Comparing the Prognostic Value of the Eighth Edition of the American Joint Committee on Cancer/Tumor Node Metastasis Staging System Between Papillary and Follicular Thyroid Cancer

Background: Recently, the eighth edition of the American Joint Committee on Cancer (AJCC)/tumor node metastasis (TNM) staging system for differentiated thyroid cancer (DTC) was published. Studies evaluating this new edition have so far only comprised patients with papillary thyroid cancer (PTC) or made no distinction between PTC and follicular thyroid cancer (FTC). Therefore, this study evaluated the prognostic value of the eighth edition of the AJCC/TNM staging system in a European population with DTC, focusing on potential differences between PTC and FTC. Methods: Adult patients with DTC who were diagnosed and/or treated at a Dutch university hospital between January 2002 and April 2016 were retrospectively studied. Overall survival (OS) and disease-specific survival (DSS) were analyzed for DTC and for PTC and FTC separately according to the seventh and eighth editions using the Kaplan-Meier method. Cox's proportional hazards model was used to compare the effect of PTC and FTC on survival. The statistical model performance was assessed using the C-index, Akaike information criterion (AIC), and the Bayesian information criterion. Results: The study included 792 patients with DTC (79% PTC, 21% FTC) with mean age of 49 years. Median follow-up was 7.2 years. Reclassification using the eighth edition resulted in the downstaging of 282 (36%) patients, an increased number of patients in stages I and II, and an equivalent decrease in patients with stages III and IV. For DTC, as well as for PTC and FTC separately, stage at diagnosis was significantly related to both OS and DSS (p<0.001). When using the seventh edition, FTC patients had a significantly lower survival rate than PTC patients in stage I and stage IV for OS, and in stage IV for DSS. This difference in survival rates disappeared using the eighth edition. In general, the statistical model performance was better for the eighth than for the seventh edition. Conclusions: In a European population of patients with DTC, the eighth edition of the AJCC/TNM staging system is a better predictor for both OS and DSS than the previous seventh edition for both PTC and FTC. Furthermore, differences in survival rates between PTC and FTC that were present using the seventh edition disappeared using the eighth edition, implying that this new edition is predicting well, regardless of DTC subtype.