Tumor Size as a Predictive Indicator for Lymph Node Metastasis in Papillary Thyroid Carcinoma: An Inverted L‐Shaped Curve Analysis Based on the SEER Database

ABSTRACT Background Papillary thyroid carcinoma (PTC) frequently metastasises to lymph nodes, with lymph node metastasis (LNM) occurring with high frequency in small, early‐stage tumors. The present study examines the inverse l ‐shaped relationship between tumor size and the likelihood of LNM in patients diagnosed with PTC. Methods We performed a detailed retrospective cohort analysis of 48,021 cases of papillary thyroid cancer using data from the Epidemiology, and End Results (SEER) database from 1992 to 2019. Our study used various analytical methods, including logistic regression, spline curve fitting, and variable interaction assessment, to clarify the association between tumor size and LNM rates. We rigorously controlled for potential confounders such as patient age, sex, ethnicity, tumor size, extrathyroidal extension (ETE), histopathological characteristics and distant metastases. In addition, we thoroughly investigated and quantitatively assessed the relationship between adjusted tumor size measurements and the likelihood of LNM development. Results The median tumor size among the 48,021 patients diagnosed with PTC was 1.3 cm. Among these patients, 12,365 (25.75%) had LNM, with a median tumor size of 1.9 cm in this group. A comparative analysis shows a significant difference in tumor sizes between PTC patients who were LNM‐positive and those who were LNM‐negative. The relationship between tumor size and the likelihood of LNM exhibits a distinct nonlinear pattern. Specifically, below a diameter threshold of 1.978 cm, the probability of LNM significantly increases with larger tumor sizes (odds ratio [OR] = 2.363, 95% confidence Interval [CI]: 2.214–2.523). Once this threshold is surpassed, the effect of tumor size on LNM incidence levels off (OR = 1.031, 95% CI: 1.003–1.061). Conclusion The results of this study confirm that tumor size significantly determines the likelihood of LNM in patients with PTC. We found an inverse l ‐shaped relationship between tumor size and the probability of LNM. As the tumor size increased below 1.978 cm, the likelihood of LNM increased, but not with tumor size above that threshold. These findings provide new insights into the complex relationship between tumor size and LNM in PTC.