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A Monte Carlo-Based Analytic Model of Neutron Dose Equivalent for a Mevion Gantry-Mounted Passively Scattered Proton System

M BaradaranGhahfarokhi*, F Reynoso, B Sun, A Darafsheh, M Prusator, S Mutic, T Zhao, Washington University School of Medicine, Saint Louis, MO

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
To develop a Monte Carlo-based analytical model to estimate in- and out-of-field neutron dose equivalent in Craniospinal Irradiation (CSI) using a Mevion gantry-mounted passively scattered proton therapy system.

Methods:
The MCNPX Monte Carlo 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 equivalents. Using the benchmarked results as a reference condition, a correction-based analytical model was reconstructed using the R software to fit the data to derive model parameters at 95% confidence interval. Sensitivity analysis of brass aperture opening, thickness of the Lucite range compensator, and modulation width was performed to obtain correction parameters for non-reference conditions.

Results:
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 Monte Carlo calculations. The correction factors may provide a conservative estimation of neutron dose, especially for depth = 5 cm and regions underneath the brass aperture.

Conclusion:
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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