Optimization of the gold layer in multifunctional theranostic core-shell magnetic nanoparticles (MNP@Au) for radiation dose enhancement in an MRI-guided proton therapy system: A Monte Carlo simulation

Magnetic Resonance Imaging (MRI)-guided proton therapy has advanced significantly in pre-clinical stages. This study investigates the potential of multifunctional theranostic magnetic nanoparticles (MNPs) to enhance radiation dose while serving as MRI contrast agents. We propose MNP@Au nanoparticles, consisting of a magnetic core coated with a gold (Au) layer, and aim to optimize the Au layer thickness to maximize dose enhancement during proton therapy while preserving magnetic properties. Using Monte Carlo simulations in the TOPAS toolkit, we simulated a spread-out Bragg peak (SOBP) within a 3 cm hypothetical tumor in a water phantom using a proton beam with a maximum energy of 150 MeV. MNP@Au nanoparticles with superparamagnetic iron oxide (SPION) or gadolinium oxide (Gd2O3) cores of varying diameters and gold layer thicknesses were analyzed. Parameters such as secondary particle energy and total energy deposition were used to determine the energy efficiency for optimizing the gold layer thickness. The results show that the optimal thickness of the gold layer increases exponentially with the increase in the diameter of the magnetic core until it reaches its saturation value. For the Gd2O3 core, the thickness of the optimal gold layer, with a growth rate of 0.0238 nm-1, reaches 3.27 nm at its saturation value. These values for the SPION core are 0.0125 nm-1 and 12.04 nm, respectively. Overall, the maximum value of energy efficiency in simulated nanoparticles is less than 50%. Among single and hybrid nanoparticles with similar mass, the energy efficiency is highest in gold nanoparticles, followed by Gd2O3@Au, SPION@Au, SPION, and hypothetical water nanoparticles. In conclusion, MNP@Au nanoparticles with smaller core diameters and thinner gold layers exhibit higher energy efficiency, making them promising multifunctional agents for MR-guided proton therapy. By optimizing the Au layer, these nanoparticles can enhance tumor contrast and radiation dose, advancing their potential in cancer treatment.