Extent of mesorectal tumor invasion as a prognostic factor after curative surgery for T3 rectal cancer patients

Objective: To determine the significance of the extent of mesorectal tumor invasion as a prognostic factor for T3 rectal cancer patients. Summary Background Data: There is controversy as to which primary lesion characteristics, other than regional lymph node involvement, in T3 rectal cancer are reliable prognostic factors. Patients and Methods: The extent of mesorectal tumor invasion was evaluated using 2 data sets comprising 196 and 247 patients undergoing curative surgery at separate institutes. When the outer aspect of the muscular layer was not identifiable, an estimate was obtained by drawing a straight line between the 2 break points of the muscular layer. Results: We selected 6 mm as the optimal value for subclassification of T3 rectal patients into 2 groups, based on the extent of mesorectal invasion, using the first data set. The overall 5-year survival rate was significantly higher in patients with < 6 mm than in those with >= 6 mm of mesorectal invasion (72% versus 50%; P < 0.01). Similarly, in the second data set, the overall 5-year survival rates of patients with mesorectal invasion < 6 min and >= 6 mm were 59% and 37%, respectively (P < 0.01). In both data sets, multivariate analyses verified the extent of mesorectal invasion to be an independent prognostic factor, together with nodal involvement. Regarding positive nodal involvement and mesorectal invasion >= 6 mm as risk factors, the overall 5-year survival rates with none, one, and both of these factors were 84%, 61%, and 38%, respectively, in the first data set (P < 0.01). Prognostic results were similar for the second data set. Conclusion: Extent of mesorectal invasion, based on a 6-mm cutoff value, is useful for subclassification of T3 rectal cancer patients.