Purpose

To analyze the clinical response of iodine-125 seed implantation (¹²⁵I-SI) in patients with radioactive iodine-refractory differentiated thyroid cancer (RAIR-DTC), and to explore the association between metabolic parameters derived from pre-operative 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) and treatment efficacy.

Material and methods

This retrospective study included 48 RAIR-DTC patients, who underwent ¹²⁵I-SI for metastatic lesions at Jiangxi Cancer Hospital from November 2019 to June 2025. Correlations between parameters, such as gender, age, metabolic tumor volume (MTV), standardized uptake value maximum (SUV_max), peak standard uptake value (SUV_peak), maximum standardized uptake value normalized by lean body mass (SUL_max), peak standardized uptake value normalized by lean body mass (SUL_peak), total lesion glycolysis (TLG), tumor-to-liver ratio (TLR), number of iodine-125 (¹²⁵I) seeds, activity of ¹²⁵I seeds, and 90% of dose received by gross tumor volume (D₉₀), with efficacy of ¹²⁵I-SI were analyzed. Statistical analyses included two independent-sample t-tests, Mann-Whitney U test, Fisher’s exact test, and logistic and Cox regression analysis. Performance was assessed using area under the curve (AUC), calibration, and decision curve analysis (DCA).

Results

Patients were regularly followed up after treatment. At 6 months, 18 patients achieved complete metabolic response (CMR), 18 partial metabolic response (PMR), and 12 stable metabolic disease (SMD). D₉₀ was an independent predictor of 6-month efficacy (p = 0.031, OR = 0.798, 95% CI: 0.428-0.925), with an AUC of 0.742. DCA and calibration curves confirmed the model’s robust predictive capability. Cox regression identified TLG as a significant predictor of local progression-free survival (LPFS) (HR = 4.106, 95% CI: 2.850-5.620, p = 0.036).

Conclusions

¹²⁵I seed implantation is an effective treatment for locally metastatic RAIR-DTC lesions. D₉₀ is a reliable predictor of 6-month efficacy, and TLG is significantly associated with LPFS following ¹²⁵I-SI.