Concordance and survival implications of preoperative subclassification of T3 rectal cancers by depth of mesorectal invasion using a 5-mm cut-off point with endorectal ultrasound and magnetic resonance imaging

Background: Validated rectal cancer staging groups T3 tumours in a single stage and depth of mesorectal invasion subclassification is not standard practice. Our aim is to report concordance between magnetic resonance imaging (MRI) and endorectal ultrasound (ERUS) for preoperative staging of T3 rectal tumours using a 5-mm cut-off point and possible survival implications. Methods: Prospective cohort study including patients staged preoperatively as cT3 by ERUS or magnetic resonance imaging. The maximum depth of penetration beyond the outer longitudinal muscle layer was measured according to a 5-mm cut-off point. Concordance rate and Kappa coefficient were calculated for both techniques. Primary end-points were disease free survival and overall survival (OS) for both groups. Results: A total of 97 patients were included. Disease-free survival in depth of mesorectal invasion <= 5 and >5 mm measured by ERUS was 130.80 (119.20-142.30) vs. 88.38 (56.13-120.64) months (P=0.020), respectively, and 129.90 (117.90-141.90) vs. 93.60 (64.50-122.70) months (P=0.045) when measured by magnetic resonance imaging. Depth of mesorectal invasion mm measured by ERUS and MRI was a prognostic factor for both OS [ERUS P=0.009; MRI P=0.019] and DFS (ERUS P=0.026; MRI P=0.054) after Cox regression analysis. Conclusions: T3 subclassification above and below 5mm is feasible by ERUS, shows good concordance with validated magnetic resonance and can easily be incorporated into the diagnostic workup for these patients with possible survival implications.