Risk Factors for Lymph Node Metastasis (LNM) in Patients with Papillary Thyroid Microcarcinoma (PTMC): Role of Preoperative Ultrasound

Objective This study aimed to investigate the risk factors and clinical value of lymph node metastasis (LNM) and missed central lymph node metastasis (CLNM) using preoperative ultrasound (US) in patients with papillary thyroid microcarcinoma (PTMC). Methods This retrospective study included 521 patients who underwent thyroidectomy for confirmed PTMC based on a final histological examination between January 2014 and June 2015. Based on the presence of LNM, 521 cases were divided into two groups: metastasis (218) and non-metastasis (303). Univariate and multivariate logistic regression analyses were used to analyse the US and clinical characteristics of the primary tumour. Results We defined LNM based on the tumour diameter with an optimal critical value of 0.55 cm using ROC analysis with a sensitivity of 65.6% and specificity of 59.6%. We defined US-missed CLNM based on the optimal critical value of 0.65 cm using diagnostic ROC analysis with a sensitivity of 66.0% and specificity of 73.0%. The odds ratios of significant factors with LNM by US were 10.3 (95% confidence interval [95% CI], 6.2-17.0), 5.3 (95% CI, 3.3-8.7), 2.7 (95% CI, 1.1-6.5), 4.3 (95% CI, 1.7-10.5), 2.5 (95% CI, 1.5-4.1), and 2.7 (95% CI, 1.7-4.4) for extrathyroidal invasion, blood flow, multifocality, tumour diameter greater than 0.55 cm, male sex, and age younger than 47 years, respectively. Conclusions US characteristics, such as extrathyroidal invasion, blood flow, tumour diameter, sex, and age, may improve the efficacy of predicting LNM and facilitating diagnosis of PTMC. Furthermore, tumour invasion to the extracapsular thyroid and a diameter greater than 0.65 cm indicate CLNM.