Abstract

Introduction: The generation of secondary neutrons in proton therapy produces a biologically signif-icant out-of-field dose, which is associated with an elevated risk of secondary cancers. This study was designed to quantify the absorbed and equivalent doses from these neutrons within a water phantom,specifically to establish how these doses depend on the initial energy of the proton beam.
Methods: We utilized a validated Monte Carlo model on the Geant4/GATE platform to simulate a pencil proton beam incident on a water phantom at energies ranging from 70 to 250 MeV. Model accuracy was verified by comparing the simulated Bragg peak positions (R80) against NIST reference data. The secondary neutron absorbed dose was isolated using the Dose Actor and Digitizer tools. Subsequently, the neutron equivalent dose (H) was calculated using energy-dependent radiation weighting factors (wR), as recommended in ICRP Publication 103. All dose calculations were normalized to a clinically standard therapeutic dose of 2 Gy delivered to the Bragg peak.
Results: The simulations revealed a substantial increase in the neutron dose with increasing incident proton energy. As the proton energy was increased from 70 to 250 MeV, the mean neutron absorbed dose escalated from 0.031 to 0.138 mGy per 2 Gy therapeutic fraction. This caused the mean neutron equivalent dose to climb from 0.25 to 1.45 mSv. Furthermore, spatial analysis showed that the majority of this neutron dose is deposited in the entrance channel, upstream of the Bragg peak.
Conclusion: The secondary neutron equivalent dose delivered during proton therapy shows a strong dependence on treatment energy, with a nearly sixfold increase observed across the clinical energy range investigated. These quantitative findings are vital for clinicians and medical physicists in evaluating the risk-benefit ratio of treatments requiring high-energy proton beams and in developing more accurate algorithms for treatment planning systems.