Feasibility of triple gamma ray imaging ofC for range verification in ion therapy.

In carbon ion therapy, the visualization of the range of incident particles in a patient body is important for treatment verification. In-beam positron emission tomography (PET) imaging is one of the methods to verify the treatment in ion therapy due to the high quality of PET images. We have shown the feasibility of in-beam PET imaging of radioactiveO andC ion beams for range verification using our OpenPET system. Recently, we developed a whole gamma imager (WGI) that can simultaneously work as PET, single gamma ray and triple gamma ray imaging. The WGI has high potential to detect the location ofC, which emits positrons with a simultaneous gamma ray of 718 keV, within the patient's body during ion therapy.In this work, we focus on investigating the performance of WGI forC imaging and its feasibility for range verification in carbon ion therapy. First, the performance of the WGI was studied to image aC point source using the Geant4 toolkit. Then, the feasibility of WGI was investigated for an irradiated polymethyl methacrylate (PMMA) phantom with aC ion beam at the carbon therapy facility of the Heavy Ion Medical Accelerator in Chiba.The average spatial resolution and sensitivity for the simulatedC point source at the centre of the field of view were 5.5 mm FWHM and 0.010%, respectively. The depth dose of theC ion beam was measured, and the triple gamma image ofC nuclides for an irradiated PMMA phantom was obtained by applying a simple back projection to the detected triple gammas.The shift between Bragg peak position and position of the peak of the triple gamma image in an irradiated PMMA phantom was 2.8 ± 0.8 mm, which demonstrates the capability of triple gamma imaging using WGI for range verification ofC ion beams.