SU‐E‐T‐254: Tissue Specific Characteristics of Prompt Gamma in Proton Radiotherapy: A GEANT4 Simulation Study

Abstract

Purpose: An in‐vivo range verification system for proton radiotherapy utilizes prompt gamma photon emitted from the de‐excitation of daughter fragments (e.g. 12C, 16O) produced in an interaction of incident proton in materials with time scales in the range of nano seconds. The goal of the present investigation is to study depth dependency of the yield of emitted prompt gamma rays from energetic proton interactions in different materials. Methods: A coincidence detector system for identifying prompt gamma was simulated with Geant4 Monte Carlo toolkit version 9.4.9 p02. The interaction of 110 MeV protons with target phantom (5 cm × 5 cm × 50 cm) was used. A virtual detector (5 cm × 5 cm × 1 mm) was replicated 500 times to form a bank of detectors. A lower bank was placed 1 cm above the phantom while another bank was placed 1 cm above the lower bank. A coincidence requirement was met when a gamma had passed through both a bottom detector and the corresponding top detector segments directly above it. Our investigation used built‐in physics list consisting of electromagnetic and nuclear interactions. Three different types of phantom materials (Water, PMMA and Bone) were used. Results: The emitted prompt gamma yields from 12C (energy range 4.21 to 4.59 MeV) and 16O (energy range 5.79 to 6.31 MeV) versus depth showed similar distributions irrespective of materials. The depths of the maximum yields from 12C were superficial to that from 16O and Bragg peak depths. The yields from 12C were greater to that from 16O up to 5 mm superficial to Bragg peak; whereas around Bragg peak the yields of 16O dominated. Distal to Bragg peak, the yields from 12C started to dominate. Conclusion: A depth dependent relationship may exist between the characteristic prompt gamma production and Bragg peak. More careful analysis has now been planned.