Intercomparison of age-dependent thyroid phantoms for thyroid monitoring in nuclear or radiological emergencies

The 2020 updated Strategic Research Agenda (SRA) of the European Radiation Dosimetry Group (EURADOS), recommends improving the calibration of in vivo monitoring techniques for children, particularly in case of radiological emergencies. Consequently, members of the WG7 task group on Internal Dosimetry for Emergency have organized and carried out an intercomparison exercise at the Whole Body Counter Laboratory of CIEMAT aimed at comparing different age-dependent thyroid neck phantoms for in-vivo monitoring of 131I in the thyroid, specifically targeting children and adults. Seven laboratories, BfS (Germany); SCK CEN (Belgium), SURO (Czech R.), IRSN (France), STUK (Finland), CIEMAT (Spain) and QST (Japan) have participated in the exercise. Before starting the exercise, a technical questionnaire for gathering information and a protocol of measurements were agreed by members of the task group. Measurements were carried out with a Broad Energy germanium detector (BEGe) and three geometries of measurements were agreed for comparing efficiency values at short (2 cm), medium (10 cm) and long (15 cm) detector-phantom distance. A fixed counting time (600 s) was selected for calculating detection limits with blank vials. Prior to the beginning of the measurement campaign, the participants answered a technical questionnaire with a detailed description of their phantoms, dimensions of the vials for thyroid simulation, radioactive sources, calibration and measurement procedures, etc. The intercomparison measurements, all the data collected in the technical questionnaire and the results of the measurements (efficiency calibration curves and detection limits) were shared with the participants for agreement. The provided information confirmed significant variations in the design of the phantoms and their characteristic such as overlying tissue thickness, thyroid volume, tissue-equivalent material and dimensions of vials simulating thyroid lobes. Such differences highlight the necessity of harmonization in this field. Regarding the intercomparison measurements, a comparative study has been conducted on the efficiency values obtained for each age group in the three defined geometries. The results evealed significant variability in the efficiency values, particularly at short and medium detector-phantom distances, where discrepancies can reach up to 50%. Following this intercomparison it seems important to establish a consensus regarding the design of reference age-dependent thyroid phantoms in order to standardize calibration and measurement . procedures for enhanced accuracy and harmonization