A Study on Proton Track Reconstruction in a Pixel-Based Telescope Using a Clinical Proton Beam

Proton computed tomography (pCT) is a novel 3-D medical imaging technique proposed for the pretreatment diagnosis of patients undergoing proton therapy. The two main components of our pCT prototype are a proton tracker and a calorimeter. A proton tracker allows precise tracking of proton trajectories, and a proton calorimeter provides accurate measurement of their energies. For our design, we use a specific monolithic active pixel sensor (MAPS) called Alice pixel detector (ALPIDE) as an essential part of both a proton tracker and a proton calorimeter. Our pCT prototype has been tested with the Varian ProBeam Compact Proton Therapy System at King Chulalongkorn Memorial Hospital (KCMH), Bangkok, Thailand. The integrated field-programmable gate array (FPGA) trigger has been developed to synchronize between the proton beam and the pCT components. The trigger signals are transmitted to produce detection events for the ALPIDE chips and to regulate the proton gating. With this setup, we have studied the possibility of reconstructing proton tracks from activated pixels in a two-sigma region of a Gaussian profile. This study focused primarily on the proton beam, which had the lowest energy of 70 MeV, and the transmission beam used for pCT imaging, which carried an energy of 200 MeV. For both energies, almost 70% of the tracks could be effectively reconstructed from every layer of the ALPIDE chips. The track results of the MAPS telescope run under our trigger system and confirm that the ALPIDE chips can be used as a tracker for the pCT prototype.