Investigating the Effect of Magnetic Field on Radiation Dose Distribution in Radiotherapy using Photon and Electron Beams of Linear Therapeutic Accelerators

Background and Objectives Magnetic fields can be used in radiation therapy to reduce electron contamination and improve dose delivery accuracy. MRIgRT systems use magnetic fields to track the position of the tumor during treatment and precisely deliver the dose from electron beams to the tumor, which will lead to improved treatment outcomes and reduced side effects. Subjects and Methods The MCNP 6.1 Monte Carlo code was used to simulate the Varian 2100 C/D LINAC in both photon and electron modes. Percentage depth dose curves, dose profiles, and the fluence of contaminating electrons and photons were calculated. Dose profile penumbra and dose differences were calculated for different modes. In the second phase of the study, a constant 1.5 Tesla longitudinal magnetic field was applied to a water phantom that was aligned with the direction of the radiation beam. Results The MD reduced the surface dose by 8.3% and the dose profile penumbra by 5.6% at the surface of the water phantom. The MD removes all contaminating electrons from the radiation field without affecting the number of photons. The application of a 1.5 Tesla longitudinal magnetic field increased the dose by 4% in the maximum dose depth region and reduced the penumbra by 20% and the off-axis dose by 57% at the same depth. Conclusion The MD reduces surface dose, off-axis dose, and dose profile penumbra. The longitudinal magnetic field reduces penumbra and off-axis dose in electron beams.


Extended Abstract
Introduction adiotherapy is a fundamental cancer treatment utilizing ionizing radiation such as photons and electrons.It seeks to eradicate cancer cells while minimizing harm to surrounding healthy tissues.Precision in delivering radiation dose to the target area is vital for effective treatment.
One challenge in radiotherapy is electron contamination, where charged electrons in photon beams can increase skin dose and harm surrounding tissues.Factors contributing to this include flattening filter, air gap, and ionization chamber.Magnetic fields effectively reduce electron contamination by deflecting electrons, preventing them from reaching healthy tissues.Studies show a 90% reduction in contamination and a 20% decrease in skin dose with a magnetic deflector (MD).
Recent research focuses on developing systems that integrate electron radiation with magnetic resonance imaging (MRI), known as MRI-guided Radiation Therapy (MRIgRT).These systems offer advantages, allowing precise tumor tracking and optimized dose planning.Magnetic fields guide electron radiation, which will lead to enhanced accuracy and reducing side effects.However, limited research has been conducted on longitudinal magnetic fields in electron therapies.
The application of magnetic fields in radiotherapy holds significant potential for improving treatment outcomes and reducing side effects.Ongoing research is expected to broaden the future applications of magnetic fields in radiotherapy.Therefore, this study aims to investigate the application and effects of magnetic fields on dosimetric parameters using Monte Carlo simulation, focusing on reducing electron contamination and applying longitudinal magnetic fields in electron beams.

Methods
The study employed the Monte Carlo MCNP code version 6.1.0to simulate the Varian 2100 C/D Linear Accelerator (LINAC) in both photon (18 MV) and electron (9 MeV) modes.The LINAC components, such as the electron source, target, primary collimator, vacuum window, flattening filter (for photon mode), scattering foil (for electron mode), ionization chamber housing, mirror, and secondary collimator were meticulously simulated.Dosimetric data were calculated for a source-to-surface distance (SSD) of 100 cm and a field size of 10 × 10 cm², with electron and photon energy cut-offs set at 0.5 and 0.01 MeV, respectively.To ensure accuracy (<2% relative error), a billion initial electrons were used for flux and absorbed dose calculations.Dosimetric calculations were conducted with voxel dimensions of 2 × 2 × 2 mm² to compute percentage depth dose (PDD) curves and dose profiles in a water phantom (50×50×50 cm³).To calculate the dosimetric parameters caused by applying the longitudinal magnetic field in the water phantom as a result of electron radiation, the PDD and the dose profile were calculated for each MC program.
The study also incorporated magnetic fields using MCNP 6.1.0,applying a constant 1 Tesla magnetic field under the LINAC's secondary collimators to eliminate the contaminating electrons.Subsequently, a 1.5 Tesla longitudinal magnetic field was applied to the water phantom in alignment with the radiation beam direction.
The analysis involved calculating penumbra and dose differences using specified equations.Origin 2021 software was employed in plotting curves and figures, with PDD curves normalized to the central axis dose for the standard reference field, and dose profiles normalized at each depth to the central axis dose.

Results
The study investigated the impact of a MD on reducing electron contamination in 18 MV photon radiation and the effects of a longitudinal 1.5 Tesla magnetic field on 9 MeV electron radiation.
In the 18 MV photon scenario, PDD curves were analyzed for a 10 × 10 cm² field at SSD=100 cm.The MD usage resulted in an 8.3% surface dose reduction, with a partial dose reduction in the build-up region and no alteration in dmax in the standard flattening filter (FF) condition.Absorbed dose profiles at the phantom surface revealed a 5.6% penumbra reduction with MD, and off-axis doses at 6.5 cm were decreased by 6.5%.The MD showed significant effectiveness in reducing electron contamination without impacting photon quantity, as evident in the energy spectra.
For 9 MeV electron radiation with a longitudinal 1.5 Tesla magnetic field, PDD curves displayed a 4% dose increase at the maximum dose depth, particularly in the build-up region.Absorbed dose profiles indicated a 20% reduction in penumbra at the maximum dose depth and a substantial 57% decrease in off-axis dose at 6.5 cm depth.The findings suggest that the longitudinal magnetic field effectively influences the dose distribution in electron radiation.

Conclusion
The study employed a Monte Carlo model to precisely analyze dosimetric parameters for an 18 MV-Varian LINAC with a MD.The MD implementation effectively reduced surface and off-axis doses by eliminating contaminated electrons, and maintaining photon output integrity.This reduction extended up to the build-up region, showcasing MD's efficacy in dose optimization.Furthermore, MD showed marginal effects on decreasing flatness and penumbra in surface dose profiles.

R
The investigation expanded to dosimetric parameters for electron beams in the presence of a longitudinal magnetic field, unveiling the substantial impact of Lorentz force on particle trajectory.When the initial particle velocity exhibited a component perpendicular to the magnetic field, the Lorentz force induced a helical motion of electrons around the magnetic field axis.As a result, laterally scattered electrons moved parallel to the magnetic field, leading to decreased penumbra and off-axis dose.Simultaneously, the surface dose increased due to the longitudinal magnetic field configuration.These findings highlight the potential of magnetic fields in optimizing dosimetry for radiotherapy applications.