Background: According to the 8th edition American Joint Committee on Cancer staging system, extrathyroidal extension (ETE) and primary tumor size remain the principle determinants of T stage. However, impact of gross ETE into strap muscles on survival remains controversial. Patients and Methods: A retrospective review of 2084 patients with ≤ 4 cm nonmetastatic differentiated thyroid cancer who underwent surgery between 2000 and 2015 was conducted. Patients were divided into three groups according to degree of ETE: no ETE (group 1), ETE into perithyroidal soft tissue (group 2), and gross ETE into strap muscle (group 3). Survivals were analyzed using Kaplan–Meier method and compared using log-rank test. Factors predictive of survival were analyzed using Cox proportional hazard model. Results: Ten-year disease-free survival (DFS) of patients in groups 1–3 was 90, 82, and 83%, respectively (p = 0.003). On multivariate analysis, age ≥ 55 years, male sex, and pathologic N1b category predicted significantly worse DFS, while ETE into perithyroidal soft tissue or gross strap muscle invasion did not predict worse DFS. Overall survival (p = 0.957) and disease-specific survival (p =0.910) were not significantly different between the three groups. There was a statistically significant difference in locoregional recurrence-free survival between groups 1 and 2 [HR 2.02, 95% CI 1.06–3.94]. Conclusion: Gross strap muscle invasion may not be an important survival prognostic factor for staging purposes. Although both gross strap muscle invasion and perithyroidal soft tissue extension may be predictive for locoregional recurrence, the distinction between them may not be as important for postoperative risk stratification.
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