Association of Preoperative Ultrasound-Measured Tumor Proximity to Trachea and Posterior Capsule with Tracheal and RLN Invasion in Papillary Thyroid Microcarcinoma.

Abstract

Active surveillance is increasingly adopted for low-risk papillary thyroid microcarcinoma (PTMC ≤1 cm). However, a subset of tumors exhibit tracheal or recurrent laryngeal nerve (RLN) invasion, resulting in upstaging and functional morbidity. Reliable preoperative predictors are needed to optimize patient selection. This retrospective study analyzed 10,112 consecutive PTMC patients (mean age, 44.7 years; 73.8% female) who underwent surgery at a high-volume center between June 2021 and February 2025. Preoperative high-resolution ultrasonography was performed for all the patients. The shortest distances from the tumor to the trachea and posterior thyroid capsule were measured and categorized. Tracheal and RLN invasion were confirmed intraoperatively. Odds ratios (ORs) were calculated across risk groups. Tracheal invasion occurred in 1.5% of the patients and was not observed when the tumor-trachea distance exceeded 3 mm. Among tumors ≤1 mm from the trachea, invasion rates increased with tumor-trachea angle (acute 7.6%, right 11.3%, obtuse 17.2%), corresponding to a 47.9-fold higher risk (95% confidence interval, 29.5-77.7) compared with tumors larger than 1 mm. Recurrent laryngeal nerve invasion occurred in 2.3% of the patients and was rare when the tumor-capsule distance exceeded 3 mm (0.05%). Tumors ≤2 mm from the posterior capsule with capsular disruption showed a 34.6% invasion rate, including 9.8% clinically significant RLN invasion, with significantly increased risks of any RLN invasion (OR 45.6) and clinically significant RLN invasion (OR, 66.2). Preoperative ultrasound assessment of tumor proximity and interface with adjacent structures robustly predicts tracheal and RLN invasion in PTMC. These parameters may improve selection for active surveillance and guide surgical decision-making.