Survival in Differentiated Thyroid Cancer: Comparing the AJCC Cancer Staging Seventh and Eighth Editions

The American Joint Committee on Cancer Cancer Staging System for differentiated thyroid cancer has been recently revised. The purpose of this study was to compare survival prognostication between the seventh and eighth editions.We retrospectively reviewed 2579 differentiated thyroid cancer patients who underwent surgery at MD Anderson Cancer Center between 2000 and 2015. Disease-specific survival (DSS) and overall survival were estimated using the Kaplan-Meier method and compared using log rank test. The effect of potential predictor was estimated using Cox proportional hazards model. Power of survival prediction was estimated using Harrell's C concordance index (C-index), and predictive capacities for DSS were estimated using proportion of variance explained (PVE).Revision of tumor-node-metastasis (TNM) categories, age cutoff, and stage reassignment in the 8th edition caused reclassification of overall stage compared with the seventh edition. The proportion of patients in stage I and II increased from 62% to 83% and 5% to 12%, respectively, while the proportion of patients in stage III and IV decreased from 20% to 2% and 14% to 3%, respectively. Ten-year DSS for stages I-IV based on the seventh edition were 100%, 97.5%, 98.3%, and 82.6%, respectively, while 10-year DSS for the corresponding stage in the eighth edition were 99.8%, 88.3%, 72.4%, and 71.9%, respectively. In multivariate analysis for both seventh edition (C-index 0.94, PVE 4.6%) and eighth edition (C-index 0.94, PVE 4.8%), the factors predictive of worse outcome for DSS were older age, advanced tumor size category, and distant metastasis while cervical lymph node metastases did not predict worse survival. For the eighth edition, patients <55 years of age with stage II disease had significantly worse DSS (p < 0.001) than patients ≥55 years with stage II disease but appeared to be similar to patients ≥55 years with stage III (p = 0.742) and IV disease (p = 0.566). Patients ≥55 years old with T3a and T3b disease had 10-year DSS of 67% and 92%, respectively (p = 0.390).The AJCC eighth edition is similar to the seventh edition in disease-specific survival prediction. Potential modifications that may improve disease-specific survival prediction in future renditions include reconsideration of T3b tumor category and upstaging classification of patients <55 years of age with distant metastases.