Proton therapy has recently garnered significant attention as an effective treatment for cancer that can effectively damage tumor tissues while reducing doses to healthy tissues and organs. However, secondary neutrons generated by interactions with protons and brass collimators or the patient's body may cause harm to the human body. Nevertheless, the risks are yet to be precisely assessed or reflected in actual treatment plans. In this study, we develop a novel neutron camera that enables the visualization of spatial dose distribution by secondary neutrons. First, a neutron camera comprising two layers of units is constructed; each of the layer is composed of a plastic scintillator coupled with a photomultiplier tube. As a preliminary experiment, it is confirmed that the developed camera can visualize fission neutrons from a 252Cf source. Subsequently, secondary neutrons are observed from a brass (imitating the brass collimator) and water (imitating the patient's body) phantom irradiated with a 70 MeV proton beam, in which the neutron camera is placed at 0°, 15° (only in the case of brass) and 30° from each phantom 30 cm ahead. Following neutron/gamma event selections using two methods, i.e., time of flight and pulse shape discrimination, each of the reconstructed images successfully converged at the correct positions, with average angular resolutions of 21° (brass, full width at half maximum (FWHM)) and 19° (water, FWHM). Furthermore, dosimetry using the G(E) function method is applied to the images, enabling a dose distribution image to be created using secondary neutrons. As a result, the evaluated dose rate is 307 μSv/min for brass and 88.1 μSv/min for water, which are 86 % and 64 % those of the simulated dose rates, respectively. Eventually, future tasks for the utilization of the camera in actual proton therapy are discussed.
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