The impact assessment of a magnetic field dedicated to MRI during a carbon-therapy on treatment efficiency: Application to 1.5 T magnetic field

Abstract

Radiation therapy is increasingly becoming more targeted in how the doses are delivered. Online MR imaging, when combined with particle therapy such as proton or carbon ion therapy will offer a novel and exciting opportunity for radiotherapy. Integration of radiotherapy with MR imaging for carbon ion therapy is an active area of research, but its clinical implementation is still a way off. It is known that the employment of extremely high magnetic fields in particle therapy poses several challenges, in particular those related to treatment planning, workflow, dose delivery, and dosimetry . The objective of this study is to validate the results of simulation calculations of the interaction of a carbon ion beam in water by comparative analysis with existing literature data and examination of Bragg curves. This involves evaluating the initial percentage of ion fragmentation and elucidating the variations of dose rates along the propagation axis of a carbon ion beam for specified values of the Z-axis (lateral side). In addition, the study aims to investigate the beam deflection and the displacement of the dose profile curves with respect to the lateral axis in a magnetic field oriented perpendicular to the beam axis. A carbon ion beam was delivered on a voxelized phantom of rectangular geometry simulated by applying a GEANT4 simulation code and ROOT 6.24/08 for the analysis. The beam deflection was calculated for a 270 MeV/u beam energy for three different magnetic fields of 0.35, 1.5 and 2 T. While the rates of decrease and increase in dose, as well as lateral shifts of dose profiles were performed in the presence of 1.5 T magnetic field (MF). The results of the simulation exhibit a satisfactory correlation with the experimental data, demonstrating that the Bragg peak area remained stable under a 1.5 T magnetic field and for a beam energy of 270 MeV/u. Furthermore, 52.32% of the carbon ions were fragmented into lighter particles before reaching the designated area. The analysis encompassed Bragg curves at various off-axis positions, the Rate of Decreasing Dose (RDD), the Rate of Increasing Dose (RID), Lateral Dose-Profile, and the lateral deflection of the primary beam dose-profile along the X-axis. The findings revealed a 45.919% reduction in dose at the center of the treatment field and a lateral displacement of 4.57675 mm under the influence of a 1.5 T magnetic field. Additionally, the displacement variation at the Bragg peak area as a function of the magnetic field followed a linear relationship. The simulation model demonstrates satisfactory performance. When administering treatment with carbon ions under a 1.5 T magnetic field, it is imperative to consider a lateral displacement of 4.57675 mm of the Bragg peak. Additionally, the percentage reduction in dose along the lateral axis must be accounted for. Furthermore, the calibration curve obtained must be utilized to accurately determine the extent of carbon ion beam deviation within the Bragg peak region. • Simulation of a therapeutic beam of carbon ions and a voxelized phantom. • Transport of carbon ion beams in different perpendicular magnetic fields. • Bragg curves in different off-axis positions and the relative dose perturbation by the presence of a 1.5 T magnetic field. • The decreasing and increasing dose rates along the carbon ion beam propagation axis. • The calibration line as a function of magnetic field values at the Bragg peak area.