PHITS Simulation Analysis of the Performance of Pediatric Thyroid Screening Measurements Using NaI(Tl) Spectrometer

Background: A NaI(Tl) survey meter was used in the pediatric thyroid screening conducted after the Fukushima Daiichi Nuclear Power Plant accident. However, this measurement method has the weakness that it is difficult to selectively identify 131I. In this study, we analyzed the performance of an energy-analyzable NaI(Tl) spectrometer using the Particle and Heavy Ion Transport code System (PHITS), which allows Monte Carlo simulation of radiation transport.Materials and Methods: The spectrum of energy emitted by the NaI(Tl) spectrometer was simulated for a total energy absorption peak by adjusting factors that affect pulse wave height. From these simulation results, the detection limits of the NaI(Tl) spectrometer were obtained using Monte Carlo simulation.Results and Discussion: The energy spectrum results were reproduced with an accuracy of 0.1% to 44.0% for total energy absorption peaks. The calculated detection limit for 131I activity equivalent to 100 mSv in the thyroid under a 0.2 μSv· hr−1 ambient dose rate was approximately 80–90 Bq, which could be detected for up to 38 days after 131I intake in a 1-year-old child.Conclusion: This study demonstrated that pediatric thyroid screening using an NaI(Tl) spectrometer can practically provide greater accuracy than NaI(Tl) survey meters.