An individual dosimetry system is essential for the evaluation of precise doses in nuclear medicine. The purpose of this study was to develop a system for calculating not only absorbed doses but also EQDX(α/β) from the PET-CT images of patients for targeted alpha therapy (TAT), considering the dose dependence of the relative biological effectiveness, the dose-rate effect, and the dose heterogeneity. A general-purpose Monte Carlo particle transport code PHITS was employed as the dose calculation engine in the system, while the microdosimetric kinetic model was used for converting the absorbed dose to EQDX(α/β). PHITS input files for describing the geometry and source distribution of a patient are automatically created from PET-CT images, using newly developed modules of the radiotherapy package based on PHITS (RT-PHITS). We examined the performance of the system by calculating several organ doses using the PET-CT images of four healthy volunteers after injecting 18F-NKO-035. The deposition energy map obtained from our system seems to be a blurred image of the corresponding PET data because annihilation γ-rays deposit their energies rather far from the source location. The calculated organ doses agree with the corresponding data obtained from OLINDA 2.0 within 20%, indicating the reliability of our developed system. Test calculations by replacing the labeled radionuclide from 18F to 211At suggest that large dose heterogeneity in a target volume is expected in TAT, resulting in a significant decrease of EQDX(α/β) for higher-activity injection. As an extension of RT-PHITS, an individual dosimetry system for nuclear medicine was developed based on PHITS coupled with the microdosimetric kinetic model. It enables us to predict the therapeutic and side effects of TAT based on the clinical data largely available from conventional external radiotherapy.

中文翻译：

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基于 PHITS 与微剂量动力学模型相结合的靶向 α 治疗个体剂量测定系统

个体剂量测定系统对于核医学中精确剂量的评估至关重要。本研究的目的是开发一种系统，不仅可以计算吸收剂量，还可以从用于靶向 α 治疗 (TAT) 的患者 PET-CT 图像中计算 EQDX(α/β)，考虑到相对生物有效性的剂量依赖性，剂量率效应和剂量异质性。系统采用通用蒙特卡罗粒子输运代码PHITS作为剂量计算引擎，并采用微剂量动力学模型将吸收剂量转换为EQDX(α/β)。使用基于 PHITS (RT-PHITS) 的放射治疗包新开发的模块，从 PET-CT 图像自动创建用于描述患者几何形状和源分布的 PHITS 输入文件。我们通过使用四名健康志愿者注射 18F-NKO-035 后的 PET-CT 图像计算多个器官剂量来检查系统的性能。从我们的系统获得的沉积能量图似乎是相应 PET 数据的模糊图像，因为湮灭 γ 射线将其能量沉积在距离源位置相当远的地方。计算出的器官剂量与OLINDA 2.0获得的相应数据一致在20%以内，表明我们开发的系统的可靠性。通过将标记的放射性核素从 18F 替换为 211At 进行的测试计算表明，TAT 中预期目标体积中存在大剂量异质性，导致更高活性注射的 EQDX(α/β) 显着降低。作为RT-PHITS的扩展，基于PHITS结合微剂量动力学模型，开发了核医学个体剂量测定系统。它使我们能够根据主要来自传统外部放射治疗的临床数据来预测 TAT 的治疗和副作用。