Establishment of screening levels for thyroid contamination assessment in adults following a nuclear accident

Rapid screening for thyroid contamination is crucial for implementing radiation protection measures in affected populations. Direct thyroid measurements using handheld radiation instruments are primarily performed during the early phases of a nuclear accident owing to their immediate results and operational convenience. This study aimed to derive dose conversion factors to convert measurements from handheld radiation instruments into thyroid equivalent doses. Thyroid screening levels were established for the South Korean Radiation Emergency Medical System. Instrument counting efficiencies were determined using a physical thyroid phantom and radioactive materials. Dose conversion factors were then derived by integrating the counting efficiencies with 131 I retention functions and thyroid equivalent dose coefficients, under various intake scenarios. Screening levels were established using the derived conversion factors in conjunction with recommendations from the International Atomic Energy Agency, accounting for the time elapsed since radioiodine intake. The effectiveness of these levels was experimentally validated under various conditions, including detector performance, distance, and positioning. The derived screening levels can be used for offsite thyroid contamination measurements, facilitating rapid screening and dose assessment during radiation emergencies. Further studies are warranted to improve applicability in pediatric populations and reduce uncertainties in the current measurement system.