Optimizing nodal and staging classification in low rectal cancers with lateral node metastasis: multicentre retrospective cohort study

Background Patients with lateral node metastasis in low rectal cancers have a poor prognosis. However, variability in patient survival in terms of lateral metastatic status has not been thoroughly investigated. This study was conducted to assess the prognostic value of lateral node involvement and to review nodal classification. Methods Patients with stage III low rectal cancers who underwent lateral node dissection were retrospectively reviewed. Two cohorts were set: the first one (1995-2006) was selected using a Japanese multi-institutional database and was used for development of a new nodal system, and the second (2007-2013) was collected from referral institutions for validation of findings. Variables correlated with poor prognosis were investigated. Next, a modified classification of lateral-positive nodal cancers was created. Finally, this new classification was compared with TNM and Japanese classification-based systems according to the Akaike information criterion (AIC) and concordance index (c-index). Results Overall, 742 and 508 patients were selected for cohorts 1 and 2, respectively. Based on the analyses on cohort 1, patients with two or more lateral metastatic nodes partially spreading into regions outside of internal iliac area exhibited poor prognosis; accordingly, a modified N classification was created, where TNM-N1 and N2a cancers with this feature were upgraded, respectively, to N2a and N2b. The modified N classification yielded the most favourable indices (AIC = 2661.08; c-index = 0.6477) compared with the TNM (AIC = 2662.36; c-index = 0.6457) and Japanese classification-based systems (AIC = 2684.06; c-index = 0.6302). All findings were confirmed by analysing cohort 2. Conclusion A modified nodal system is proposed to account for the significance of lateral node metastasis. According to the literature, there is a gap in the appropriateness of existing grading systems for low rectal cancer with lateral node metastasis. Here, we demonstrated that a newly arranged N classification system, where the TNM-N status of cancers with two or more lateral involved nodes partially spreading outside the internal iliac area was upgraded, exhibited excellence in survival stratification. We believe that this new system could be helpful in deciding on patient-adjusted postoperative treatment strategies.