Staging of Differentiated Thyroid Cancer from a Single Institution : Comparison of the 7 th and 8 th Editions of AJCC / UICC Staging

Purpose: Traditional clinicopathological variables are used to predict prognosis in differentiated thyroid cancer, including age at diagnosis, tumor size, and extrathyroidal extension (ETE). The American Joint Committee on Cancer/Union for International Cancer Control (AJCC/UICC) 8th edition for thyroid cancer was released in 2016 and included major revisions regarding tumor, node, and metastasis (TNM) stages, resulting in significant stage migration. The aim of our study was to describe this stage migration of the 7th and 8th editions of the AJCC/UICC staging system using a cohort of differentiated thyroid cancer patients from a single institution. Methods: We reviewed 2,447 patients who underwent thyroidectomy between September 2008 and December 2011. Of these, the complete medical records of 2,234 differentiated thyroid cancer patients were retrospectively included in the study. TNM stage was analyzed using the 7th and 8th editions of the AJCC/UICC staging system. Results: TNM stage showed notable changes owing to changes in age cutoff point, and the definitions of ETE and N0. Upon reclassification, 835 patients of T3 were downgraded to T1 and T2, resulting in increased numbers of T1 and T2 patients (49.3% to 85.0% and 1.3% to 3.0%, respectively). Reclassification of nodal stage resulted in 191 cases (8.6%) of NX, according the 7th edition, to be re-categorized into N0; therefore, N0 cases increased from 49.5% to 58.1%. Additionally, stage I and II tumor cases increased from 60.6% to 89.3%, and 0.2% to 9.8%, respectively, and stage III and IV cases showed corresponding decreases from 28.2% to 0.9%, and 5.4% to 0.1%, respectively. Conclusion: Staging of differentiated thyroid cancer cases using the AJCC/UICC staging system 8th edition results in marked TNM staging migration in the direction of downstaging.