Monte Carlo Simulations of the Effect of Magnetic field on Deposited dose in Proton Therapy

Proton therapy is well known to have a greater effectiveness for cancer cell disruption within targeted tumors as compared to X-ray and gamma rays, while minimizing entry and exit dose damage to healthy tissues. The combining of MRI with proton therapy has been proposed in order to allow for better beam guidance. However, the presence of the magnetic field associated with the MRI system both influences the beam trajectory, as well as having some effect on the deposited dose. This work focusses on the change in deposited dose with field strength applied either along the beam direction or perpendicular to it. We used Monte Carlo code PHITS (v.3.28) to calculate the absorbed dose contributed from primary and secondary particles inside a cylindrical water phantom during proton therapy, using proton pencil beam along Z-axis with energies ranging from 70±0.1 MeV to 220±0.1 MeV with a step of 10 MeV. Magnetic fields were applied either parallel to, or perpendicular to, the beam direction. Primary and secondary protons are the main contributor in the total dose with about 83% and 17%, respectively. The rest of the contribution comes mainly from heavy ions like deuterons, tritons, 3He, 4He and a small amount of neutrons, electrons and photons. For a 220MeV beam, the deposited dose at the Bragg Peak, is increased by about 0.5% per Tesla for perpendicular fields, while fields parallel experienced modified the deposited dose substantially less. These effects are discussed for various beam energies and possible mechanisms discussed.