Radioactivity Retention of Liquid Iodine-131 in Residual Syringes: Implications for Radiation Exposure and Radioactive Waste Management in Radiopharmaceutical Preparation

Residual radioactivity in syringes used during the preparation of liquid iodine-131 radiopharmaceuticals may contribute to avoidable radiation exposure and increase the radioactive waste burden. Although radioactivity retention in syringes has been reported for several radiopharmaceutical preparations, safety-oriented evaluations of residual iodine-131 activity in liquid formulations remain limited. This study aimed to investigate the relationship between residual radioactivity retention and radiation exposure rates in syringes following liquid iodine-131 preparation and to assess the implications for radioactive waste management. This experimental study used sodium iodide labeled with iodine-131 (Na¹³¹I) in liquid form. Initial and residual syringe activities were measured using a radionuclide dose calibrator, while ambient dose equivalent rates were measured using a gamma radiation survey meter. Residual activity, percentage radioactivity retention, radiation dose rate, and estimated decay-in-storage duration were analyzed descriptively. Pearson's correlation analysis and simple linear regression were performed to evaluate the relationship between radioactivity retention and measured radiation dose rates. Sequential rinsing with water for injection and 0.9% sodium chloride solution was evaluated as an approach to reducing residual radioactivity. Mean residual syringe activity increased from 0.270 ± 0.012 mCi at an initial activity concentration of 5 mCi/mL to 0.435 ± 0.016 mCi at 15 mCi/mL. Radioactivity retention increased from 12.90 ± 0.63% to 20.97 ± 0.74%, accompanied by an increase in the measured radiation dose rate from 2.703 ± 0.102 to 4.131 ± 0.237 µSv/h. Radioactivity retention exhibited a strong positive correlation with the radiation dose rate (Pearson's r = 0.9412, R² = 0.8858, p < 0.0001). Sequential rinsing reduced the measured dose rate from 4.160 ± 0.186 to 1.329 ± 0.034 µSv/h using water for injection and from 4.190 ± 0.117 to 1.314 ± 0.023 µSv/h using 0.9% sodium chloride solution. These findings demonstrate a strong association between residual syringe radioactivity and measured radiation dose rates following liquid iodine-131 preparation, highlighting the contribution of residual activity to the initial radioactive waste burden. Controlled rinsing procedures may help reduce residual syringe activity and support radiation protection, contamination control, and radioactive waste management practices in nuclear medicine facilities.