Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in-beam MR scanner

Background: As it promises more precise and conformal radiation treatments, magnetic
resonance imaging-integrated proton therapy (MRiPT) is seen as the next development 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.
Purpose: To 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.
Methods: An 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 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 cm2 and 30×20 cm2) at four lo-
cations 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 determine
the magnetic field effects, a 2D Gaussian fit was applied to each individual dose spot to
determine the central position (X, Y ), minimum and maximum lateral standard deviation
(σmin and σmax), orientation (θ), and the eccentricity (ε) was calculated.
Results: The 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 to 186 mm along the
beam path. Within the central imaging field of the MR scanner, the maximum relative dose
spot size σmax decreased by up to 3.66%, while σmin 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 theta was rotated by a maximum of 5.39◦. At the higher
proton energies, the same effects were still seen, although to a lesser degree.
Conclusions: The 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 treat-
ment 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.