First Patient Study of In-Beam OpenPET for Range Verification in Carbon-Ion Therapy

In-beam PET measurement is recognized as one of the most practical methods for range verification of particle therapy, in which fragmentation reactions produce positron emitters. As the positron emitters are produced in small amounts and have short half-lives, measurement during or immediately after the irradiation is required to obtain as many events as possible. Therefore in-beam PET is desired, but conventional in-beam PET systems with a dual-head geometry were designed mainly for 2D imaging. Therefore, we invented the OpenPET, which has an open space for the beam to pass through even with a full-ring geometry enabling 3D imaging. Last year, we installed the $5^{\text {th }}$ prototype of the OpenPET into a treatment room of the heavy-ion medical accelerator in Chiba (HIMAC) and we reported results of the first phantom study. Following the in-beam phantom measurement, here, we report the first patient study for head and neck cancers. The patients were treated with two-port vertical and horizontal irradiations for each fraction. Because the OpenPET system is designed only for horizontal irradiation, the vertical irradiation took place without the OpenPET, and the in-beam PET measurement was performed only for the following horizontal irradiation. The measurement, which started at the beginning of the beam irradiation, was continued for 10 min after finishing the irradiation. As the number of counts in the measured coincidence data is low, we employed a maximum a posteriori ordered subset expectation maximization algorithm with median root prior for image reconstruction. As a result, we could visualize the positron emitter distribution produced in the patient body by carbon-ion irradiation.