Calibration and Performance Characterization of a Thyroid Bioassay System Using I-131 Under Philippine Clinical Conditions

Purpose: Occupational monitoring of internal exposure to radioiodine is required under international standards (e.g. IAEA GSR Part 3) and national regulations (e.g. PNRI CPR Part 3–Section 29); however, locally–validated calibration data for thyroid bioassay systems remain limited. This study aims to perform a systematic calibration and performance characterization of a thyroid bioassay system — under clinical conditions — focusing on counting efficiency, resolving time, and detection limits relevant to occupational bioassay. Methods: The Captus 4000e multichannel analyzer Thyroid Uptake System was calibrated using a neck phantom (ANSI N44.3, 1973) at a fixed 20 cm detector–phantom distance to simulate in–vivo thyroid geometry. Liquid iodine-131 samples (2µCi to 10µCi) were prepared via dilution, and then assayed using system. Background correction was applied using repeated initial and final measurements. The counting efficiency was calculated from net count rates, with consideration of a gamma yield of 0.82 for the emission of 365 keV photons. The resolving time was determined using the two-source method. Measurement uncertainty was evaluated by combining Type A and Type B components. The lower limit of detection (LLD) and minimum detectable activity (MDA) were also calculated. Results: The counting efficiencies of the different radioactivity (2µCi, 4µCi, 6µCi, 8µCi and 10µCi) of Iodine-131 are 0.0046, 0.0038, 0.0036, 0.0035 and 0.0034, respectively, demonstrating a non-linear decrease with increasing activity. The practical counting efficiency is approximately 0.004+0.0009 (26% expanded uncertainty). The low efficiency and non-linear response imply that counting rate losses due to the resolving time (dead time) at approximately 3microseconds. The system demonstrated an LLD of 52 counts and an MDA of around 131Bq (0.003µCi). Conclusion: Calibration results show deviation from ideal detector response due to dead time. These findings emphasize that calibration requires local performance characterization. Facility-specific calibration is essential for correct internal dose assessment and reliable occupational bioassay implementation.