Prognostic Implication of N1b Classification in the Eighth Edition of the Tumor-Node-Metastasis Staging System of Differentiated Thyroid Cancer

Background: Regional lymph node metastases (LNM) have prognostic significance in differentiated thyroid cancer (DTC). However, there was no distinction between N1a and N1b in the final staging classification in the eighth edition of the tumor-node-metastasis (TNM) staging system. This study aimed to evaluate the prognostic implication of N1b classification for predicting disease-specific survival (DSS) in DTC patients with stage I/II disease. Methods: A total of 3089 patients with stage I/II DTC who underwent thyroid surgery between 1996 and 2005 were included. DSS was evaluated according to N classification and number of LNM. A modification of the TNM was assessed that classified N1b cases in patients aged 55 years as stage IIB and the remaining cases as stage IIA. Results: The mean patient age was 45.6 years, and the median follow-up period was 10.0 years. In patients aged 55 years, patients with N1b had significantly poorer DSS compared to those with N0 (hazard ratio [HR]=11.0; p<0.001) and N1a (HR=4.2; p=0.013). The large-volume LNM group had significantly poorer DSS compared to the N0 (HR=10.1; p<0.001) and small-volume LNM (HR=3.9; p=0.019) groups. When patients were reclassified using the modified TNM staging system, DSS was significantly poorer in stage IIB patients than in stage IIA patients (HR=2.9; p=0.030). Conclusions: N1b classification has a significant prognostic implication in patients with stage I/II DTC, especially in older patients. Modified TNM staging employing N1b classification could be more useful for the prediction of DSS.