Prognostic Analysis of Iodine-avid Residual Metastatic Lymph Nodes in Differentiated Thyroid Cancer

Abstract Backgroud : In clinical practice, even with equivalent administered radioiodine( 131 I) activities, metastatic lymph node lesions exhibit heterogeneous responses to initial 131 I therapy among differentiated thyroid cancer (DTC )patients. Objectives To evaluate the efficacy of initial 131 I treatment in DTC patients with iodine-avid residual metastatic lymph nodes (reLNs) and to develop a model for predicting the functional 131 I therapy response. Methods 220 DTC patients who underwent initial 131 I therapy with iodine-avid reLNs on Rx-WBS (post-therapy whole-body scan with SPECT/CT) were included retrospectively in this study. Imaging features of reLNs, including number, maximum short-axis diameter, calcification, and semiquantitative 131 I uptake indices, together with pre-therapy stimulated thyroglobulin (ps-Tg) and administrated 131 I activity were analyzed. 131 I therapy response was assessed according to the 2015 American Thyroid Association criteria. Functional 131 I therapy response was defined as two groups by disappearance or persistence of iodine uptake on reLNs according to the follow-up scans. To predict the functional 131 I therapy response, logistic regression was used to identify independent predictors and a nomogram was constructed and validated. Results After initial 131 I therapy, patients exhibited excellent response (ER, 44.5%), indeterminate response (IDR, 30.0%), biochemically incomplete response (IR, 11.4%), and structurally incomplete response (SIR,14.1%), respectively. In terms of functional outcomes, 86.8% of patients had complete resolution of radioactive iodine uptake, while persistent iodine uptake was observed in the remaining 13.2% of cases. Compared with the disappearance group, the persistent 131 I uptake group had higher administrated 131 I activity, higher ps-Tg levels, more iodine-avid lymph nodes, larger maximum short-axis diameter and more frequent calcification of the reLNs. Multivariable analysis identified ps-Tg and number of iodine-avid reLNs as independent risk factors for 131 I persistent uptake. In the imaging-only model, lymph node number and maximum short-axis diameter were positively associated with 131 I persistent uptake. The nomogram achieved an AUC of 0.792 (95% CI, 0.678–0.906). Conclusion Initial 131 I therapy yielded favorable outcomes in DTC patients with iodine-avid reLNs. The number and size of reLNs serve as key predictors of persistent radioactive iodine uptake. A novel nomogram incorporating ps-Tg, metastatic nodal number, and maximum short-axis diameter was constructed to preliminarily achieve individualized prediction of functional therapeutic response.