Date of Award

Winter 12-15-2021

Author's School

Graduate School of Arts and Sciences

Author's Department

Physics

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

This thesis presents a study of neutron production in two types of proton-therapy systems: passive scattering Mevion S250, and active scanning Mevion S250i. The scattering system has been operating at Washington University in St. Louis since 2013. The scanning system was installed in the same building in 2020. The scanning system has the advantage of producing fewer neutrons due to a focused scanned beam and the absence of beam scatterers, collimators, and a static brass aperture. A systematic particle transport Monte Carlo analysis using the Geant4 toolkit has been performed to track the neutrons produced in various components of each of the two proton-therapy systems. The main analysis is based on the neutron fluence spectra incident onto a water phantom. The water phantom models the body of the patient. The total neutron fluence is decomposed into several neutron fluence contributions received from each component in the proton beam nozzle and those items in the treatment room. Dividing the fluence spectra into multiple contributions has been made possible by the TTree method in the ROOT data analysis software, and its implementation in Geant4. Each fluence spectrum has been transformed into the total neutron absorbed dose-equivalent per treatment Gy to the isocenter. Pie charts for the neutron dose-equivalent contributions of various components are obtained, and the variations of the contributions of the various components of the system as a function of both the delivered beam energy and the field size are analyzed. Neutron dose-equivalent profiles as a function of position inside the treatment room are presented. Simulations of both systems are benchmarked against the experimental data of the proton profiles, neutron spectra, and neutron total dose-equivalent measurements. Two measurement techniques are discussed: Bonner spheres, for obtaining the neutron fluence spectrum, and WENDI chamber, for determination of the neutron total, i.e. integrated, dose-equivalent. The main conclusions from the study involved understanding the dependence of the internal neutron ratio on the beam energy and the field size. In addition, the amount of reduction of the secondary neutron production by the scanning system compared to the scattering system was a part of the aims of the presented analysis.

Language

English (en)

Chair and Committee

Lee G. Sobotka

Committee Members

Arash Darafsheh

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