The prognostic value of the proposed eighth edition of the American Joint Committee on Cancer (AJCC) tumor, node, and metastasis (TNM) classification is currently unclear. The aim of the study was to evaluate the prognostic value of the eighth edition of the AJCC TNM classification.

We retrospectively assessed 3176 patients with differentiated thyroid carcinoma (DTC) who underwent thyroidectomy at a tertiary Korean hospital from 1996 to 2005. Cancer-specific survival (CSS) was analyzed using the Kaplan–Meier method and compared using the log-rank test. Performance of the eighth edition TNM with respect to prediction of CSS was assessed against the current seventh edition.

Upon reclassification according to the eighth edition, 37.6% of patients were down-staged. The proportions of stage I and II tumors increased from 61.9% to 81.1% and from 1.7% to 16.0%, respectively, whereas those of stage III and IVB (formerly IVC in the seventh edition) decreased from 27.6% to 2.3% and 0.8% to 0.5%, respectively. The proportions of variance explained (PVEs) for the ability of the eighth and the seventh edition to predict CSS were 3.9% and 2.9%, respectively. The C-index values were 0.765 (95% confidence interval 0.764–0.766) for the eighth edition and 0.736 (0.735–0.737) for the seventh edition.

Our results demonstrate that the eighth edition TNM more accurately predicts CSS for patients with DTC than does the seventh edition.