Background: It is important to identify prognostic factors for lateral lymph node metastasis (LLNM) in papillary thyroid microcarcinoma (PTMC) because they determine the extent of surgery. Several similarly designed studies have investigated predictors of LLNM, but with no more than 1000 cases. In addition, there are no recommendations or guidelines covering the differences in risk by tumor location. This study is the largest, using a papillary thyroid microcarcinoma population with 2967 patients. The purpose of this study is to address predictive factors of LLNM, focusing on lesion location.
Patients and methods: We retrospectively reviewed the data of 2967 PTMC patients who underwent total thyroidectomy and central neck dissection and/or lateral neck dissection (unilateral or bilateral) between January 1997 and June 2015.
Results: On multivariate analysis, superior lesion [adjusted odds ratio (OR) 3.32, p < 0.000], male gender (adjusted OR 1.39, p = 0.0047), age under 45 years (adjusted OR 1.42, p = 0.015), and central lymph node metastasis (adjusted OR 3.40, p < 0.000) were significant predictors of high-risk LLNM. Superior lesion [hazard ratio (HR) 2.32, p = 0.005] and central lymph node metastasis (CLNM, HR 7.12, p < 0.000) were significant risk factors for locoregional recurrence (LRR). To reduce the effect of selection bias, we performed propensity score matching analysis with regard to tumor location. With a total of 1138 patients with matched data and 569 patients for each location, superior lesion (adjusted OR 3.17, p < 0.000), age under 45 years (adjusted OR 1.73, p = 0.005), and CLNM (adjusted OR 2.77, p < 0.000) were independent predictive factors of LLNM. Superior lesion (HR 2.28, p = 0.04) and CLNM (HR 5.32, p = 0.001) were significant risk factors for LRR.
Conclusions: In addition to young age, male gender, and CLNM identified in previous studies, meticulous assessment for LLNM is required in PTMC patients when lesions are located in the superior pole of the thyroid during preoperative evaluation or postoperative follow-up, because superior located papillary microcarcinoma is a risk factor for LLNM and LRR.