T1a Versus T1b Differentiated Thyroid Cancers: Do We Need to Make the Distinction?

The 7th edition of the American Joint Committee on Cancer (AJCC) staging system trialed a subdivision of T1 tumors into T1a (<1 cm) and T1b (1.0-2 cm). The 2009 American Thyroid Association (ATA) guidelines recommended total thyroidectomy for tumors >1 cm, and lobectomy for those ≤1 cm. These AJCC staging parameters remain a focus of debate, and ATA guidelines are in transition. The aim of this study was to determine if the T1 staging subdivision is associated with different treatment strategies and patterns of patient survival.

All adult patients with AJCC pT1 differentiated thyroid cancer (DTC) from the National Cancer Data Base (NCDB; 1998-2012) and Surveillance, Epidemiology, and End Results (SEER) program (2004-2012) were divided into two groups based on tumor size: T1a versus T1b. Demographic, clinical, and pathologic features were evaluated. Multivariate regression analysis was used to determine factors associated with undergoing total thyroidectomy and radioactive iodine. Cox proportional hazards models were performed to determine factors associated with overall and disease-specific survival.

Among 149,912 DTC patients, 98,111 (65.4%) were T1a and 51,801 (34.6%) T1b in the NCDB; in SEER, among 18,381 patients, 11,208 (61.0%) had T1a and 7173 (39.0%) T1b tumors. Patients with T1b cancers were younger (48 vs. 51 years T1a) and more likely to have private insurance (76.2% vs. 74.1%), no comorbidities (86.0% vs. 83.8%), and undergo treatment at academic medical centers (41.4% vs. 40.3%; all p < 0.01). They also were more likely to undergo total thyroidectomy (87.7% vs. 74.3%), and had more lymphovascular invasion (10.2% vs. 3.3%), positive surgical margins (7.9% vs. 3.8%), metastatic lymph nodes (35.8% vs. 23.8%), and distant metastases (0.4% vs. 0.3%; all p < 0.01). Factors associated with radioactive-iodine use included younger patient age, lower income, having insurance, positive surgical margins, and T1b stage (p < 0.01). After adjustment, overall (p = 0.23) and disease-specific survival (p = 0.93) were similar among patients with T1a versus T1b tumors.

These results illustrate that patients with pT1a versus pT1b tumors undergo different treatment strategies. Based on the newly published 2015 ATA guidelines, whereby either lobectomy or total thyroidectomy can be performed for low-risk tumors, it might be anticipated that treatment differences will diminish over time. Therefore, division of AJCC T1 staging into T1a versus T1b subgroups might become obsolete over time.