Calculation of magnetic field effects on dose distribution in MR-guided Helium Ion-therapy using FLUKA Monte Carlo simulation code.

Nowadays, in one hand, the potential clinical application of helium ion beams is being discussed in the literature. In other hand, MR-guided radiotherapy has been welcomed by the radiotherapy oncology community so far. The idea of ​​combining an MRI system with a helium-ion beam delivery gantry has challenges, one of which is the dose changes in the patient's body that occur due to the deviation of the initial beam under a magnetic field. In order to quantitatively evaluate these dose changes, in this study, variation in the depth- and transverse dose profile curves inside a water phantom at therapeutic energies under magnetic fields of 0.35, 1.5 and 3 T were calculated using the FLUKA Monte Carlo code. The percentage of dose fluctuation in the direction of the central axis due to the application of magnetic field at minimum, medium and maximum energies was calculated and it was shown that the maximum dose changes occur at 220.5 MeV / n energy and at a depth of 30.9 cm phantom. Also, the maximum longitudinal retraction and deflection of helium beams in the presence of 3 Tesla magnetic field and at 220 MeV energy were 5.2 mm and 4.7 mm, respectively. To evaluate the accuracy of the simulations, depth-dose curves of helium ion beams in a water phantom were compared with the experimental data without the magnetic field influence. The maximum difference between the simulated and the measured values was 0.962 mm which is less than the permissible limit for predicting the Bragg peak of the ion beam for clinical dosimetry purposes.