Département de Radio-Oncologie, Centre Hospitalier Universitaire de Québec, Québec, Canada.
Med Phys. 2011 Oct;38(10):5307-10. doi: 10.1118/1.3626482.
To perform absorbed dose calculations based on Monte Carlo simulations for a hypothetical (170)Tm source and to investigate the influence of encapsulating material on the energy spectrum of the emitted electrons and photons.
GEANT4 Monte Carlo code version 9.2 patch 2 was used to simulate the decay process of (170)Tm and to calculate the absorbed dose distribution using the GEANT4 Penelope physics models. A hypothetical (170)Tm source based on the Flexisource brachytherapy design with the active core set as a pure thulium cylinder (length 3.5 mm and diameter 0.6 mm) and different cylindrical source encapsulations (length 5 mm and thickness 0.125 mm) constructed of titanium, stainless-steel, gold, or platinum were simulated. The radial dose function for the line source approximation was calculated following the TG-43U1 formalism for the stainless-steel encapsulation.
For the titanium and stainless-steel encapsulation, 94% of the total bremsstrahlung is produced inside the core, 4.8 and 5.5% in titanium and stainless-steel capsules, respectively, and less than 1% in water. For the gold capsule, 85% is produced inside the core, 14.2% inside the gold capsule, and a negligible amount (<1%) in water. Platinum encapsulation resulted in bremsstrahlung effects similar to those with the gold encapsulation. The range of the beta particles decreases by 1.1 mm with the stainless-steel encapsulation compared to the bare source but the tissue will still receive dose from the beta particles several millimeters from the source capsule. The gold and platinum capsules not only absorb most of the electrons but also attenuate low energy photons. The mean energy of the photons escaping the core and the stainless-steel capsule is 113 keV while for the gold and platinum the mean energy is 160 keV and 165 keV, respectively.
A (170)Tm source is primarily a bremsstrahlung source, with the majority of bremsstrahlung photons being generated in the source core and experiencing little attenuation in the source encapsulation. Electrons are efficiently absorbed by the gold and platinum encapsulations. However, for the stainless-steel capsule (or other lower Z encapsulations) electrons will escape. The dose from these electrons is dominant over the photon dose in the first few millimeter but is not taken into account by current standard treatment planning systems. The total energy spectrum of photons emerging from the source depends on the encapsulation composition and results in mean photon energies well above 100 keV. This is higher than the main gamma-ray energy peak at 84 keV. Based on our results, the use of (170)Tm as a brachytherapy source presents notable challenges.
基于蒙特卡罗模拟对假设的（170）Tm 源进行吸收剂量计算，并研究封装材料对发射电子和光子能谱的影响。
使用 GEANT4 蒙特卡罗代码版本 9.2 补丁 2 模拟（170）Tm 的衰变过程，并使用 GEANT4 Penelope 物理模型计算吸收剂量分布。基于 Flexisource 近距离治疗设计的假设（170）Tm 源，活性核心设定为纯铥圆柱体（长 3.5 毫米，直径 0.6 毫米）和不同的圆柱形源封装（长 5 毫米，厚度 0.125 毫米），由钛、不锈钢、金或铂制成。根据不锈钢封装的 TG-43U1 公式，计算线源逼近的径向剂量函数。
对于钛和不锈钢封装，94%的韧致辐射是在核心内部产生的，分别有 4.8%和 5.5%在钛和不锈钢壳体内，而在水中不到 1%。对于金壳，85%在核心内部产生，14.2%在金壳内，而在水中可忽略不计（<1%）。与金封装相比，铂封装导致韧致辐射效应相似。与裸源相比，不锈钢封装将β粒子的射程减小了 1.1 毫米，但组织仍将从源胶囊几毫米处的β粒子接收剂量。金和铂胶囊不仅吸收了大部分电子，而且还衰减了低能光子。从核心和不锈钢壳逃逸的光子的平均能量为 113keV，而对于金和铂，平均能量分别为 160keV 和 165keV。
（170）Tm 源主要是韧致辐射源，大部分韧致辐射光子在源核心内产生，在源封装中几乎没有衰减。电子被金和铂封装有效地吸收。然而，对于不锈钢胶囊（或其他低 Z 封装），电子将逸出。这些电子产生的剂量在最初的几毫米内超过光子剂量，但目前的标准治疗计划系统并未考虑到这一点。从源中逸出的光子的总能量谱取决于封装组成，导致平均光子能量远高于 100keV。这高于 84keV 的主要伽马射线能量峰。根据我们的结果，将（170）Tm 用作近距离治疗源存在显著挑战。
