Effect of Tumor Size and Minimal Extrathyroidal Extension in Patients with Differentiated Thyroid Cancer

Introduction: Risk-stratified treatment strategies have become a focus in the treatment of differentiated thyroid cancer (DTC). In the 2015 American Thyroid Association treatment guidelines, adjuvant treatment with radioactive iodine (RAI) is considered in the presence of minimal extrathyroidal extension (mETE). This study aimed to investigate the prognostic significance of mETE and tumor size in patients with DTC. Methods: A retrospective review was undertaken of 2323 consecutive patients treated surgically for T1-T3 (defined per seventh edition of the American Joint Committee on Cancer staging criteria) and M0 DTC from 2000 to 2015 at The University of Texas MD Anderson Cancer Center. Patients were divided into four groups according to the size of the tumor (4cm vs. >4cm) and the presence of mETE. Predictors of disease-free survival (DFS), disease-specific survival, locoregional failure (LRF), and distant metastatic failure (DMF) were compared using the log-rank test and Cox's proportional hazards models. Results: There were only seven DTC-related deaths, limiting the clinical significance of the analysis, especially of overall and disease-specific survival. Following multivariate analysis, patients with tumors >4cm did worse than patients with tumors 4cm with respect to DFS (group 3 [>4cm without mETE] adjusted hazard ratio (HRadj)=2.1 [confidence interval (CI) 1.1-3.8]; group 4 [>4cm with mETE] HRadj=2.9 [CI 1.6-5.1]). However, patients did not differ according to DFS, regardless of the presence of mETE within each size category (group 2 [4cm with mETE] vs. group 1 [4cm without mETE] HRadj=1.3 [CI 0.9-1.8]; group 4 [>4cm without mETE] vs. group 3 [>4cm with mETE] HRadj=1.0 [CI 0.5-2.3]). For LRF and DMF, size but not mETE was also an independent risk factor. Conclusion: Tumor size, but not the presence of mETE, was an independent predictor of DFS, LRF, and DMF in DTC.