Jelen Urszula, Dong Bin, Begg Jarrad, Roberts Natalia, Whelan Brendan, Keall Paul, Liney Gary
Department of Medical Physics, Ingham Institute for Applied Medical Research, Liverpool, NSW, Australia.
Liverpool Cancer Therapy Centre, Radiation Physics, Liverpool, NSW, Australia.
Front Oncol. 2020 Feb 14;10:136. doi: 10.3389/fonc.2020.00136. eCollection 2020.
Unique characteristics of MRI-linac systems and mutual interactions between their components pose specific challenges for their commissioning and quality assurance. The Australian MRI-linac is a prototype system which explores the inline orientation, with radiation beam parallel to the main magnetic field. The aim of this work was to commission the radiation-related aspects of this system for its application in clinical treatments. Physical alignment of the radiation beam to the magnetic field was fine-tuned and magnetic shielding of the radiation head was designed to achieve optimal beam characteristics. These steps were guided by investigative measurements of the beam properties. Subsequently, machine performance was benchmarked against the requirements of the IEC60976/77 standards. Finally, the geometric and dosimetric data was acquired, following the AAPM Task Group 106 recommendations, to characterize the beam for modeling in the treatment planning system and with Monte Carlo simulations. The magnetic field effects on the dose deposition and on the detector response have been taken into account and issues specific to the inline design have been highlighted. Alignment of the radiation beam axis and the imaging isocentre within 2 mm tolerance was obtained. The system was commissioned at two source-to-isocentre distances (SIDs): 2.4 and 1.8 m. Reproducibility and proportionality of the dose monitoring system met IEC criteria at the larger SID but slightly exceeded it at the shorter SID. Profile symmetry remained under 103% for the fields up to ~34 × 34 and 21 × 21 cm at the larger and shorter SID, respectively. No penumbra asymmetry, characteristic for transverse systems, was observed. The electron focusing effect, which results in high entrance doses on central axis, was quantified and methods to minimize it have been investigated. Methods were developed and employed to investigate and quantify the dosimetric properties of an inline MRI-Linac system. The Australian MRI-linac system has been fine-tuned in terms of beam properties and commissioned, constituting a key step toward the application of inline MRI-linacs for patient treatments.
MRI直线加速器系统的独特特性及其组件之间的相互作用，给其调试和质量保证带来了特殊挑战。澳大利亚的MRI直线加速器是一个原型系统，它探索了辐射束与主磁场平行的轴向配置。这项工作的目的是调试该系统与辐射相关的方面，以便将其应用于临床治疗。对辐射束与磁场的物理对准进行了微调，并设计了辐射头的磁屏蔽，以实现最佳的束流特性。这些步骤以束流特性的研究测量为指导。随后，根据IEC60976/77标准的要求对机器性能进行了基准测试。最后，按照美国医学物理师协会任务组106的建议，获取了几何和剂量学数据，以表征束流，用于治疗计划系统中的建模以及蒙特卡罗模拟。考虑了磁场对剂量沉积和探测器响应的影响，并突出了轴向设计特有的问题。实现了辐射束轴与成像等中心在2毫米公差范围内的对准。该系统在两个源轴距（SIDs）下进行了调试：2.4米和1.8米。剂量监测系统的重复性和比例性在较大的源轴距下符合IEC标准，但在较短的源轴距下略超出标准。在较大和较短的源轴距下，对于尺寸分别达约34×34厘米和21×21厘米的射野，剖面对称性分别保持在103%以下。未观察到横向系统特有的半值层不对称性。对导致中心轴上高入射剂量的电子聚焦效应进行了量化，并研究了将其降至最低的方法。开发并采用了一些方法来研究和量化轴向MRI直线加速器系统的剂量学特性。澳大利亚的MRI直线加速器系统在束流特性方面进行了微调并完成了调试，这是轴向MRI直线加速器应用于患者治疗的关键一步。
