Dose perturbation calculation due to the different inhomogeneities in the presence of ۱.۵ Tesla magnetic field with ۱۸ MV flattening filter free (FFF) photon beam

Background or Introduction: Magnetic resonance image-guided radiation therapy (MRIgRT) systems enhance tumor imaging and treatment precision by combining linear accelerators and MR scanners for real-time tumor movement observation. Two MRIgRT systems are being developed: one with a transverse magnetic field to the beam axis and the other with a longitudinal magnetic field to the beam axis. The transverse magnetic field system can lead to an increase in dose on the exit surface due to the "electron return effect (ERE)" and shift dose distributions along the direction of the Lorentz force. The longitudinal magnetic field system increases the skin dose in the patient due to incident electrons from the upper side of the multileaf collimator. In the presence of inhomogeneities such as air cavities and implants like PEEK, the dose distribution can be significantly affected. Monte Carlo program (MCNP ۶.۱) will be used for dose perturbation calculations to investigate the effect of magnetic field in dose distribution in the presence of these inhomogeneities.Material and Methods: The dose distributions in a virtual lung phantom under the influence of magnetic fields were calculated using the MCNP ۶.۱ Monte Carlo code. The phantom size was ۵۰×۵۰×۵۰ cm۳. Different inhomogeneities included PolyetherEtherKetone (PEEK), and an air cavity was incorporated into it each time, positioned at a depth of ۲ cm and with a thickness of ۲ cm. The PEEK and air cavity density were set to ۱.۳۰ and ۰.۰۱۲۰۵ g/cm۳. The uniform magnetic flux density of ۱.۵ T was used for ۱۰×۱۰ cm۲ fields at a source-to-surface distance of ۱۰۰ cm, using an ۱۸ MV photon spectrum without flattening filter (FFF).Results & Discussion: The dose in front of the inhomogeneities, water–PEEK, and water-air cavity interfaces in the phantom increased by ۴.۳۳% and ۳۱.۱۲% for the ۱.۵ T transverse magnetic field, respectively. The dose behind the water-air cavity interface decreased by ۳۶.۴۲%. However, the dose behind the water-PEEK interface increased by ۳.۵۵%. The PEEK inhomogeneity was prone to creating hot spots on both sides. However, the air cavity inhomogeneity was prone to creating hot and cold spots in front and behind the inhomogeneity.Conclusion(s): The dose perturbation due to the inhomogeneity in the presence of the transverse magnetic field might be intensive. The PEEK with a high mass density produces hot spots, and the air cavity makes both cold and hot spots.

Keywords:

Magnetic field , Dose perturbation , Inhomogeneity , MCNP ۶.۱ , ۱۸ MV photon beam