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 (

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I) seeds implantation in the treatment of RAIR-DTC. Methods: Thirty-nine RAIR-DTC patients with cervical lymph node metastasis were treated with 

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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

P0.05); 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 (P0.001). There was a weak correlation between the number of 

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I seeds and the reduction of short diameter of the target lesions (P0.05)

but not with the reduction of Tg level (P0.05). No serious adverse event associated with the treatment was observed. Conclusion: 

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I seeds implantation is an effective and safe way in the treatment of cervical lymph node metastasis of RAIR-DTC.