Oborn B M, Kolling S, Metcalfe P E, Crozier S, Litzenberg D W, Keall P J
Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, Australia.
Sydney Medical School, University of Sydney, NSW 2006, Australia.
Med Phys. 2014 May;41(5):051708. doi: 10.1118/1.4871618.
A potential side effect of inline MRI-linac systems is electron contamination focusing causing a high skin dose. In this work, the authors reexamine this prediction for an open bore 1 T MRI system being constructed for the Australian MRI-Linac Program. The efficiency of an electron contamination deflector (ECD) in purging electron contamination from the linac head is modeled, as well as the impact of a helium gas region between the deflector and phantom surface for lowering the amount of air-generated contamination.
Magnetic modeling of the 1 T MRI was used to generate 3D magnetic field maps both with and without the presence of an ECD located immediately below the MLC's. Forty-seven different ECD designs were modeled and for each the magnetic field map was imported into Geant4 Monte Carlo simulations including the linac head, ECD, and a 30 × 30 × 30 cm(3) water phantom located at isocenter. For the first generation system, the x-ray source to isocenter distance (SID) will be 160 cm, resulting in an 81.2 cm long air gap from the base of the ECD to the phantom surface. The first 71.2 cm was modeled as air or helium gas, with the latter encased between two windows of 50 μm thick high density polyethlyene. 2D skin doses (at 70 μm depth) were calculated across the phantom surface at 1 × 1 mm(2) resolution for 6 MV beams of field size of 5 × 5, 10 × 10, and 20 × 20 cm(2).
The skin dose was predicted to be of similar magnitude as the generic systems modeled in previous work, 230% to 1400% of D(max) for 5 × 5 to 20 × 20 cm(2), respectively. Inclusion of the ECD introduced a nonuniformity to the MRI imaging field that ranged from ∼20 to ∼140 ppm while the net force acting on the ECD ranged from ∼151 N to ∼1773 N. Various ECD designs were 100% efficient at purging the electron contamination into the ECD magnet banks; however, a small percentage were scattered back into the beam and continued to the phantom surface. Replacing a large portion of the extended air-column between the ECD and phantom surface with helium gas is a key element as it significantly minimized the air-generated contamination. When using an optimal ECD and helium gas region, the 70 μm skin dose is predicted to increase moderately inside a small hot spot over that of the case with no magnetic field present for the jaw defined square beams examined here. These increases include from 12% to 40% of [Formula: see text] for 5 × 5 cm(2), 18% to 55% of D(max) for 10 × 10 cm(2), and from 23% to 65% of D(max) for 20 × 20 cm(2).
Coupling an efficient ECD and helium gas region below the MLCs in the 160 cm isocenter MRI-linac system is predicted to ameliorate the impact electron contamination focusing has on skin dose increases. An ECD is practical as its impact on the MRI imaging distortion is correctable, and the mechanical forces acting on it manageable from an engineering point of view.
在线MRI直线加速器系统的一个潜在副作用是电子污染聚焦导致皮肤剂量过高。在这项工作中，作者重新审视了为澳大利亚MRI直线加速器项目建造的开放式1T MRI系统的这一预测。对电子污染偏转器（ECD）清除直线加速器头部电子污染的效率进行了建模，以及偏转器与体模表面之间的氦气区域对降低空气产生的污染量的影响。
利用1T MRI的磁场建模生成有无位于多叶准直器（MLC）正下方的ECD时的三维磁场图。对47种不同的ECD设计进行了建模，并将每种设计的磁场图导入Geant4蒙特卡罗模拟中，模拟包括直线加速器头部、ECD以及位于等中心的30×30×30 cm³水模体。对于第一代系统，X射线源到等中心的距离（SID）将为160 cm，导致从ECD底部到体模表面有81.2 cm长的气隙。前71.2 cm建模为空气或氦气，后者封装在两个50μm厚的高密度聚乙烯窗口之间。对于5×5、10×10和20×20 cm²的6 MV射野，以1×1 mm²的分辨率计算体模表面的二维皮肤剂量（在70μm深度处）。
预测皮肤剂量与先前工作中建模的通用系统幅度相似，对于5×5至20×20 cm²分别为D(max)的230%至1400%。ECD的引入给MRI成像场带来了不均匀性，范围从约20至约140 ppm，而作用在ECD上的净力范围从约151 N至约1773 N。各种ECD设计在将电子污染清除到ECD磁体组方面效率为100%；然而，一小部分被散射回射束并继续到达体模表面。用氦气取代ECD与体模表面之间大部分延长的气柱是一个关键因素，因为它显著减少了空气产生的污染。当使用最佳的ECD和氦气区域时，对于此处检查的下颌定义方形射束，预测在一个小热点内70μm皮肤剂量将比无磁场情况适度增加。这些增加包括5×5 cm²时为[公式：见原文]的12%至40%，10×10 cm²时为D(max)的18%至55%，以及20×20 cm²时为D(max)的23%至65%。
预计在160 cm等中心MRI直线加速器系统的MLC下方耦合高效的ECD和氦气区域可改善电子污染聚焦对皮肤剂量增加的影响。ECD是可行的，因为其对MRI成像畸变的影响是可校正的，并且从工程角度来看作用在其上的机械力是可管理的。
