National Radiation Emergency Center, Korea Institute of Radiological and Medical Sciences, Seoul, Republic of Korea.
Department of Nuclear Engineering, Hanyang University, Seoul, Republic of Korea.
Health Phys. 2019 Oct;117(4):388-395. doi: 10.1097/HP.0000000000001070.
The counting efficiencies obtained using a physical neck phantom are typically used in the measurement of I activity in the thyroid. It is well known, however, that the geometrical discrepancies between the physical neck phantom and the anatomy of the subject can significantly influence the counting efficiencies. Thus, it is necessary to consider the anatomical characteristics of individuals if we need to accurately determine the activity of I in the thyroid. This study aims to produce individualized counting efficiencies for thyroid measurement, considering the age, sex, and overlying tissue thickness of the subject being measured by Monte Carlo simulation. Simulations were performed using a series of computational human phantoms of different ages and sexes. The difference in counting efficiencies, depending on the age and sex of the phantom, were found to range from -26 to 3% for the phantoms and monitoring systems considered in the present study. The overlying tissue thickness of the computational phantoms was also modified to find the relationship between the counting ratio of the 80.2 and 364 keV gammas from I and the overlying tissue thickness. The equations for estimating the overlying tissue thickness of a subject were then derived from the relationships between counting ratios and overlying tissue thickness. Finally, in the present study, a set of equations representing the variation in counting efficiencies for the 364 keV peak as a function of the overlying tissue thickness were derived, which can be used to determine individualized counting efficiencies for the subject being measured. These individualized counting efficiencies considering the overlying tissue thickness given a subject's age and sex can provide accurate estimates of I activity for internal dosimetry.
使用物理颈模型获得的计数效率通常用于测量甲状腺中的 I 活度。然而，众所周知，物理颈模型与受试者解剖结构之间的几何差异会显著影响计数效率。因此，如果我们需要准确确定甲状腺中的 I 活度，则有必要考虑个体的解剖学特征。本研究旨在通过蒙特卡罗模拟，针对测量的个体的年龄、性别和覆盖组织厚度，为甲状腺测量生成个体化的计数效率。使用一系列不同年龄和性别的计算人体模型进行了模拟。结果发现，对于本研究中考虑的模型和监测系统，计数效率的差异范围为 -26%至 3%，取决于模型的年龄和性别。还修改了计算模型的覆盖组织厚度，以找到 I 的 80.2keV 和 364keV 伽马射线计数比与覆盖组织厚度之间的关系。然后，从计数比与覆盖组织厚度之间的关系推导出用于估算主体覆盖组织厚度的方程。最后，在本研究中，推导出了一组表示 364keV 峰计数效率随覆盖组织厚度变化的方程，可用于确定测量对象的个体化计数效率。考虑到主体的年龄和性别，这些考虑覆盖组织厚度的个体化计数效率可以为内部剂量测定提供 I 活度的准确估计。
