MMP9 and metastatic malignant neoplasm in the lymph nodes: The univariate Cox proportional hazard regression model showed that the following factors were significantly correlated with SPRD: age (P=0.005), tumor size (P=0.007), capsule invasion (P=0.031), central lymph node metastasis (P=0.026), extrathyroidal invasion (P=0.001), vascular invasion (P<0.001), advanced TNM stage (P<0.001), and serum MMP-9 level (P<0.001).