Prognostic significance of pathological sub-classification of pT3 rectal cancer

Purpose: Patients with pT3 rectal cancer represent a heterogeneous prognostic group. A more accurate histological sub-classification of pT status has been suggested as an improvement of the TNM staging system. The aim of the study was to evaluate the prognostic implication of a histopathologic sub-classification of pT3 rectal cancer. Methods: In this retrospective single-center study, pT3 rectal cancer patients who underwent surgery from January 2000 to December 2018 were evaluated. The maximum depth of tumor invasion beyond the muscularis propria was recorded. A ROC curve identified the best prognostic cutoff value to classify patients in two prognostic groups. Survival curves were estimated by the Kaplan-Meier method, and univariate and multivariate analyses with the Cox regression model were used to find independent factors influencing survival. Results: Overall, 203 patients were included. Four millimeters was identified as the best cutoff value: 82 patients showed a depth of invasion < 4 mm (group A) and 121 ≥ 4 mm (group B). Both the estimated 5-year OS and DFS were statistically better in group A than in group B (OS: 83.9% vs 62.2%, p < 0.01; DFS: 78.3% vs 40.6%, p < 0.01). The depth of tumor invasion was an independent risk factor for OS (HR 2.25, 95% CI 1.26-3.99, p = 0.006) and DFS (HR 2.30, 95% CI 1.40-3.78, p = 0.001). Conclusion: Our findings suggest that a sub-classification of pT3 rectal cancer, based on the depth of tumor invasion, should be considered to be introduced in the TNM staging system.