在磁共振成像引导放疗中，研究1.5T横向磁场存在时治疗束的剂量扰动效应。

Shao Wencheng, Tang Xiaobin, Bai Yanling, Shu Diyun, Geng Changran, Gong Chunhui, Guan Fada

Department of Nuclear Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing; Department of Radiation Physics, Harbin Medical University Cancer Hospital, Harbin, China.

Department of Nuclear Science and Engineering, Nanjing University of Aeronautics and Astronautics; Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Nanjing, China.

J Cancer Res Ther. 2018 Jan;14(1):184-195. doi: 10.4103/jcrt.JCRT_1349_16.

BACKGROUND

Magnetic resonance imaging (MRI)-guided radiotherapy is a promising image-guided cancer radiotherapy method. For MRI-guided radiotherapy, the proper energy of a therapeutic beam is important for beam-designing processes, and the magnetic-induced dose perturbation would be mainly influenced, especially the perturbation surrounding the tissue-air or air-tissue interfaces. Thus, it was necessary to investigate the impact of beam energy from photon, proton, and carbon ion beams on the magnetic-induced dose perturbations.

MATERIALS AND METHODS

Using a phantom of a water-air-water structure, the dose distributions were calculated with or without the presence of a 1.5 T uniform magnetic field through GEANT4. Based on the calculated doses, magnetic-induced dose perturbations were then obtained. For investigating the effects of beam energies on magnetic-induced dose perturbations, low-, middle-, and high-beam energies were adopted for each beam type.

RESULTS AND DISCUSSION

For photon beams, the dose perturbations were increased as the beam energies increased. At the up water-air interface, the maximum perturbations exceeded 50%. Near the edge of the radiation field, perturbations of 5%-20% were achieved. For proton and carbon ion beams, their Bragg peaks were shifted from original positions, and the shifting distances were increased with the increased beam energies. However, no evident magnetic-induced dose perturbations were noted at the up water-air interface and bottom air-water interface for all the beam energies. To some extent, this study provided references for assessing the effects of beam energies on magnetic-induced dose perturbations, especially the perturbations around the air cavities inside cancer patients.

CONCLUSION

In MRI-guided cancer radiotherapy, the dose perturbation effects for therapeutic beams are relatively obvious, and the beam energies of therapeutic beams have large impacts on the magnetic-induced dose perturbations with the presence of a 1.5 T transverse magnetic field.

背景

磁共振成像（MRI）引导的放射治疗是一种很有前景的图像引导癌症放射治疗方法。对于MRI引导的放射治疗，治疗束的合适能量对于束设计过程很重要，并且磁致剂量扰动会受到主要影响，尤其是在组织 - 空气或空气 - 组织界面周围的扰动。因此，有必要研究光子、质子和碳离子束的束能量对磁致剂量扰动的影响。

材料与方法

使用水 - 空气 - 水结构的模体，通过GEANT4计算在有或没有1.5 T均匀磁场存在的情况下的剂量分布。基于计算出的剂量，然后获得磁致剂量扰动。为了研究束能量对磁致剂量扰动的影响，每种束类型采用低、中、高束能量。

结果与讨论

对于光子束，剂量扰动随着束能量的增加而增加。在上部水 - 空气界面处，最大扰动超过50%。在辐射场边缘附近，实现了5% - 20%的扰动。对于质子和碳离子束，它们的布拉格峰从原始位置发生了偏移，并且偏移距离随着束能量的增加而增加。然而，对于所有束能量，在上部水 - 空气界面和底部空气 - 水界面处均未观察到明显的磁致剂量扰动。在一定程度上，本研究为评估束能量对磁致剂量扰动的影响提供了参考，特别是癌症患者体内气腔周围的扰动。