Abstract: Background and purpose: This study aimed to investigate the relationship between lymph node metastatic rate (LR) and response to radioiodine therapy in patients with papillary thyroid carcinoma (PTC). Methods: A total number of 143 PTC patients after radioiodine therapy were included and classified into 4 groups [Ⅰ(0%-10%),Ⅱ(＞10%-25%), Ⅲ(＞25%-50%), Ⅳ(＞50%)] according to the lymph node metastatic rate, and the responses to initial radioiodine therapy after a median follow-up period of 20.7 months were evaluated. They were classified into 4 groups [excellent response (ER), indeterminate response (IDR), biochemical incomplete response (BIR), and structural incomplete response (SIR)] according to the guideline proposed by 2015 American Thyroid Association. One-Way analysis of variance, χ² test and Kruskal-Wallis test were used to evaluate the differences in basic clinicopathological features and clinical responses among the 4 groups. The ROC curve was analyzed to evaluate the clinical value of lymph node metastatic rate for predicting ER and optimal cut-off point. Results: There were no significant differences in gender and T-stage among 4 groups (P>0.05). However,Ⅰ group was significantly older than the other 3 groups (P=0.001). With the increase of lymph node metastatic rate, the number of ER cases decreased, while cases of BIR and SIR generally increased. Compared with the other 3 groups, less cases of ER (27.8%), while more BIR (27.8%) or SIR (11.1%) were observed in group Ⅳ (H=18.816, P=0.000). Cut-off value of lymph node involved rate was 52.27%, with a better specificity of predicting ER. Area under the ROC curve was 0.668. Conclusion: The higher lymph node metastatic rate in patients with PTC, the worse clinical outcome it could be. A cut-off value of lymph node metastatic rate 52.27% is a specific independent predictor for the clinical outcome in PTC patients treated with radioiodine therapy.

Key words: Papillary thyroid carcinoma, Lymph node metastatic rate, Radioiodine remnant ablation, Clinical outcome