Development and evaluation of an in-beam PET system for proton therapy monitoring.

Objective: In-beam positron emission tomography (PET) has important development prospects in real-time monitoring of proton therapy. However, in the beam-on operation, the high bursts of radiation events pose challenges to the performance of the PET system.

Approach: In this study, we developed a dual-head in-beam PET system for proton therapy monitoring and evaluated its performance. The system has two PET detection heads, each with 6 Χ 3 Plug&Imaging (PnI) detection units. Each PnI unit consists of 6 Χ 6 lutetium-yttrium oxyorthosilicate crystal arrays. The size of each crystal strip is 3.95 Χ 3.95 Χ 20 mm^3, which is one-to-one coupled with a silicon photomultiplier. The overall size of the head is 15.3 Χ 7.65 cm^2.

Main Results: The in-beam PET system achieved a single count rate of 48 Mcps at the activity of 144.9 MBq, an absolute sensitivity of 2.717%, and a spatial resolution of approximately 2.6 mm (full width at half maximum) at the center of the field-of-view. When imaging a Derenzo phantom, the system could resolve rods with a diameter of 2 mm. Time-dynamic [18F]-Fluorodeoxyglucose mouse imaging was performed, demonstrating the metabolic processes in the mouse. This shows that the in-beam PET system has the potential for biology-guided proton therapy. The in-beam PET system has been used to monitor the range of a 130 MeV proton beam irradiating a polymethyl methacrylate (PMMA) phantom, with a beam intensity of 6.0*10^9 p/s and an irradiation duration of one minute. PET data were acquired only during the one-minute irradiation. We simulated the range shift by moving the PMMA and adding an air gap, showing that the error between the actual and the measured range is less than 1 mm.

Significance: The results demonstrate that the system has a high count rate and the capability of range monitoring in beam-on operation, which is beneficial for achieving real-time range verification of proton beams in the future.