Energy Spread Estimation of Radioactive Carbon Ion Beams Using Optical Imaging

Radioactive ion (RI) beams (e.g. 11C and 15O) combined with in-beam positron emission tomography (PET) allow treatment of cancer with exact range verification. However, one drawback of the RI beams (generated as secondary beams) over conventional stable heavy ion beams is the wide momentum distribution (i.e. energy spread) thereby causing the Bragg peak to shift as well as the distal-falloff length to widen. Hence, the energy spread of RI beams should be carefully measured during the quality assurance. In this study, we propose an optical imaging technique for the energy spread estimation of RI 11C ion beams. A PMMA phantom (10.0×10.0×9.9 cm3) was irradiated with a 11C beam (energy = 188.5 MeV/u, sigma = 4.5 MeV/u). Three different energy spreads of 1.1 MeV/u, 2.2 MeV/u and 4.0 MeV/u were obtained by using the momentum acceptances of 1%, 2% and 4%, respectively. An optical system consisting of a lens and a cooled CCD camera was used to acquire the optical images. The in-beam luminescence light images were obtained to extract the Bragg peak and distal falloff length information whereas the offline beam Cerenkov light images were obtained for the estimation of the stopping positions of the 11C ion beams. The energy spread of the 11C ion beam was estimated by two optical parameters: (1) distal-50% falloff length of the luminescence signals and (2) full-width at half maximum (FWHM) of the Cerenkov light in the beam direction. These parameters estimated the energy spread with the mean squared errors of 1.47×10-3 MeV/u and 9.8×10-5 MeV/u, respectively. In conclusion, the optical imaging can estimate the energy spread of RI 11C beams with high accuracy. The optical imaging (luminescence + Cerenkov) is a promising solution for the quality assurance of RI beams.2021 Virtual IEEE Nuclear Science Symposium and Medical Imaging Conferenc