Abstract
Purpose: Recent studies have suggested that the characteristics of prompt gammas (PG) emitted from excited oxygen nuclei during proton therapy are advantageous for determining beam range during treatment delivery. Since PGs are only emitted while the beam is on, the feasibility of using oxygen PGs for treatment verification depends greatly on the design of highly efficient detectors. The purpose of this work is to characterize the oxygen PG emission emitted from the patient to help drive the design of clinically viable detectors. Methods and Materials: Using a validated Monte Carlo (MC) model of PG emission during proton irradiation, we studied the oxygen PG detection efficiency in a high purity Germanium detector. PG spectra recorded by the detector were calculated as a function of distance for a patient irradiated with clinical proton pencil beams. Changes to the fluence of 6.13 MeV oxygen PGs incident on the detector were characterized as a function of distance from patient isocenter and proton beam energy. Results: The detection efficiency was found to be 2.33×10−8 oxygen PGs per proton at 10 cm distance at 40 MeV. Falloff of the measured PG signal was found to decrease rapidly as a function of distance. However, compared to the efficiency at 40 MeV, the detection efficiency increased by 60% at 70 MeV and by up to 90% for energies greater than 150MeV. Conclusion: Due to the characteristics of PG emission from oxygen with respect to proton energy (i.e. depth in the patient), the falloff of detected 6.13 MeV PGs varied with the distance from the patient, the solid angle subtended by the detector, and the energy of the proton beam. The falloff of the PG signal was described well by that of a point source at isocenter irradiating a cylindrical detector of a known diameter. National Institute of Health, Grant# R21CA137362
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