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Secondary Particle Interactions in a Compton Camera Designed for in Vivo Range Verification of Proton Therapy

R Panthi1*, P Maggi2 , S Peterson3 , D Mackin1 , J Polf2 , S Beddar1 , (1) The University of Texas M.D. Anderson Cancer Center, Houston, TX, (2) University of Maryland, Baltimore, MD, (3) University of Cape Town, Rondebosch, SOUTH AFRICA


(Monday, 7/15/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 6

Purpose: To determine the types, proportions, and energies of the secondary radiation interactions in a Compton camera during the delivery of a clinical proton beam.

Methods: The delivery of a 150 MeV clinical proton pencil beam incident on a water phantom was simulated using Geant4 software. The simulation included a Compton camera with 64 cadmium zinc telluride crystals, matching the configuration of a Polaris J Compton camera designed for in vivo range verification. The interaction positions of the secondary radiation particles (prompt gamma rays, annihilation gammas, radioactive-decay gammas, x-rays, neutrons, electrons, positrons, and protons) were scored.

Results: The total number of secondary particles created in the water target that interacted in the Compton camera was 127,683 per 10â?¸ protons. The interaction rates in the detector modules ranged from 4,451 to 12,284 per 10â?¸ protons (~1 Gy). The minimum interaction rate occurred in the Compton camera aligned with the beam entrance to the target, and consisted of 2,675 neutrons, 536 scattered prompt gammas, 589 annihilation gammas, 344 prompt gammas, 211 electrons, 43 x-rays, 14 radioactive-decay gammas, 26 protons, and 13 positrons per 10â?¸ protons. Prompt gamma, which correlates most closely with the dose distribution, produced only 6.7% of the interactions. The largest fractions of the interactions in the Compton camera were produced by neutrons (37.5%), annihilation gammas (20.8%), scattered prompt gammas (16.3%), and electrons (13.5%).

Conclusion: Strategies for using Compton cameras for proton range verification should include methods of reducing the large neutron and electron backgrounds. The proportions of interaction types differ by module, and this difference may provide information useful for background suppression.


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