Discrepancy Characteristics of Positron Imaging Monitoring and Depth-Dose Distributions in Carbon Ion Radiotherapy

For heavy ion radiotherapy of tumors, a persistent worldwide challenge is the inability to accurately measure the actual depth of ion beams and to assess the precision of irradiation during treatment. Positron-emitters radionuclides such as 10 C, 11 C, and 15 O, generated during the treatment process, are deposited along the beam trajectory, enabling Positron Emission Tomography (PET) imaging and the monitoring of the dose distribution primary beam. Although the distribution of positron-emitters activity can be reconstructed by a PET system, establishing a correlation between this activity and the dose is a prerequisite for employing the system to assess carbon ion beam dose distribution. In this study, a dual flat panels In-beam PET scanner was developed to investigate the relationship between the spatial activity distribution of positron products and the spatial dose distribution of carbon ions. Experiments revealed that along the Y-axis perpendicular to the beam direction, the positron activity peak shows a positional deviation of less than 0.5 mm, allowing direct beam positioning. In the beam direction, however, while the two are correlated, they do not directly coincide under the same beam energy, a millimeter-level discrepancy exists between the positron activity peak and the dose peak, and this deviation increases with higher beam energy. Based on these findings, the study proposes that a dose monitoring model using positron activity distribution as input and incorporating machine learning methods can be established, paving a new pathway for in vivo range verification and precise dose control in carbon ion therapy.