评估直线加速器多叶准直器和机架下垂对容积调强弧形治疗的影响。

Evaluation of the Impact of the Linac MLC and Gantry Sag in volumetric modulated arc therapy.

机构信息

Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, 6009, Australia.

Department of Physics, University of Western Australia, Crawley, WA, 6009, Australia.

出版信息

Med Phys. 2019 May;46(5):1984-1994. doi: 10.1002/mp.13491. Epub 2019 Apr 1.

DOI:10.1002/mp.13491

PMID:30870581

Abstract

PURPOSE

Mechanical sag in the radiotherapy linear accelerator gantry and multi-leaf collimator (MLC) carriage effectively causes systematic deviations in the isocenter with respect to gantry angle. To minimize the impact of this error on treatment, a tolerance value of a 1-mm mechanical isocenter shift is commonly accepted for intensity-modulated radiation therapy quality assurance (QA). However, this tolerance value has not been firmly established for volumetric modulated arc therapy (VMAT) treatments. The purpose of this study is therefore to evaluate the impact of gantry and MLC carriage sag on VMAT clinical performance.

METHODS

A published dataset of Elekta and Varian sag measurements served as a starting point for the investigation. Typical sag profiles were chosen and modeled as continuous isocenter deviations in three dimensions. The data were then incorporated into existing Digital Imaging and Communications in Medicine protocol, extended for radiotherapy plans via a "beam-splitting" algorithm. Three treatment sites were investigated in parallel: head and neck, prostate, and prostate with surrounding lymph nodes. Monte Carlo-simulated dose distributions were obtained for varying magnifications of the modeled sag. The resulting dose distributions, including that for no error, were compared qualitatively and quantitatively, against multiple metrics.

RESULTS

The dose-volume histograms (DVHs) for all plans exhibited a decrease in planning target volume (PTV) dose uniformity with increasing sag magnification, whereas dose to organs at risk exhibited no coherent trend. The prostate plan was shown to be the most vulnerable to mechanical sag across all considered metrics. However, all plans with peak isocenter deviation less than 1 mm were well within typical cutoff points for each metric.

CONCLUSIONS

All avenues of investigation presented substantiate the commonly accepted tolerance value of a 1-mm peak isocenter shift in annual linac QA.

摘要

目的

放疗直线加速器龙门架和多叶准直器（MLC）滑架的机械下垂会导致等中心相对于龙门架角度的系统偏差。为了最大限度地减少该误差对治疗的影响，通常接受强度调制放疗（IMRT）质量保证（QA）的 1 毫米机械等中心移位容差值。然而，对于容积调强弧形治疗（VMAT）治疗，尚未确定该容差值。因此，本研究的目的是评估龙门架和 MLC 滑架下垂对 VMAT 临床性能的影响。

方法

使用 Elekta 和 Varian 下垂测量的已发表数据集作为研究的起点。选择典型的下垂曲线并将其建模为三维连续等中心偏差。然后将数据纳入现有的数字成像和通信医学协议中，并通过“光束分裂”算法扩展为放射治疗计划。同时研究了三个治疗部位：头颈部、前列腺和周围淋巴结的前列腺。对模型下垂的不同放大倍数获得了蒙特卡罗模拟的剂量分布。将包括无误差的剂量分布与多种指标进行定性和定量比较。