Prognostic Value of Extramural Depth of Tumor Invasion to Thickness of Mesorectum Ratio at Pretreatment Endorectal Ultrasound in T3 Rectal Cancer Patients

Abstract

Objective: To evaluate the prognostic value of the extramural depth (EMD)/thickness of mesorectum (TM) ratio in patients with cT3 rectal cancer (RC).

Methods: This study enrolled 382 treatment-naive cT3 RC patients undergoing endorectal ultrasound from January 2018 to January 2020. The last follow-up date was January 31, 2025. The EMD/TM ratio was calculated. T3 substages (T3a-c) were classified using 5-mm/10-mm EMD cutoffs. Survival analysis employed Kaplan-Meier and Cox regression. Predictive performance was evaluated via the Akaike Information Criterion (AIC) and time-dependent area under the curve (AUC).

Results: The optimal cutoff value for the EMD/TM ratio was 0.3 based on ROC analysis. Patients with an EMD/TM ratio >0.3 had significantly lower 5-y PFS and OS rates (PFS: 60.45% vs. 77.63%, OS: 74.14% vs. 92.92%, both p < 0.001) compared to those with a ratio ≤0.3. A significant difference in 5-y OS was observed among the T3 subgroups (p = 0.004), but there was no significant difference in 5-y PFS (p = 0.180). Multivariable analysis indicated that the EMD/TM ratio was an independent predictor of PFS (HR: 2.91, 95% CI: 1.41-6.00, p = 0.018) and OS (HR: 2.91, 95% CI: 1.41-6.00; p = 0.004). Furthermore, high- and low-risk subpopulations of T3a RC were identified using the EMD/TM ratio and demonstrated distinctive 5-y PFS and OS (PFS: 62.43% vs. 77.20%, p = 0.006; OS: 80.95% vs. 92.80%, p = 0.008). The EMD/TM ratio showed superior predictive performance, with a lower AIC and better time-independent AUC than the T3 substage.

Conclusion: The EMD/TM ratio outperforms T3 substage in prognostic stratification for cT3 RC patients, offering a practical tool for clinical risk assessment.

Keywords: Endorectal ultrasound; Extramural depth; Rectal cancer; T3; Thickness of mesorectum.