The effects of a transverse magnetic field on the dose enhancement of nanoparticles in a proton beam: a Monte Carlo simulation.

High-Z nanoparticles (NP) as radiosensitization agents provide the feasibility of dose localization within the tumor in radiotherapy. Dose enhancement of NPs in the presence of a magnetic field (MF) could be challenged when magnetic resonance imaging (MRI) systems are used as an image-guided system. The MF can influence dose enhancement of NPs at their interfaces and surrounding medium and affect their dose deposition behavior. In the TOPAS Monte Carlo code, gold nanoparticle (GNP) and superparamagnetic iron oxide nanoparticle (SPION) were irradiated using 70 and 150 MeV proton beams, in presence of transverse MF strengths with 0, 1, 3, and 7 T. The changes in the liberated secondary electrons from NPs and their dose enhancement ratio (DER), magnetic dose enhancement ratio (MDER), and angular dose distribution in 10 nm shell thicknesses up to 500 nanometers from their centers were measured. The central plane of NPs was considered as a scorer that its thickness was 2 nm and divided to 6-degree sectors with 10 nm radial length. The dose deposition in this voxelated scorer was calculated. The values of the deposited doses around NPs decrease rapidly while the DERs resulted from the secondary electrons are increased. MDERs are changed within ±5% and ±7% for 20 and 50 nm radius NPs, respectively. The variation in the angular dose distribution around a singular NP was not considerable with different applied MF strengths. The dose values in the voxelated central plane show very similar results for the same NPs types in the different MF strengths. The typically used MF in the MRI systems, could not cause an electron return effect (ERE) at the interfaces of NPs and water. The DERs of NPs in a water medium resulted from emerged secondary electrons, experience a low degree of perturbation in the presence of an MF. The results of this study show that the NPs as dose enhancement agents can be used also in an MF without pronounced modification in their efficacy caused by electron return effect.