Feasibility of on-line multiple scanning proton beam range verification with PET imaging: Monte Carlo simulation studies

We investigate the feasibility for intra-fraction online multiple scanning proton beam range verifications (BRVs) with in-situ PET imaging, which is beyond the single-beam BRV we had previously studies. We simulated various 132-MeV proton beams with 4-mm diameter and different beamseparations irradiating PMMA phantoms by GATE simulations. The generated nuclear interaction-induced positrons, which were distributed in either a planar or a curved positron activity range profile, were imaged by a PET (∼2-mm resolution, 17% sensitivity in the energy range from 350 keV to 650 keV, 250×250×128 mm³ field-of-view). We calculated the activity ranges (ARs) from reconstructed PET images and compared them with the corresponding ARs of the original positron distributions. Preliminary studies show that: 1) for two separated beams with overlapped positrons, errors of AR measurement for each beam varied from 0.2 mm to 2.3 mm as center-to-center beam separation and AR difference were in the range of 8-12 mm and 2-8 mm respectively. It is feasible to achieve < 1.0-mm accuracy for two beams with a 100-second PET acquisition and 12-mm beam-separation; 2) for densely distributed beams with a continuous profile of AR distributions, PET image was blurred further at non-planar AR region than planar region which led to worsened AR measurement accuracy at non-planar region. Further studies will focus on identifying key factors that impact the AR measurement with multi-beam irradiations, such as the PET resolution model and the ways to reduce the error in nonplanar regions, sensitivity, configuration, image process and enhancement, to improve beam range verifications.