Objective: MR-linac machines are being developed for image-guided radiation therapy but the magnetic field of such machines could affect dose distributions. The purpose of this work was to evaluate the effect of a magnetic field on linac beam dosimetric parameters including penumbra for circular cones used in radiosurgery. Methods: Monte Carlo simulation was conducted for a linac machine with circular cones at 6 MV beam. A homogenous magnetic field of 1.5 T was applied transversely and parallel to the radiation beam. Percentage depth dose (PDD) and beam profiles in a water phantom with and without the magnetic field were calculated. Results: The results have shown that when the magnetic field is applied transversely, the PDDs in the water phantom differ in the buildup region and distant part of PDD curves. The beam profiles at three different depths are all significantly different from those without the magnetic field. The penumbra is greater when a magnetic field has been applied. Conclusion: Linear accelerator-based SRT and SRS use small circular cones. The beam penumbra for these cones can change in the presence of a magnetic field. The perturbation of dose distribution has been also observed in a patient plan due to the presence of a magnetic field. The results of this study show that dose distributions in the presence of a magnetic field must be considered for MR-guided radiotherapy treatments.

目的：正在开发用于图像引导放射治疗的 MR-linac 机器，但此类机器的磁场可能会影响剂量分布。这项工作的目的是评估磁场对直线加速器光束剂量学参数的影响，包括放射外科中使用的圆锥体的半影。方法：蒙特卡罗模拟在 6 MV 光束下对具有圆锥的直线加速器进行。横向并平行于辐射束施加 1.5 T 的均匀磁场。计算了有和没有磁场的水体模中的百分比深度剂量 (PDD) 和光束剖面。结果：结果表明，当横向施加磁场时，水体模型中的PDDs在PDD曲线的堆积区域和远处部分不同。三个不同深度的光束轮廓与没有磁场的光束轮廓都有很大的不同。当施加磁场时，半影更大。结论：基于直线加速器的 SRT 和 SRS 使用小圆锥。这些锥体的光束半影可以在存在磁场的情况下发生变化。由于磁场的存在，在患者计划中也观察到了剂量分布的扰动。这项研究的结果表明，对于 MR 引导的放射治疗，必须考虑磁场存在下的剂量分布。