In-beam whole gamma imaging of a 10C ion beam: a phantom study

For range verification in carbon therapy, in-beam positron emission tomography (PET) imaging of radioactive ion (RI) beams is ideal because the beam stopping position can be directly visualized. Among such RI beams, a ¹⁰C beam is appropriate because its short half-life (19.3 s) results in high signal to noise ratio even for an in-beam PET scan of about 1 min. ¹⁰C emits a 718 keV gamma ray almost simultaneously with the positron decay, and we have shown the feasibility of its direct localization based on the b⁺-gamma coincidence (i.e., triple-gamma imaging) by our whole gamma imaging (WGI). The WGI system is realized as a dual-detector ring geometry, which can work in three modes of PET, Compton imaging and triple-gamma imaging. However, range verification performance for each of these three modes has not been assessed yet. In particular, the potential of Compton imaging, which is not influenced by the relatively long positron range of ¹⁰C, has not been shown. Therefore in this work, for the ¹⁰C beam range verification, we compared the performance of Compton imaging with those of PET and triple-gamma imaging using the WGI. First, a current WGI prototype was modeled by Geant4, and imaging performance was assessed for a ¹⁰C point source and a ¹⁰C pencil beam. Then, a polymethyl methacrylate (PMMA) phantom was irradiated with a ¹⁰C beam under in-beam WGI measurement in the carbon therapy facility of the Heavy Ion Medical Accelerator in Chiba, and the measured images were compared with the depth dose, which was measured in water. Small shifts (< 3 mm) of the Bragg peak position and the positions of the peaks for Compton, PET and triple-gamma imaging were observed, which confirm the feasibility of Compton imaging for range verification of a ¹⁰C beam. The worst shift was observed for the Compton imaging which may be explained by the energy resolution of the scatterer detectors. The availability of three imaging modalities may offer better range verification.