Beam trajectory correction and dose distribution in the presence of fringe fields in magnetic resonance imaging-guided proton therapy
Objective To explore the correction of beam trajectories in the presence of fringe fields in magnetic resonance imaging-guided proton therapy and dose changes in the body before and after correction.Methods The open-source treatment planning software matRad was used to design plans for brain tumor,liver tumor,and prostate cancer cases,and simulation studies were conducted in the Monte Carlo simulation toolkit TOPAS to calculate proton dose distribution.A proton beam trajectory correction model suitable for three-dimensional magnetic fields was established,and a beam trajectory correction algorithm was developed.The deflection of the proton Bragg peak in the presence of fringe fields was analyzed.Furthermore,3 treatment plans were simulated and dose correction was carried out when the fringe field existed.Gamma analysis method is used to evaluate the correction effect;and the dose changes in the target area and organs-at-risk after correction were quantitatively analyzed.Results The perturbation of the magnetic field would cause lateral deflection of the proton beam trajectory,and the presence of fringe fields would significantly increase this effect,which increased with the increasing of beam energy.When the fringe field existed,the treatment plans for brain tumor,liver tumor,and prostate cancer were corrected.Under the 3%/3 mm criterion,the gamma passing rates for target area were 94.844%,92.054%,and 97.863%,respectively,and after correction,the total dose in the body was increased by 2.8%,2.5%,and 1.5%,respectively.The increased dose was mainly contributed by incident protons.Conclusion In magnetic resonance imaging-guided proton therapy,the effects of fringe fields should be considered.The increase in incident proton beam energy after correction will lead to an increase in the total dose in the body.Since the beam trajectory still has curvature,the dose changes differently in different organs-at-risk.
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