用于肝脏转移立体定向体部放射治疗的术前质量保证中，在自由匀场射束下对数字兆伏级图像进行剂量学验证。

Dosimetric Validation of Digital Megavolt Imager for Flattening Filter Free Beams in the Pre-Treatment Quality Assurance of Stereotactic Body Radiation Therapy for Liver Metastases.

机构信息

Department of Radiation Oncology, Apollo Cancer Institute, Chennai, Tamilnadu, India.

Department of Physics, School of Advanced Sciences, VIT University, Vellore, Tamilnadu, India.

出版信息

Asian Pac J Cancer Prev. 2020 Jun 1;21(6):1659-1665. doi: 10.31557/APJCP.2020.21.6.1659.

DOI:10.31557/APJCP.2020.21.6.1659

PMID:32592361

原文链接:https://pmc.ncbi.nlm.nih.gov/articles/PMC7568874/

Abstract

AIM

The aim is to evaluate the use of digital megavolt imager (DMI) aS1200 in portal dosimetry with flattening filter free (FFF) beams.

MATERIALS AND METHODS

Dosimetric properties of DMI is characterized at 6MV FFF beams for signal saturation, dose linearity, dependency on dose-rate and source-detector distance (SDD), signal lag (ghosting), and back scatter. Portal dosimetry is done for twenty volumetric modulated arc therapy (VMAT) based stereotactic body radiotherapy (SBRT) plans for the treatment of liver metastases and the results are compared with repeated measurements of Octavius 4D.

RESULTS

The detector signal to monitor unit (MU) ratio drops drastically below 25MU. The detector linearity with dose is within 1% and no evidence of signal saturation as such. The aS1200 response variation across various dose rates and SDD is <0.4% and <0.2% respectively. The effect of ghosting increased distinctly at higher dose rate but however it is negligible (0.1%). The impact of back scatter is <0.3% because of additional shielding provided at the back of the detector. The portal dosimetry results of SBRT QA plans evaluated at the gamma criteria of 2mm/2% (DTA/DD) both under global and local mode analysis has shown an average gamma passing rate of area gamma (<1) 97.9±0.8% and 96.4±0.9%. The SBRT QA results observed in aS1200 are inline and consistent with Octavius 4D measured results.

CONCLUSION

The characteristics of aS1200 evaluated at FFF beams have shown its potential ability as QA tool and can be used in SBRT QA for liver metastases with greater confidence.

摘要

目的

旨在评估数字兆伏成像仪（DMI）aS1200 在使用无均整过滤器（FFF）射束时的适形调强治疗（VMAT）立体定向体部放疗（SBRT）中的剂量验证中的应用。

材料和方法

在 6MVFFF 射束下对 DMI 的剂量学特性进行了特征描述，包括信号饱和、剂量线性、剂量率和源-探测器距离（SDD）依赖性、信号滞后（鬼影）和背散射。对 20 例肝脏转移立体定向体部放疗（SBRT）的容积调强弧形治疗（VMAT）计划进行了剂量验证，结果与 Octavius 4D 的重复测量结果进行了比较。

结果

探测器信号与监测单位（MU）比值在低于 25MU 时急剧下降。探测器的剂量线性在 1%以内，没有信号饱和的证据。aS1200 在不同剂量率和 SDD 下的响应变化分别小于 0.4%和 0.2%。在较高的剂量率下，鬼影的影响明显增加，但可以忽略不计（0.1%）。由于探测器背面增加了额外的屏蔽，背散射的影响小于 0.3%。SBRT QA 计划的剂量验证结果在全局和局部模式分析下的 2mm/2%（DTA/DD）伽玛标准下评估，平均区域伽玛通过率（面积伽玛（<1））为 97.9±0.8%和 96.4±0.9%。aS1200 观察到的 SBRT QA 结果与 Octavius 4D 测量结果一致。