Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
Phys Med Biol. 2019 Jun 5;64(11):115029. doi: 10.1088/1361-6560/ab0aa0.
The out-of-field surface dose contribution due to backscattered or ejected electrons, focused by the magnetic field, is evaluated in this work. This electron streaming effect (ESE) can contribute to out-of-field skin doses in orthogonal magnetic resonance guided radiation therapy machines. Using the EGSnrc Monte Carlo package, a phantom is set-up along the central axis of an incident 10 [Formula: see text] 10 cm 7 MV FFF photon beam. The phantom exit or entry surface is inclined with respect to the magnetic field, and an out-of-field water panel is positioned 10 cm away from, and centered on, the isocenter. The doses from streaming backscattered or ejected electrons, for either a 0.35 T or 1.5 T magnetic field, are evaluated in the out-of-field water panel for surface inclines of 10, 30, and 45°. The magnetic field focuses electrons emitted from the inclined phantom. Dose distributions at the surface of the out-of-field water panel are sharper in the 1.5 T magnetic field as compared to 0.35 T. The maximum doses for the 0.35 T simulations are 23.2%, 37.8%, and 39.0% for the respective 10, 30, and 45° simulations. For 1.5 T, for the same angles, the maximum values are 17.1%, 29.8%, and 35.8%. Dose values drop to below 2% within the first 1 cm of the out-of-field water phantom. The phantom thickness is an important variable in the magnitude of the ESE dose. The ESE can produce large out-of-field skin doses and must be a consideration in treatment planning in the MRgRT work-flow. Treatments often include multiple beams which will serve to spread out the effect, and many beams, such as anterior-posterior, will reduce the skin dose due to the ESE. A 1 cm thick shielding of either a bolus placed on the patient or mounted on the present RF coils would greatly reduce the ESE dose contributions. Further exploration of the capabilities of treatment planning systems to screen for this effect is required.
本文评估了由磁场聚焦的背散射或喷射电子引起的场外表面剂量贡献。这种电子流（ESE）效应可导致正交磁共振引导放射治疗机的场外皮肤剂量增加。使用 EGSnrc 蒙特卡罗程序包，在入射 10[Formula: see text]10 cm 7 MV FFF 光子束的中心轴上设置一个体模。体模出口或入口表面相对于磁场倾斜，一个场外水面板放置在距等中心 10 cm 处，并位于其中心。对于 0.35 T 或 1.5 T 磁场，评估了从倾斜体模中散射或喷射出的电子的流散射或喷射出的电子的剂量，对于表面倾斜角为 10、30 和 45°的场外水面板。磁场聚焦从倾斜体模发射的电子。与 0.35 T 相比，1.5 T 磁场下水面板表面的剂量分布更尖锐。0.35 T 模拟的最大剂量分别为 10、30 和 45°模拟的 23.2%、37.8%和 39.0%。对于 1.5 T，对于相同的角度，最大值分别为 17.1%、29.8%和 35.8%。在水模场外的前 1 cm 内，剂量值降至 2%以下。体模厚度是 ESE 剂量大小的一个重要变量。ESE 可产生大量的场外皮肤剂量，因此必须在 MRgRT 工作流程的治疗计划中考虑。治疗通常包括多个光束，这将有助于分散这种影响，而许多光束，如前后光束，由于 ESE，会降低皮肤剂量。在患者身上放置的治疗垫或安装在现有的 RF 线圈上的 1 cm 厚的屏蔽物会大大降低 ESE 剂量贡献。需要进一步探索治疗计划系统的能力，以筛选这种效应。
