Accuracy and Clinical Significance of Intraoperative Gross Extrathyroidal Extension (T3b) Assessment in Differentiated Thyroid Carcinoma

Objective: In the eighth edition of the American Joint Committee on Cancer tumor–node–metastasis staging system, gross extrathyroidal extension (ETE) into the strap muscles is classified as T3b when identified during surgery. In clinical practice, this invasion is primarily assessed intraoperatively by the surgeon and documented in the operative report, forming the basis of the final T3b staging. Because this evaluation is inherently subjective, its diagnostic accuracy remains uncertain. This study evaluated the accuracy of intraoperative gross ETE assessment and whether misclassification affects recurrence outcomes. Methods: In total, 4987 patients who underwent thyroidectomy at Seoul St. Mary’s Hospital during 2017–2022 were analyzed. Patients were categorized by concordance between intraoperative findings and final pathology: confirmed gross ETE (Group A), intraoperative overestimation (Group B), and intraoperative underestimation (Group C). Clinical characteristics, recurrence rates, and predictors of inaccurate assessment were compared. Results: Of the cohort, 179 patients (3.6%) were judged intraoperatively to have gross ETE, classified as Group A (141 patients), Group B (38), and Group C (33). Recurrence rates were not significantly different among groups (6.4%, 2.6%, and 3.0% in Groups A, B, and C, respectively). Other than lymphatic invasion and tumor size, baseline characteristics were comparable among groups. Multivariate analysis identified age (odds ratio [OR]: 0.961; 95% confidence interval [CI]: 0.932–0.990; p = 0.009), tumor location (OR: 0.182; 95% CI: 0.056–0.591; p = 0.005), and lymphatic invasion (OR: 0.292; 95% CI: 0.118–0.719; p = 0.007) as independent predictors of inaccurate intraoperative evaluation. Conclusions: Among 179 patients suspected of gross ETE intraoperatively, 21.2% showed no muscle invasion on pathology. Although recurrence rates were similar across groups, recurrence-free survival tended to be lower in Group A relative to Group B, indicating the potential prognostic relevance of accurate intraoperative T3b identification. Long-term follow-up is needed to confirm this trend.