Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, 30 Fruit St, Boston, MA 02114, USA.
Phys Med Biol. 2012 Sep 7;57(17):N329-37. doi: 10.1088/0031-9155/57/17/N329. Epub 2012 Aug 15.
The role of MR imaging for image-guided radiation therapy (IGRT) is becoming more and more important thanks to the excellent soft tissue contrast offered by MRI. Hybrid therapy devices with integrated MRI scanners are under active development for x-ray therapy. The combination of proton therapy with MRI imaging has only been investigated at the theoretical or conceptual level. Of concern is the deflection of the proton beam in the homogeneous magnetic field. A previous publication has come to the conclusion that the impact of a 0.5 T magnetic field on the dose distribution for proton therapy is very small and lateral deflections stay well below 2 mm. The purpose of this study is to provide new insights into the effects of magnetic fields on a proton beam coming to rest in a patient. We performed an analytical calculation of the lateral deflection of protons with initial energies between 50 MeV and 250 MeV, perpendicular to the beam direction and the magnetic field. We used a power-law range-energy relationship and the Lorentz force in both relativistic and non-relativistic conditions. Calculations were done for protons coming to rest in water or soft tissue, and generalized to other uniform and non-uniform media. Results were verified by comparisons with numerical calculations and Monte Carlo simulations. A key result of our calculations is that the maximum lateral deflection at the end of range is proportional to the third power of the initial energy. Accordingly, due to the strong dependence on the energy, even a relatively small magnetic field of 0.5 T will cause a deflection of the proton beam by 1 cm at the end of range of a 200 MeV beam. The maximum deflection at 200 MeV is more than 10 times larger than that of a 90 MeV beam. Relativistic corrections of the deflection are generally small but they can become non-negligible at higher energies around 200 MeV and above. Contrary to previous findings, the lateral deflection of a proton beam can be significant (1 cm and above) even in relatively small magnetic fields of 0.5 T. However, the curved path of a proton beam in a magnetic field is easily predictable and it should be possible to account for this in treatment planning.
磁共振成像在图像引导放射治疗（IGRT）中的作用变得越来越重要，这要归功于 MRI 提供的出色软组织对比度。带有集成 MRI 扫描仪的混合治疗设备正在为 X 射线治疗进行积极开发。质子治疗与 MRI 成像的结合仅在理论或概念层面进行了研究。值得关注的是质子束在均匀磁场中的偏转。先前的一篇出版物得出的结论是，0.5 T 磁场对质子治疗剂量分布的影响非常小，侧向偏移远低于 2 毫米。本研究的目的是提供有关磁场对停留在患者体内的质子束的影响的新见解。我们对初始能量在 50 MeV 至 250 MeV 之间、垂直于光束方向和磁场的质子进行了侧向偏转的分析计算。我们在相对论和非相对论条件下使用幂律能谱关系和洛伦兹力。计算针对停留在水中或软组织中的质子，并推广到其他均匀和非均匀介质。计算结果通过与数值计算和蒙特卡罗模拟的比较进行了验证。我们计算的一个关键结果是，射程末端的最大侧向偏移与初始能量的立方成正比。因此，由于强烈依赖于能量，即使是相对较小的 0.5 T 磁场也会导致 200 MeV 束射程末端的质子束发生 1 厘米的偏移。200 MeV 时的最大偏移量是 90 MeV 束的 10 倍以上。偏转的相对论修正通常很小，但在 200 MeV 左右及以上的较高能量下可能变得不可忽略。与先前的发现相反，即使在相对较小的 0.5 T 磁场中，质子束的侧向偏移也可能很明显（1 厘米及以上）。然而，质子束在磁场中的弯曲路径很容易预测，在治疗计划中应该可以考虑到这一点。
