Oborn B M, Dowdell S, Metcalfe P E, Crozier S, Mohan R, 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.
Shoalhaven Cancer Care Centre, Nowra, NSW 2541, Australia.
Med Phys. 2015 May;42(5):2113-24. doi: 10.1118/1.4916661.
This paper investigates, via magnetic modeling and Monte Carlo simulation, the ability to deliver proton beams to the treatment zone inside a split-bore MRI-guided proton therapy system.
Field maps from a split-bore 1 T MRI-Linac system are used as input to geant4 Monte Carlo simulations which model the trajectory of proton beams during their paths to the isocenter of the treatment area. Both inline (along the MRI bore) and perpendicular (through the split-bore gap) orientations are simulated. Monoenergetic parallel and diverging beams of energy 90, 195, and 300 MeV starting from 1.5 and 5 m above isocenter are modeled. A phase space file detailing a 2D calibration pattern is used to set the particle starting positions, and their spatial location as they cross isocenter is recorded. No beam scattering, collimation, or modulation of the proton beams is modeled.
In the inline orientation, the radial symmetry of the solenoidal style fringe field acts to rotate the protons around the beam's central axis. For protons starting at 1.5 m from isocenter, this rotation is 19° (90 MeV) and 9.8° (300 MeV). A minor focusing toward the beam's central axis is also seen, but only significant, i.e., 2 mm shift at 150 mm off-axis, for 90 MeV protons. For the perpendicular orientation, the main MRI field and near fringe field act as the strongest to deflect the protons in a consistent direction. When starting from 1.5 m above isocenter shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed. Further to this, off-axis protons are slightly deflected toward or away from the central axis in the direction perpendicular to the main deflection direction. This leads to a distortion of the phase space pattern, not just a shift. This distortion increases from zero at the central axis to 10 mm (90 MeV) and 5 mm (300 MeV) for a proton 150 mm off-axis. In both orientations, there is a small but subtle difference in the deflection and distortion pattern between protons fired parallel to the beam axis and those fired from a point source. This is indicative of the 3D spatially variant nature of the MRI fringe field.
For the first time, accurate magnetic and Monte Carlo modeling have been used to assess the transport of generic proton beams toward a 1 T split-bore MRI. Significant rotation is observed in the inline orientation, while more complex deflection and distortion are seen in the perpendicular orientation. The results of this study suggest that due to the complexity and energy-dependent nature of the magnetic deflection and distortion, the pencil beam scanning method will be the only choice for delivering a therapeutic proton beam inside a potential MRI-guided proton therapy system in either the inline or perpendicular orientation. Further to this, significant correction strategies will be required to account for the MRI fringe fields.
本文通过磁场建模和蒙特卡罗模拟，研究在分体式孔腔MRI引导质子治疗系统中向治疗区域内输送质子束的能力。
将分体式孔腔1T MRI直线加速器系统的场图用作geant4蒙特卡罗模拟的输入，该模拟对质子束在其通往治疗区域等中心路径上的轨迹进行建模。模拟了沿直线（沿MRI孔腔）和垂直（穿过分体式孔腔间隙）方向。对从等中心上方1.5米和5米处起始的能量为90、195和300 MeV的单能平行和发散束进行了建模。使用详细描述二维校准图案的相空间文件来设置粒子起始位置，并记录它们穿过等中心时的空间位置。未对质子束的散射、准直或调制进行建模。
在直线方向上，螺线管式边缘场的径向对称性使质子绕束的中心轴旋转。对于从等中心1.5米处起始的质子，这种旋转为19°（90 MeV）和9.8°（300 MeV）。还观察到向束中心轴的轻微聚焦，但仅对于90 MeV质子显著，即在离轴150毫米处有2毫米的偏移。对于垂直方向，主要的MRI场和近边缘场在使质子沿一致方向偏转方面作用最强。当从等中心上方1.5米处起始时，观察到90 MeV质子有135毫米的偏移和300 MeV质子有65毫米的偏移。此外，离轴质子在垂直于主偏转方向的方向上会稍微向中心轴或远离中心轴偏转。这导致相空间图案发生畸变，而不仅仅是偏移。这种畸变从中心轴处的零增加到离轴150毫米处的质子的10毫米（90 MeV）和5毫米（300 MeV）。在两个方向上，平行于束轴发射的质子和从点源发射的质子之间在偏转和畸变图案上存在微小但细微的差异。这表明MRI边缘场的三维空间变化性质。
首次使用精确的磁场和蒙特卡罗建模来评估通用质子束向1T分体式孔腔MRI的传输。在直线方向上观察到显著的旋转，而在垂直方向上看到更复杂的偏转和畸变。本研究结果表明，由于磁偏转和畸变的复杂性以及能量依赖性，笔形束扫描方法将是在潜在的MRI引导质子治疗系统中以直线或垂直方向输送治疗性质子束的唯一选择。此外，需要重大的校正策略来考虑MRI边缘场。
