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内容記述 |
Radioactive ion beams can be considered for range verification in carbon therapy as an ideal method using in-beam positron emission tomography (PET) imaging since ion beam ranges can be directly visualized. In particular, a PET image with acceptable quality can be obtained for a10C ion beam even for an in-beam PET scan time of approximately 1 min, owing to the short half-life (19.3 s) of 10C. In addition, a 718 keV photon is emitted almost simultaneously with the positron decay, and the feasibility of its direct localization based on triple gamma imaging (i.e., the β+-gamma coincidence) has been shown by whole gamma imaging (WGI). The WGI system is realized as a dual-detector ring geometry, which can work in three modes: PET, Compton imaging and triple gamma imaging. However, the range verification performance for all three modes has not yet been assessed. In particular, the potential of Compton imaging, which is not influenced by the relatively long positron range of 10C, has not been shown. In this work, we focused on comparing the performance of Compton imaging with that of PET for a10C ion beam using WGI. First, we irradiated a polymethyl methacrylate (PMMA) phantom with a10C beam under in-beam WGI measurement in the carbon therapy facility of the heavy ion medical accelerator in Chiba (HIMAC) and compared the measured images with the depth dose. We then modelled our WGI system by Geant4 to evaluate the influence of the scatterer energy resolution on the imaging performance of the WGI for a10C point source and on the accuracy of range verification for a10C pencil beam. Energy resolutions of 17 % (current WGI) and 1 % (advanced WGI) at 511 keV photons were considered for the scatterer detectors. The measurements revealed small shifts between the Bragg peak position and the positions of the peaks for PET, Compton and triple gamma imaging. The worst shift of 3.1 ± 0.3 mm was observed at 80 % of the peaks for Compton imaging, which may be explained by the undesirable poor energy resolution of the scatterer detectors. In the simulation, spatial resolutions of 5.6 mm and 2.4 mm were obtained for the Compton mode of the current and advanced WGIs, respectively, for the simulated 10C point source at the Center of the field of view (FOV). The shifts in the Bragg peak position and the position of the Compton peak at the 80 % level considerably improved for the advanced WGI for the simulated PMMA phantom irradiated with a10C pencil beam. The shifts at the 80 % peak level were 0.1 mm for the PET and Compton modes of the advanced WGI. The positron range-free did not compensate for the limited energy resolution in Compton imaging, and the accuracy of the Compton image was comparable to that of the PET image when the energy resolution of the scatterer detector was improved. Sensitivity enhancement by combining PET with Compton imaging and simultaneous tumor damage assessment with range verification are expected to be possible applications of WGI in 10C beam irradiation. |
