Implications of Extrathyroidal Extension Invading Only the Strap Muscles in Papillary Thyroid Carcinomas

Based on the new TNM classification, differentiated thyroid carcinomas with extrathyroidal extension (ETE) into the strap muscles are designated as T3b. The current study analyzed the clinical significance of this new stage in patients with papillary thyroid cancer (PTC). This retrospective cohort study included 4045 eligible PTC patients. The patients were divided into four groups according to the extent of ETE: no ETE, ETE into perithyroidal tissue, T3b (ETE into the strap muscles), and ETE beyond the strap muscles. Differences in the clinical characteristics and prognosis of the four groups were assessed. Of these 4045 patients, 2300 (56.9%) had no ETE, 1004 (24.8%) had ETE into perithyroidal tissue, 371 (9.2%) had ETE into the strap muscles, and 370 (9.1%) had ETE beyond the strap muscles. Tumor size, multifocality, lymph node metastasis (LNM), distant metastasis (DM), recurrence, and recurrence-free survival (RFS) between patients with ETE into perithyroidal tissue and patients without ETE did not show differences, but significant differences were observed between patients with T3b and patients without ETE, and only patients with ETE beyond the strap muscles had significantly worse overall survival (OS) than the other three groups. However, T3b did not independently predict worse RFS in univariable and multivariable analyses. T3b, distinguished from minimal ETE, may be related to larger tumor size, higher prevalence of multifocality, LNM, DM, recurrence, and worse RFS than no ETE, but it may not affect OS and may not independently predict recurrence in PTC patients.