Tumor size amplifies the effect of minimal extrathyroidal extension on lymph node metastasis in papillary thyroid carcinoma: a population-based study

How minimal extrathyroidal extension (minETE) and tumor size jointly relate to lymph node metastasis (LNM) in papillary thyroid carcinoma (PTC) remains unclear. We analyzed 65,404 patients with PTC from the Surveillance, Epidemiology, and End Results database (2004–2021). Predictors were ranked by permutation importance and assessed by multivariable logistic regression. Restricted cubic splines were used to model the effect of minETE across tumor size. Interaction was tested on multiplicative and additive scales (relative excess risk due to interaction, RERI), with sensitivity analyses by lymph node compartment and period of diagnosis. minETE was present in 8.9% of patients and showed the strongest association with LNM (adjusted OR 3.53, 95% CI 3.32–3.75). This effect varied non-linearly with tumor size: the OR was stable at 3.5–4.0 for tumors up to 20 mm and exceeded 7.0 for tumors larger than 60 mm. Patients with minETE and 0–10 mm tumors had a higher LNM rate than patients without extension but with tumors larger than 40 mm (54.7% vs. 45.3%; adjusted OR 1.83, 95% CI 1.57–2.14). The interaction was significant on the additive scale (RERI 2.15, 95% CI 1.49–2.80) but not on the multiplicative scale, and was consistent across periods of diagnosis. minETE was strongly associated with LNM and showed a super-additive interaction with tumor size, most evident above 20 mm. Because minETE is usually established on final pathology, these findings mainly inform postoperative risk assessment (nodal staging, adjuvant radioactive iodine, and surveillance) rather than the primary decision on prophylactic central neck dissection.