CT-guided iodine-125 brachytherapy for advanced thymic epithelial tumors: a preliminary observational study

This study aimed to provide a preliminary description of the application of CT-guided iodine-125 (¹²⁵I) seed implantation, a form of brachytherapy, in patients with advanced or recurrent thymic epithelial tumors (TETs).

We retrospectively reviewed seven patients with advanced or recurrent TETs who underwent CT-guided ¹²⁵I seed implantation. Based on the extent of disease at presentation, one patient had widespread distant metastases, while the remaining six had localized lesions. Treatment planning was performed using a treatment planning system(TPS, Beijing Astro Technology Co., Ltd.) with a prescribed dose of 60-70 Gy. Follow-up evaluations included regular contrast-enhanced CT scans. Tumor response was assessed according to RECIST 1.1 criteria, and safety was evaluated using CTCAE v5.0.

A total of 12 lesions were treated across seven patients. All procedures were technically successful. Over a median follow-up of 46 months (range: 9-69 months),four patients (57.1%) attained a complete response (CR) and three patients (42.9%) attained a partial response (PR) at the patient level. Patient-level response was defined by composite assessment of all target lesions. At the lesion level, six lesions (50%) demonstrated a CR and six lesions (50%) demonstrated a partial response PR. Notably, lesion-level observations are statistically non-independent given that half of treated lesions derived from a single patient. Complications included pneumothorax in three patients (one of whom required drainage) and grade 1 radiation pneumonitis in one patient; all adverse events resolved without long-term sequelae. No severe radiation-related toxicities (e.g., pericarditis) were observed. In a descriptive subgroup excluding the patient with widespread metastases, four of six patients achieved CR and two achieved PR; no local progression was documented during available follow-up, but this finding remains descriptive and non-comparative.

CT-guided ¹²⁵I radioactive seed implantation appears feasible and well-tolerated in patients with advanced or recurrent TETs. The observed objective tumour responses and toxicity profile are hypothesis-generating but must be interpreted with caution given the very small sample size, heterogeneous cohort, non-independent lesion-level data, and lack of a control group. This preliminary study cannot establish durable local control or superiority over external beam radiotherapy. Further larger, prospective studies with standardized dosimetry and time-to-event endpoints are warranted.