Abstract: Background and purpose: With the increasing incidence of thyroid cancer, radioactive iodine-refractory differentiated thyroid cancer (RAIR-DTC) has become difficult to diagnose and treat. The purpose of this study was to explore the efficacy and safety of iodine-125 ( ¹²⁵ I) seeds implantation in the treatment of RAIR-DTC. Methods: Thirty-nine RAIR-DTC patients with cervical lymph node metastasis were treated with  ¹²⁵ I seeds in the Affiliated Cancer Hospital of Xiangya School of Medicine, Central South University from 2013-2017. The level of serum thyroglobulin (Tg) and the changes of the target lesions were monitored every 3 months. We evaluated the response according to the response evaluation criteria in solid tumors version 1.1 (RECIST 1.1). Meanwhile, the adverse events were monitored during the therapy according to common terminology criteria for adverse events version 5.0 (CTCAE 5.0). Results: After treatment, the objective remission rate of 39 patients was 43.6%, and the disease control rate was 92.3%. The level of serum Tg at 3 months after treatment was significantly lower than that before treatment (t=2.2, P<0.05); 6 months versus 3 months after treatment, 9 months versus 6 months after treatment, and 12 months versus 9 months after treatment, there was no statistically significant difference in serum Tg level. The maximum short diameters of lymph nodes at 3, 6, 9 and 12 months after treatment were (15.1±1.1) mm, (13.8±1.0) mm, (12.9±1.0) mm and (12.9±1.0) mm, respectively, significantly lower than that before treatment (18.3±1.2) mm (P<0.001). There was a weak correlation between the number of  ¹²⁵ I seeds and the reduction of short diameter of the target lesions (P<0.05), but not with the reduction of Tg level (P>0.05). No serious adverse event associated with the treatment was observed. Conclusion:  ¹²⁵ I seeds implantation is an effective and safe way in the treatment of cervical lymph node metastasis of RAIR-DTC.

Key words: Radioactive iodine-refractory differentiated thyroid cancer, Iodine-125 seeds, Brachytherapy, Lymph node