Minimal Extrathyroidal Extension in Bilateral Papillary Thyroid Carcinoma Is Associated With Postoperative Structural Recurrence.

[BACKGROUND] The updated American Joint Committee on Cancer (AJCC) staging system has excluded minimal extrathyroidal extension (mETE) from the T3 category. However, mETE remains classified as an intermediate-risk feature for recurrence in thyroid cancer. The prognostic significance of mETE and its association with recurrence risk remain subjects of ongoing debate.

[METHODS] This study analyzed a retrospective cohort of 1870 papillary thyroid carcinoma (PTC) patients who underwent total thyroidectomy (TT) with central lymph node dissection (CLND) between 2015 and 2020. Cox proportional hazards regression models and subgroup analyses were employed to evaluate the association of mETE with structural recurrence.

[RESULTS] After a median follow-up of 27.9 months, 124 patients (6.6%) experienced structural recurrence. The recurrence rate was significantly higher in patients with mETE than in those without (11.0% vs. 5.6%, p < 0.001). Subgroup analyses revealed that mETE was an independent risk factor, particularly in patients with bilateral tumors (HR: 2.99, 95% CI: 1.8-4.95) and those without Hashimoto's thyroiditis (HT) (HR: 2.33, 95% CI: 1.53-3.63). A significant interaction between tumor bilaterality and mETE was observed (p = 0.003).

[CONCLUSIONS] In patients with PTC, mETE is a significant prognostic factor of structural recurrence and is associated with decreased disease-free survival (DFS). Critically, we demonstrate for the first time that mETE elevates recurrence risk to near the ATA intermediate-high threshold (17.6%) in bilateral PTC, regardless of tumor size. This synergy of mETE and bilaterality supports upgrading risk stratification and intensifying surveillance for this subset.