This study aimed to determine the optimal categorization of extramural tumor deposits lacking residual lymph node (LN) structure (EX) in colorectal cancer staging.
The TNM classification system categorizes EX on the basis of their contour characteristics (the contour rule).
We conducted a multicenter, retrospective, pathological review of 1716 patients with stage I to III curatively resected colorectal cancer who were treated at 11 institutions (1994–1998). In addition, 2242 patients from 9 institutions (1999–2003) were enrolled as a second cohort for validating results. EX were classified as isolated foci confined to vascular or perineural spaces (ie, lymphatic, venous, or perineural invasion) or as tumor nodules (ND). N- and T-staging systems employing different categories for staging were compared in terms of their prognostic power. In addition, the diagnoses of extramural, discontinuously spreading lesions made by 11 observers from different institutions were assessed for interobserver agreement.
EX were observed in 18.2% of patients in the first cohort. The method of categorization of EX in tumor staging has a stronger impact on N than T staging. The N-staging system in which all ND types were classified as N factor (the ND rule) could more effectively stratify the survival outcome than the contour rule (Akaike information criterion, 3040.8 vs 3059.5; the Harrell C-index, 0.7255 vs 0.7103). EX were observed in 16.9% of patients in the second cohort. Statistically, the ND rule was more informative than the contour rule for N staging. The Fleiss kappa coefficient for distinguishing LN metastases from EX (0.74) was lower than expected for complete agreement, and it decreased further to 0.51 when calculated for the judgment of ND with smooth contours.
Classifying all ND types as N factors irrespective of contours can simplify the tumor staging system by enhancing diagnostic objectivity, resulting in improved prognostic accuracy.