S Thompson*, S Samant, J Nimmagadda, L Maloney, University of Florida, Gainesville, FL, Erik Kryck, University of Florida Health Proton Therapy Institute, St. Charles Health System

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  S Thompson

PO-GePV-T-113 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To present a proton portal imaging (PPI) system which uses in situ generated spallation neutrons to produce images of patient anatomy; and to provide an accurate Monte Carlo model which enables the optimization of the detector placement and geometry, as well as to provide spectral data on the spallation neutrons and photons in order to ensure optimal detection efficiency.

Methods: The proposed PPI system will make use of scintillation cameras (modeled as LiF-ZnS) to capture the in vivo generated neutrons. An MCNP 6.1 model was constructed to optimize the detector placement, geometry, and to ensure adequate sensitivity to neutrons while also displaying low sensitivity to gamma radiation. This model also serves to generate modulation curves, which can be compared to measurements to determine the limiting spatial resolution for the system. A line pair phantom was constructed with calcium bars of thicknesses and spacings from 2-8 mm to determine the limiting spatial resolution. The Monte Carlo model was compared to measurements obtained in a clinical double-scattering proton system with a typical treatment field.

Results: Modulation plots are presented for the limiting bar thicknesses and spacings. The bar pattern is indiscernible at about 4-5 mm, which is the limiting spatial resolution of the system. Photon spatial intensity maps are also presented to verify that neutron interactions in the scintillator dominate photon interactions, as these are the interactions of interest in this study. Neutron and photon spectra plots were also produced in the phantom to study the sensitivity of detector placement on neutron capture efficiency.

Conclusion: The limiting spatial resolution for the proposed PPI system is between 4-5 mm. The interaction of neutrons in the scintillator outweighs the contribution of photons to the acquired signal, which is essential for PPI using in vivo generated exit neutrons.

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