Author: Kimyli Lemieux, Xingyu Nie 👨🔬
Affiliation: University of Kentucky/UK HealthCare 🌍
Purpose: To optimize the selection of calibration radioisotope and its activity for dose calibrator (DC) linearity testing, adhering to ALARA (As Low As Reasonably Achievable) principles while considering cost efficiency.
Methods: Conversion factors (CFs) between calibration radioisotopes (F-18, Tc-99m) and agents commonly used in radiopharmaceutical therapy and theranostic facilities were measured across clinical ranges for seven DCs from five models (Biodex Atomlab 400, 500+; Capintec CRC-7, CRC-25R, CRC-55tW) using calibrated dial settings. Required activities of calibration radioisotopes were determined based on the maximum clinical activity of each radiopharmaceutical. Hand skin radiation doses were calculated using VARSKIN+ v1.1 for the F-18 decay method and the sleeve (Calicheck tubes) method with Tc-99m. Whole-body (WB) radiation doses were estimated from unshielded gamma point-source dose rates. Radioisotope costs were assessed using supplier price charts.
Results: CFs between all radioisotope pairs were consistent within individual DC units and models but varied across manufacturers: CF(Tc-99m/F-18) = 3.51 ± 0.18, CF(Tc-99m/I-131) = 1.53 ± 0.10, CF(Tc-99m/Lu-177) = 0.33 ± 0.00, CF(Tc-99m/Y-90) = 0.09 ± 0.01. The maximum clinical activity of I-131 typically dictated the required calibration radioisotope activity. For DCs dedicated to assaying Lu-177 or Y-90 radiopharmaceuticals, lower F-18 or Tc-99m activities were sufficient. The choice of testing method depended on facility scheduling and radioisotope pricing. WB radiation doses were generally low (<2 mrem/testing), but skin doses could reach ~40 mrem when using F-18 with the decay method for testing DCs which were used for assaying I-131, potentially exceeding the doses associated with the sleeve method. The sleeve method was also advantageous for testing multiple DCs, DCs without auto-decay functionality, or when faster results were needed.
Conclusion: DC conversion factors were validated, and optimized calibration radioisotope activities were determined following ALARA. The selection of calibration radioisotope should balance radiation dose minimization, cost-effectiveness, scheduling constraints, and equipment-specific limitations.