A Monte Carlo study on the impact of a transverse magnetic field on microscopic dose enhancement of nanoparticles in therapeutic proton beams.

The rapid evolution of cancer treatment modalities has positioned proton therapy as a highly effective approach for targeting specific tumours. Proton therapy takes advantage of the Bragg peak phenomenon to deliver a concentrated dose to the tumour while minimizing exposure to surrounding healthy tissues. This feature has spurred interest in further enhancing proton therapy through the integration of advanced technologies, such as image-guided proton therapy and nanoparticle (NP) application. The incorporation of NPs into tumour tissues has emerged as a promising strategy to enhance the delivered dose in radiation therapy. This study investigates the dose enhancement factor (DEF) resulting from the presence of various NPs, when irradiated by a spread-out Bragg peak of a 120 MeV proton beam. Additionally, the magnetic dose enhancement factor (MDEF) under transverse magnetic fields of 3 T and 7 T is examined using the Geant4 simulation toolkit. The findings clarify the NP-mediated dose enhancement in proton therapy, particularly in the context of MRI-guided treatments. The highest DEF occurs within NPs (e.g., 1,341% for Ir), while the surrounding tissue exhibits negligible enhancement (< 10% up to a radial distance of 500 nm). The results indicate that magnetic fields up to 7 T do not significantly alter dose distributions around NPs. While validating the compatibility of NP-enhanced proton therapy with MRI guidance, this work provides a comparison of metallic (Au, Ir, Gd, and SPION) and non-metallic (B, C) NPs, establishing a foundation for clinical NP selection and future radiobiology studies.