259 TIME-OF-FLIGHT METHOD FOR NEUTRON REJECTION IN PROMPT GAMMA IMAGING OF BEAM RANGE AND DENSITY CHANGES IN PROTON THERAPY

A therapeutic proton beam penetrating a patient generates a large number of prompt photons as well as neutrons, resulting from proton-induced nuclear reactions. This allows for obtaining longitudinal beam-profile images via the detection of selected prompt-gamma rays emitted perpendicularly to the beam direction. The significant neutron background on top of the prompt photons signal was observed. Thus, the goal of presented research is to propose the novel in-situ time-of-flight (TOF) technique for neutron background rejection in order to get the optimum signal from prompt-gamma rays escaping the irradiated object. Density heterogeneities within the patient affect the position of the Bragg peak. Due to potential morphological changes the Bragg peak position may vary during the course of treatment, which may lead to clinically significant under-dosage in the tumor or over-dosage in the organs-at-risk. The Bragg peak shifts due to target morphological modifications, whichmay occur in realistic radiotherapy scenarios (e.g. tumor or normal tissue swelling), can be detected with the presented method to within a few millimeters. We show a clinically relevant correlation between the prompt-gamma profiles and the target (patient) density.