To calculate in- and out-of-field neutron spectra and dose equivalent, using Monte Carlo (MC) simulation, for a Mevion gantry-mounted passively scattered proton system in craniospinal irradiation. An analytical model based on the MC calculations that estimates in- and out-of-field neutron dose equivalent from proton CSI was also developed.
The MCNPX MC code was used to simulate a Mevion S250 proton therapy system. The simulated proton depth-doses and profiles for pristine and spread-out Bragg peaks were benchmarked against the measured data. Previous measurements using extended-range Bonner spheres were used to verify the calculated neutron spectra and dose equivalent. Using the benchmarked results as a reference condition, a correction-based analytical model was reconstructed by fitting the data to derive model parameters at 95% confidence interval. Sensitivity analysis of brass aperture opening, thickness of the Lucite (PMMA) range compensator, and modulation width was performed to obtain correction parameters for non-reference conditions.
For the neutron dose equivalent per therapeutic proton dose, the MCNPX calculated dose equivalent matched the measured values to within 8%. The benchmarked neutron dose equivalent at the isocenter was 41.2 and 20.8 mSv/Gy, for cranial and spinal fields, respectively. For in- and out-of-field neutron dose calculations, the correction based analytical model showed up to 17% discrepancy compared to the MC calculations. The correction factors may provide a conservative estimation of neutron dose, especially for depth ≤ 5 cm and regions underneath the brass aperture.
The proposed analytical model can be used to estimate the contribution of the neutron dose to the overall CSI treatment dose. Moreover, the model can be employed to estimate the neutron dose to the implantable cardiac electronic devices.

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