Speaker
Description
Purpose: In proton therapy, secondary particles such as neutrons and gamma rays are unavoidably produced through nuclear interactions, potentially contributing to out-of-field dose and biological risk. This study aims to evaluate the energy spectra and dose contribution of secondary neutrons and gamma rays generated within a water phantom during proton irradiation using detailed Monte Carlo simulations.
Methods: A comprehensive Geant4/GATE simulation was developed to model a clinical proton beam (70–250 MeV) incident on a water-filled Blue Phantom PT geometry. Physics processes were modeled using the QGSP_BIC_HP_EMY list, and particle interactions were recorded using Dose Actor, PhaseSpace Actor, and Digitizer Actor. Secondary particle spectra and deposited energy were analyzed using PDG-based particle classification. The Bragg peak location (R80) was validated against NIST data and prior experimental studies.
Results: The R80 values from simulations showed excellent agreement with reference data, with relative deviations within 1.6%. Secondary neutron and gamma yields increased with proton energy, reaching 34.84% and 133.39% of primary protons at 250 MeV, respectively. However, their contribution to deposited energy in the phantom remained minimal, with secondary non-proton particles contributing up to 6.88% of total deposited energy at 250 MeV. The dose contribution from neutrons and gamma rays was consistently <0.01% across all energies studied.
Conclusion: While secondary neutron and gamma production increases with incident proton energy, their direct contribution to in-field dose remains negligible. Nonetheless, their potential biological impact—especially outside the treatment field—warrants further investigation. These findings support ongoing efforts to optimize shielding and assess long-term risks in proton therapy.
