Patient data-based Monte Carlo simulation of in-beam single-ring OpenPET imaging

We are developing the OpenPET, which can provide an open space to make the patient accessible during positron emission tomography (PET) measurements. The OpenPET provides PET measurements during particle therapy such as carbon ion beam treatment, enabling visualization of the irradiation field of the patient as well as direct tumor tracking with a real-time imaging system. In the carbon ion beam treatment, positron emitters are produced by the fragmentation reactions. Therefore, by means of PET, it can be confirmed whether or not the dose distribution in the treatment plan and that in the actual irradiation are matched. In this study, we conducted Monte Carlo simulations based on clinical patient data for scanned
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C ion beam irradiation to investigate the effectiveness of the OpenPET geometry compared with offline PET measurements. Production rates of the positron emitters were calculated based on the patient CT image and physical dose information in the treatment plan by the FLUKA code. The activity distribution averaged over the measurement time was calculated from the production rates by an analytical model including activity decay and biological washout effects. Annihilation gamma ray events from the activity distribution were simulated by the Geant4 Monte Carlo toolkit. Activity distribution images were reconstructed by the list-mode maximum a posteriori ordered subset expectation maximization. As a result, we found the online OpenPET with a measurement time of 10 minutes could detect a similar number of list-mode data events as an offline PET with a measurement time of 30 minutes. Reconstructed images for the online OpenPET contained activity distribution not only around the distal edge but also at the entrance area of the irradiation field. The offline PET showed strong noise before the Bragg peak position. The simulation results showed that online OpenPET could achieve good quality of images with a short measurement time.