Technical note: Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in‐beam MR scanner

Abstract

BackgroundAs it promises more precise and conformal radiation treatments, magnetic resonance imaging‐integrated proton therapy (MRiPT) is seen as a next step in image guidance for proton therapy. The Lorentz force, which affects the course of the proton pencil beams, presents a problem for beam delivery in the presence of a magnetic field.PurposeTo investigate the influence of the 0.32‐T perpendicular magnetic field of an MR scanner on the delivery of proton pencil beams inside an MRiPT prototype system.MethodsAn MRiPT prototype comprising of a horizontal pencil beam scanning beam line and an open 0.32‐T MR scanner was used to evaluate the impact of the vertical magnetic field on proton beam deflection and dose spot pattern deformation. Three different proton energies (100, 150, and 220 MeV) and two spot map sizes (15 × 15 and 30 × 20 cm2) at four locations along the beam path without and with magnetic field were measured. Pencil‐beam dose spots were measured using EBT3 films and a 2D scintillation detector. To study the magnetic field effects, a 2D Gaussian fit was applied to each individual dose spot to determine the central position , minimum and maximum lateral standard deviation ( and ), orientation (θ), and the eccentricity (ε).ResultsThe dose spots were subjected to three simultaneous effects: (a) lateral horizontal beam deflection, (b) asymmetric trapezoidal deformation of the dose spot pattern, and (c) deformation and rotation of individual dose spots. The strongest effects were observed at a proton energy of 100 MeV with a horizontal beam deflection of 14–186 mm along the beam path. Within the central imaging field of the MR scanner, the maximum relative dose spot size decreased by up to 3.66%, while increased by a maximum of 2.15%. The largest decrease and increase in the eccentricity of the dose spots were 0.08 and 0.02, respectively. The spot orientation θ was rotated by a maximum of 5.39°. At the higher proton energies, the same effects were still seen, although to a lesser degree.ConclusionsThe effect of an MRiPT prototype's magnetic field on the proton beam path, dose spot pattern, and dose spot form has been measured for the first time. The findings show that the impact of the MF must be appropriately recognized in a future MRiPT treatment planning system. The results emphasize the need for additional research (e.g., effect of magnetic field on proton beams with range shifters and impact of MR imaging sequences) before MRiPT applications can be employed to treat patients.

Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane‐parallel ionization chamber

“…In addition to the dependence of c ⃗ B on the lateral position, as discussed by Oborn et al 14 and Gebauer et al 15 due to lateral displacement and field deformation towards the field edges, it is important to note that c ⃗ B also exhibits deviations from unity in the Bragg peak region. These deviations remained small in the central regions of the field, but the Bragg peak region showed noticeable effects on the absorbed dose-to-water influenced by the MF.…”

“…Grevillot et al 32 used the same beamline and observed only minor fluctuations in the proton field homogeneity for a shifted field. In addition, Gebauer et al 15 reported minimal relative shifts (<0.5 mm) of proton pencil beams within the central 5 × 5 cm 2 region of a proton field measured in a 0.32 T MRI machine. This suggests that potential uncertainties arising from a proton field shift are unlikely to have a substantial effect on the measurement results in our study.…”

“…On the other hand, the magnetic imaging field of the MR scanner affects the path of protons and subsequent secondary electrons, leading to bent proton trajectories and spiraling secondary electrons. 14,15 The resulting distortion of the dose distribution influences both treatment planning and ionization chamber-based dosimetry.…”