Clinical Impact of Mesorectal Extranodal Cancer Tissue in Rectal Cancer: Detailed Pathological Assessment Using Whole-Mount Sections

PURPOSE: Mesorectal cancer deposits showing no histological evidence of lymph node structure (extranodal cancer tissue) are a common feature in rectal cancer. However, optimal categorization of extranodal cancer tissue using TNM grading is not yet established. We reviewed extranodal cancer tissue in detail using whole-mount sections to clarify its clinical impact. METHODS: This retrospective study involved 214 consecutive patients with stage I-III rectal cancer. After fixation, the whole tumor mass including the mesorectum was sliced into longitudinal sections and stained. Mesorectal involvement was classified as direct tumor infiltration, lymph node involvement, or extranodal cancer tissue. Extranodal cancer tissue was classified morphologically, and its maximum size and distance from the primary tumor were measured. The clinical impact of extranodal cancer tissue was evaluated by univariate and multivariate analyses. RESULTS: A total of 498 extranodal cancer deposits were detected in 88 patients (41.1%). Multivariate Cox proportional hazards model analysis indicated that the presence of extranodal cancer tissue was an independent prognostic factor for relapse-free (P < .001) and overall survival (P < .003). The hazard ratio for extranodal cancer tissue was higher than for nodal involvement, irrespective of morphological classification. The clinical impact differed significantly with the number of histological types of extranodal cancer tissue, the number of deposits, their maximum size, and their distance from the primary tumor. CONCLUSIONS: In the present study, we have shown that extranodal cancer tissue detected by whole-mount sections has a clinical impact that is more severe than nodal involvement.