A novel cartilage-based reference point for the recurrent laryngeal nerve: implication for preoperative risk stratification in posterior subcapsular thyroid tumors

BACKGROUND: Protecting the recurrent laryngeal nerve (RLN) is central in thyroidectomy, yet little is known about preoperative localization of the RLN and its role in assessing injury risk. We evaluated whether a novel cartilage-based landmark, the inferior cricoid-tracheal (ICT) point, located at the intersection of the lateral tracheal border and the inferior margin of the cricoid cartilage, approximates the laryngeal entry point (LEP) of the RLN and enables ultrasound-based measurement of tumor-nerve distance. METHODS: The study consisted of two independent cohorts. In the first cohort, the distance between the ICT point and the LEP was evaluated intraoperatively in 34 patients. In the second cohort, 78 patients with posterior subcapsular tumors underwent preoperative ultrasound measurement of the tumor-to-ICT point distance and intraoperative neuromonitoring (IONM) of RLN signals. An adverse RLN event was assessed using a decrease in signal amplitude of ≥ 50% as the threshold. Postoperative voice status was assessed clinically. RESULTS: The mean ICT point-to-LEP distance was 2.27 ± 0.82 mm. Among the 78 patients with posterior subcapsular tumors, no IONM-defined adverse RLN events were observed in the 49 patients with a tumor-to-ICT point distance ≥ 2.5 mm. Among the 29 patients with a distance < 2.5 mm, 5 had adverse RLN events, whereas the remaining 24 had preserved RLN signals after careful dissection. CONCLUSIONS: Preoperative ultrasound measurement of the tumor-to-ICT point distance may provide an exploratory reference for assessing tumor-RLN proximity in posterior subcapsular thyroid tumors. In this study, a tumor-to-ICT point distance ≥ 2.5 mm was associated with no observed IONM-defined adverse RLN events. This cutoff may serve as a practical reference for preoperative risk assessment, although further validation in larger independent cohorts is still needed.