Proton beam behavior in a parallel configured MRI‐proton therapy hybrid: Effects of time‐varying gradient magnetic fields

Abstract

Purpose Real‐time magnetic resonance (MR) guidance is of interest to various groups globally because the superior soft tissue contrast MR images offer over other x‐ray‐based imaging modalities. Because of the precision required in proton therapy, proton therapy treatments rely heavily on image guidance. Integrating a magnetic resonance imaging (MRI) into a proton therapy treatment is a challenge. The charged particles (protons) used in proton therapy experience magnetic forces when travelling through the MRI magnetic fields. Given that it is desired that proton beams can be delivered with submillimeter accuracy, it is important that all potential sources of beam displacement are well modeled and understood. This study investigated the behavior of monoenergetic proton beams in the presence of a simulated set of realistic three‐dimensional (3D) vector magnetic gradient fields required for spatial localization during imaging. This deflecting source has not been previously investigated. Methods Three‐dimensional magnetic vector fields from a superconducting 0.5 T open bore MRI magnet model (previously developed in‐house) and 3D magnetic fields from an in‐house gradient coil model were applied to two types of computer simulations. In all simulations, monoenergetic proton pencil beams (from 80 to 250 MeV) were used. The initial directions of proton beams were varied. In all simulations, the orientation of the B0 field coincided with the positive z‐axis in the simulation geometry. The first type of simulation is based on an analytic magnetic force equation (analytic simulations) while the second type is a full Monte Carlo (MC) simulation. The analytic simulations were limited to propagating the proton beams in vacuum but could be rapidly calculated in a desktop computer while the MC simulations were calculated in a cluster computer. The proton beam locations and dose profiles at the central plane (z = 0 cm) with or without magnetic fields were extracted and used to quantify the effect of the presence of the different magnetic fields on the proton beam. Results The analytic simulations agree with MC results within 0.025 mm, thus acting as the verification of MC calculations. The presence of the B0 field caused the beam to follow a helical trajectory which resulted in angular offsets of 4.9o, 3.6o, and 2.8o for the 80, 150, and 250 MeV, respectively. Magnetic field deflections caused by a rapid MRI sequence (bSSFP, with maximum gradient strength of 40 mT/m) show a pattern of distortion which remained spatially invariant in the MR's field of view. For the 80 MeV beam, this pattern shows a maximum ranged in the y direction of 1.5 mm. The presence of the B0 field during the bSSFP simulations adds the same beam rotation to the observed during the B0 only simulations. Conclusion This investigation reveals that time‐varying gradient magnetic fields required for image generation can cause a small spread in the proton beams used in the study which are independent of the effects arising from the B0 field. Further, studies where cl...

Evaluation of proton and photon dose distributions recalculated on 2D and 3D Unet-generated pseudoCTs from T1-weighted MR head scans

“…Recently, the use of adaptive strategies for photon therapy has increased with the introduction of hybrid machines combining linear accelerators with MR scanners [3,4]. Given that advances in photon radiation therapy are typically transferred to proton radiation therapy, there has been a growing interest for MR-guided proton therapy research in the last years [5][6][7][8][9][10][11][12][13][14][15][16][17]. For online adaptive radiation therapy, replanning strategies have to be fast and reliable.…”

Benchmarking a GATE/Geant4 Monte Carlo model for proton beams in magnetic fields

“…A more detailed evaluation could potentially be performed using a finite element model 9 to describe also the field components due to beam line scanning magnets or including time-varying magnetic fields. 13 In the research beam line used in this work, scanning magnets from the PBS delivery system are located 670.0 and 740.0 cm away from the treatment isocenter. Therefore, their influence on the resultant magnetic field was neglected.…”

“…[1][2][3][4][5][6][7] First studies on the feasibility of MRPT were already performed from a technical 5,8 and dosimetric point 1,3,4 of view, showing that proper compensations for lateral beam bending due to magnetic fields are strictly required. To predict the effect of magnetic fields on beam trajectories and dose calculations, analytical 2,[9][10][11][12][13] and full-based Monte Carlo (MC) methods 1,3,4,6,14 can be employed.…”

“…The accuracy of dose calculations for a future MRPT system relies on a complete and detailed description of all single pencil beam interactions not only with homogenous, but also with fringe magnetic fields. 9,13 For each specific MRPT design, it is henceforth required to create an accurate beam model describing the amount of lateral and angular deflection of every single beam path toward and within the patient. An experimental validation of the proposed beam model is then required to ensure that all possible sources of deflection due to different magnetic fields present on the beam line and the MR core have been considered.…”

Technical Note: Design and commissioning of a water phantom for proton dosimetry in magnetic fields

“…[3][4][5][6] The impact of magnetic fields on the dose distribution, treatment delivery system as well as dosimetry were mostly studied textitin silico as only few facilities exist offering magnetic fields in a proton therapy research room. [7][8][9][10][11][12] In cooperation of the Medical University of Vienna and the MedAustron ion therapy center, a research magnet, capable of creating magnetic field strengths up to 1 T, was installed in an experimental particle therapy beam line.…”