Optimization of Iodine-125 Production for Prostate Brachytherapy Considering Iodine-126 Contamination and Irradiation Time
Abstract
Purpose
The simultaneous production of iodine-125 (¹²⁵I) and iodine-126 (¹²⁶I) during neutron irradiation directly impacts radionuclidic purity and clinical applicability of brachytherapy seeds. This study evaluated the dependence of ¹²⁵I and ¹²⁶I generation on irradiation time, aiming to identify conditions that achieve clinically adequate ¹²⁵I activity for prostate cancer treatment while maintaining acceptable ¹²⁶I contamination levels.
Methods
Natural xenon (¹²⁴Xe) samples were irradiated in the IEA-R1 research reactor under neutron fluxes of 3–8 × 10¹³ n·cm⁻²·s⁻¹ with irradiation times of 8, 16, 32, and 48 h. After optimized decay periods, iodine was chemically extracted into a fixed volume of NaI solution and analyzed by HPGe gamma spectrometry. The activity of ¹²⁵I was determined using the sum-peak method, while ¹²⁶I activity was quantified through a refined efficiency calibration curve based on a ¹⁶⁶mHo standard source.
Results
Irradiation times shorter than 32 h resulted in ¹²⁵I activities below the clinically recommended range for prostate brachytherapy seeds. Irradiations of at least 32 h yielded ¹²⁵I activities within the recommended interval of 0.2–0.8 mCi per seed, considering the NaI solution volume used. Longer irradiation times increased ¹²⁵I activity but also promoted secondary neutron capture reactions, leading to higher ¹²⁶I formation. The ¹²⁶I/¹²⁵I activity ratio showed a clear dependence on irradiation duration, remaining below clinical acceptance limits for all evaluated conditions.
Conclusion
Irradiation time is a critical parameter governing both clinically adequate ¹²⁵I activity and ¹²⁶I contamination. Under the studied conditions, a minimum irradiation time of 32 h is required to meet prostate brachytherapy activity standards while preserving radionuclidic purity.