新型旋转式 MRI 兼容屏蔽后装施源器在调强子宫颈腔内近距离放疗中的蒙特卡罗剂量学研究。

Monte Carlo dosimetry study of novel rotating MRI-compatible shielded tandems for intensity modulated cervix brachytherapy.

机构信息

Medical Physics Unit, McGill University, Montreal, QC, Canada; Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University, Baltimore, MD, USA.

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

出版信息

Phys Med. 2020 Mar;71:178-184. doi: 10.1016/j.ejmp.2020.02.014. Epub 2020 Mar 9.

DOI:10.1016/j.ejmp.2020.02.014

PMID:32163887

Abstract

PURPOSE

Intensity modulated brachytherapy (IMBT) with rotating metal shields enables dose modulation that can better conform to the tumor while reducing OAR doses. In this work, we investigate novel rotating shields, compatible with MRI-compatible tandems used for cervix brachytherapy. Three unique shields were evaluated using the traditional Ir source. Additionally, Se and Yb isotopes were investigated as alternative sources.

METHODS

Three different IMBT shields were modeled and simulated in RapidBrachyMCTPS. Each tungsten shield was designed to fit inside a 6 mm-wide MRI-compatible tandem. The active core of the source was replaced with Ir, Se and Yb. Transmission factors (TFs) were calculated and defined as the dose ratio at 1 cm on opposite sides of the shielded tandem on the transverse plane. Polar and azimuthal anisotropy plots were extracted from simulations. Dose homogeneities VV were calculated for all radionuclide-shield combinations.

RESULTS

TFs are favorable for IMBT and ranged between 12.9% and 32.2% for Ir, 4.0%-16.1% for Se and 1.2-6.4% for Yb for all shield designs. Average beam-widths in the polar and azimuthal directions were reduced to the range of 42°-112° and 27°-107°, respectively, for all shield-radionuclide combinations. Dose homogeneities for all the radionuclide-shield combinations were within 12% of the non-IMBT tandem.

CONCLUSIONS

This study has quantitatively assessed the influence of various rotating cervical cancer-specific IMBT tandem shields on dosimetry. The dynamic single-channel shields and narrow beam-widths in the polar and azimuthal direction give rise to highly anisotropic distributions. Intermediate-to-high energy radionuclides, Se and Yb substantially improve the modulation capacity of IMBT and pave the way for treating large and complex cervical cancer without interstitial needle implantation.

摘要

目的

带旋转金属屏蔽的调强近距离放射治疗（IMBT）可实现剂量调制，在降低 OAR 剂量的同时更好地适形于肿瘤。本研究旨在探讨与用于宫颈癌近距离放射治疗的 MRI 兼容施源器兼容的新型旋转屏蔽。采用传统 Ir 源评估了三种独特的屏蔽。此外，还研究了 Se 和 Yb 同位素作为替代源。

方法

在 RapidBrachyMCTPS 中对三种不同的 IMBT 屏蔽进行建模和模拟。每个钨屏蔽均设计成可置于 6mm 宽的 MRI 兼容施源器内。源的活性芯被 Ir、Se 和 Yb 取代。计算了传输因子（TF），并定义为屏蔽施源器横截面上对侧 1cm 处的剂量比。从模拟中提取极向和方位角各向异性图。计算了所有放射性核素-屏蔽组合的均匀性 VV。

结果

TF 有利于 IMBT，Ir 的 TF 范围为 12.9%-32.2%，Se 的 TF 范围为 4.0%-16.1%，Yb 的 TF 范围为 1.2%-6.4%，所有屏蔽设计均如此。对于所有屏蔽-放射性核素组合，极向和方位角方向的平均束宽分别减小至 42°-112°和 27°-107°。所有放射性核素-屏蔽组合的均匀性均在非 IMBT 施源器的 12%以内。