prognostic outcomes of high-risk stage I colorectal cancer patients were compared with those of low-risk stage I and stage II patients. Results: Multivariable analyses identified left-sided location (hazard ratio [HR]: 1.65, 95% confidence interval [CI]: 1.09–2.58), T2 tumors (HR: 1.80, 95% CI: 1.21–2.66), and lymphatic invasion (HR: 1.55, 95% CI: 1.05–2.28) as risk factors for recurrence in stage I colon cancer, and patients with these three risk factors were classified as high risk. For stage I rectal cancer, patients with poor differentiation (HR: 2.86, 95% CI: 1.21–5.69), T2 tumors (HR: 1.53, 95% CI: 1.07–2.23), and venous invasion (HR: 1.51, 95% CI: 1.08–2.13) were identified as high risk. The Kaplan–Meier analysis of cumulative recurrence rate and recurrence-free survival revealed that the high-risk stage I colorectal cancer patients have poorer clinical outcomes than the low-risk patients. Conclusion: Although stage I colorectal cancer patients generally have a favorable prognosis after curative surgery, poorer prognosis was observed in high-risk stage I colorectal cancer patients than in low-risk patients.
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