镱-169、硒-75 和铱-192 放射性同位素在高剂量率腔内近距离放射治疗中的剂量学考虑。

Dosimetric Considerations for Ytterbium-169, Selenium-75, and Iridium-192 Radioisotopes in High-Dose-Rate Endorectal Brachytherapy.

机构信息

Medical Physics Unit, McGill University, Montreal, Quebec, Canada; Research Institute of the McGill University Health Centre, Montreal, Quebec, Canada.

Department of Oncology, McGill University, Montreal, Quebec, Canada; Department of Radiation Oncology, Jewish General Hospital, McGill University, Montreal, Quebec, Canada.

出版信息

Int J Radiat Oncol Biol Phys. 2019 Nov 15;105(4):875-883. doi: 10.1016/j.ijrobp.2019.07.003. Epub 2019 Jul 19.

DOI:10.1016/j.ijrobp.2019.07.003

PMID:31330175

Abstract

PURPOSE

To investigate differences between prescribed and postimplant calculated dose in Ir high-dose-rate endorectal brachytherapy (HDR-EBT) by evaluating dose to clinical target volume (CTV) and organs at risk (OARs) calculated with a Monte Carlo-based dose calculation software, RapidBrachyMC. In addition, dose coverage, conformity, and homogeneity were compared among the radionuclides Ir, Se, and Yb for use in HDR-EBT.

METHODS AND MATERIALS

Postimplant dosimetry was evaluated using 23 computed tomography (CT) images from patients treated with HDR-EBT using the Ir microSelectron v2 (Elekta AB, Stockholm, Sweden) source and the Intracavitary Mold Applicator Set (Elekta AB, Stockholm, Sweden), which is a flexible applicator capable of fitting a tungsten rod for OAR shielding. Four tissue segmentation schemes were evaluated: (1) TG-43 formalism, (2) materials and nominal densities assigned to contours of foreign objects, (3) materials and nominal densities assigned to contoured organs in addition to foreign objects, and (4) materials specified as in (3) but with voxel mass densities derived from CT Hounsfield units. Clinical plans optimized for Ir were used, with the results for Se and Yb normalized to the D of the Ir clinical plan.

RESULTS

In comparison to segmentation scheme 4, TG-43-based dosimetry overestimates CTV D by 6% (P = .00003), rectum D by 24% (P = .00003), and pelvic bone D by 5% (P = .00003) for Ir. For Yb, CTV D is overestimated by 17% (P = .00003) and rectum D by 39% (P = .00003), and pelvic bone D is significantly underestimated by 27% (P = .007). Postimplant dosimetry calculations also showed that a Yb source would give 20% (P = .00003) lower rectum V and 17% (P = .00008) lower rectum D.

CONCLUSIONS

Ignoring high-Z materials in dose calculation contributes to inaccuracies that may lead to suboptimal dose optimization and disagreement between prescribed and calculated dose. This is especially important for low-energy radionuclides. Our results also show that with future magnetic resonance imaging-based treatment planning, loss of CT density data will only affect calculated dose in nonbone OARs by 2% or less and bone OARs by 13% or less across all sources if material composition and nominal mass densities are correctly assigned.

摘要

目的

通过评估使用基于蒙特卡罗的剂量计算软件 RapidBrachyMC 计算的临床靶区（CTV）和危及器官（OAR）的剂量，研究 Ir 高剂量率腔内近距离治疗（HDR-EBT）中计划与植入后计算剂量之间的差异。此外，还比较了 Ir、Se 和 Yb 用于 HDR-EBT 的放射性核素的剂量覆盖、适形度和均匀度。

方法与材料

使用 23 名接受 Ir 微选择器 v2（Elekta AB，斯德哥尔摩，瑞典）源和腔内模具敷贴器套件（Elekta AB，斯德哥尔摩，瑞典）治疗的 HDR-EBT 患者的 23 个计算机断层扫描（CT）图像进行植入后剂量评估，腔内模具敷贴器套件是一种可容纳钨棒用于 OAR 屏蔽的柔性敷贴器。评估了四种组织分割方案：（1）TG-43 形式，（2）为异物轮廓指定的材料和名义密度，（3）为轮廓器官和异物指定的材料和名义密度，（4）与（3）相同的材料，但体素质量密度来自 CT 亨氏单位。使用针对 Ir 优化的临床计划，将 Se 和 Yb 的结果归一化为 Ir 临床计划的 D。

结果

与方案 4 相比，基于 TG-43 的剂量学高估了 Ir 的 CTV D 6%（P=0.00003），直肠 D 24%（P=0.00003），骨盆骨 D 5%（P=0.00003）。对于 Yb，CTV D 高估了 17%（P=0.00003），直肠 D 高估了 39%（P=0.00003），骨盆骨 D 显著低估了 27%（P=0.007）。植入后剂量计算还表明，Yb 源会使直肠 V 降低 20%（P=0.00003），直肠 D 降低 17%（P=0.00008）。

结论

在剂量计算中忽略高 Z 材料会导致不准确，从而可能导致剂量优化不理想和计划剂量与计算剂量之间存在差异。这对于低能放射性核素尤其重要。我们的结果还表明，如果正确分配材料组成和名义质量密度，在未来基于磁共振成像的治疗计划中，即使丢失 CT 密度数据，对非骨 OAR 的计算剂量的影响也不会超过 2%，对所有源的骨 OAR 的影响也不会超过 13%。