To evaluate the efficacy and safety of ¹²⁵I seeds implantation for lymph nodes metastasis (LNM) from radioactive iodine-refractory differentiated thyroid carcinoma (RAIR-DTC), and to verify the computer three-dimensional treatment planning system (TPS) from the dosimetry accuracy in assisting seeds implantation to treat LNM.

Retrospective analysis was performed on 17 RAIR-DTC patients with LNM admitted to the General Hospital of Northern Theater Command from December 2016 to January 2019 (8 males, 9 females, median age 58 years). All patients underwent preoperative TPS planning design, CT-guided puncture and ¹²⁵I seeds implantation (seed activity 14.8-25.9 MBq). The dosimetric results of postoperative validation were compared with those of preoperative planning, including the dosimetric parameters such as target volume before and after surgery and the dose received by 90% and 100% gross tumor volume (GTV) (D₉₀, D₁₀₀), the percentage received by 100% and 150% of the prescription dose (V₁₀₀, V₁₅₀), homogeneity index (HI). All patients underwent CT after 6 months to compare the LNM size, serum thyroglobulin (Tg) level, and the improvement of complications before and after treatment. Efficacies were divided into complete remission (CR), partial remission (PR), stable disease (SD), and progressive disease (PD). Paired t test or Wilcoxon signed rank test were used to analyze the data.

Among 17 patients, a total of 226 ¹²⁵I radioactive seeds were implanted. Among them, 1 achieved CR, 10 achieved PR, 4 were with SD, and 2 were with PD. The diameter of LNM was 1.40(0.65, 3.05) cm before treatment and was 0.40(0.21, 0.91) cm 6 months after treatment (z=-3.95, P<0.05). The Tg before treatment was 23.50(20.94, 72.92) μg/L and was 8.90(3.20, 40.22) μg/L 6 months after treatment (z=-5.009, P<0.001). Tg antibody were all negative. There were 90.90% (20/22) of patients had slightly lowerD₉₀ than the prescribed dose ((12 378.8±3 182.0) vs (12 497.8±1 686.4) cGy; t=0.251, P>0.05). The postoperative dose parametersD₁₀₀ and V₁₅₀ ((6 881.5±1 381.8) cGy, (58.5±18.4)%) were both lower than those of preoperative plan ((8 085.8±2 330.0) cGy, (66.5±17.7)%; t values: 8.913, 3.032, both P<0.05), and the remaining indicators were not significantly different from those of the preoperative plan (t values: 0.251, 1.493, z values: from -1.604 to -0.593, all P>0.05).

According to the TPS preoperative plan, ¹²⁵I seeds implantation for treating RAIR-DTC LNM can achieve the expected dose distribution, and the short-term tumor local control is effective. It is a safe and effective treatment method.

Thyroid neoplasms; Neoplasm metastasis; Lymph nodes; Brachytherapy; Iodine radioisotopes; Radiation dosage