Geant4 Monte Carlo investigation of the magnetic field effect on dose distributions in low-density regions in magnetic resonance image-guided radiation therapy

•Investigation of the dosimetric effect of magnetic fields in the treatment of lung tumors by means of MRIgRT.•For transverse magnetic fields, the dose increases at a water–lung interface while it decreases at a lung–water interface.•Lung depth dose in longitudinal magnetic fields is enhanced by the Lorentz force, thus reducing the penumbra. It is necessary to evaluate the effect of the presence of a magnetic field when treating lung tumors with MRIgRT. In this study, the effect of transverse and longitudinal magnetic fields on dose distributions in low-density regions was quantitatively investigated. The dose distributions in a virtual lung phantom under the influence of magnetic fields were calculated using the Geant4 Monte Carlo code. The phantom size was 30 × 20 × 30 cm3, and it was composed of three layers: water (3 cm thickness), lung (12 cm thickness), and water (5 cm thickness). The density of the lung layer was set to 0.1, 0.3 and 0.6 g/cm3. The uniform magnetic flux densities of 0.35 T and 1.5 T were used for 2 × 2, 5 × 5, and 10 × 10 cm2 fields at a source-to-surface distance of 100 cm, using a 6 MV photon spectrum. The dose at the water–lung interface in a lung phantom increased by 58%, 51%, and 22% for the lung densities of 0.1, 0.3, and 0.6 g/cm3, respectively. In the 1.5 T longitudinal magnetic field, the penumbra at the lung center (at the depth of 9 cm) decreased by 5.14, 1.50 and 0.35 mm for a 5 × 5 cm2 field, respectively. The dose distributions in lowdensity regions are more affected by the magnetic field. In conclusion, the doses at the water–lung interface increased in the transverse magnetic field. The depth dose increased, and the penumbra decreased in the longitudinal magnetic field.