ABSTRACT: Extrathyroidal extension (ETE) is a key pathological feature of papillary thyroid carcinoma (PTC) associated with aggressive tumor behavior and adverse outcomes. However, clinicopathological factors associated with ETE remain incompletely characterized, particularly in Southeast Asian populations. This study aimed to identify factors associated with pathological ETE in patients with PTC. This prospective observational study included 346 consecutive patients with histopathologically confirmed PTC who underwent thyroidectomy at a tertiary referral center in Vietnam between April and August 2025. ETE was defined based on postoperative histopathological examination, including both minimal and gross ETE. Clinicopathological variables were evaluated using univariable analysis and multivariable logistic regression. ETE was identified in 66 patients (19.1%). In univariable analysis, age ≥ 55 years, symptomatic presentation, multifocality, tumor size, lymph node metastasis and thyroiditis were associated with ETE (all p < 0.05). In multivariable analysis, age ≥ 55 years (OR = 2.321; 95%CI 1.153-4.674; p = 0.018), tumor size (per 10 mm increase) (OR = 1.052; 95%CI 1.016-1.090; p = 0.005) and lymph node metastasis (OR = 2.932; 95%CI 1.565-5.492; p < 0.001) were independently associated with increased odds of ETE. In contrast, coexisting thyroiditis was associated with lower odds of ETE (OR = 0.440; 95%CI 0.231-0.839; p = 0.013). In conclusion, older age, larger tumor size and lymph node metastasis were independently associated with pathological ETE in PTC, whereas coexisting thyroiditis showed a potentially inverse association that should be confirmed in larger cohorts. These findings provide additional insight into clinicopathological factors associated with tumor invasiveness in PTC and may contribute to improved risk assessment.

Impact statement: This prospective study identifies age, tumor size and lymph node metastasis as key factors associated with extrathyroidal extension in papillary thyroid carcinoma.

Key words: Papillary thyroid carcinoma; extrathyroidal extension; lymph node metastasis; thyroiditis; tumor size.