3D SCINTILLATOR DETECTOR QUENCHING CHARACTERIZATION FOR SCANNING PROTON BEAMS

Proton pencil beam scanning is becoming the standard treatment delivery technique for proton therapy centers. Scanned proton pencil beams provide a highly conformal dose distribution. The complex dose distribution poses challenges for quality assurance measurements leading to sophisticated detector setups and time consuming measurements. Fast 3D measurements are therefore desirable to verify the complex dose distribution and to enable the utilization of the full potential of proton therapy. The overall objective of this project is to improve volumetric scintillators detectors to provide 3D measurements for applications for beam commissioning, quality assurance program, and patient-specific treatment delivery verification. Detectors based on volumetric scintillators are gaining interest for use in proton therapy because they promise fast and high-resolution proton beam measurements. However, the scintillators’ response depends on the ionization density of the incident radiation, termed ionization quenching. For protons and other heavy charged particles, the ionization density, which is quantified as the linear energy transfer (LET), varies as a function of depth. Therefore, quenching introduces a non-linear response to the absorbed dose of proton beams. To fully utilize volumetric scintillator detectors for dose verification, ionization quenching correction factors are needed. Previous studies have shown the feasibility of using multiple cameras to image volumetric scintillators for obtaining real-time measurements, and 3D information. Furthermore, ionization quenching correction models based on the widely used Birks’ equation was shown to have lower dose accuracy at the Bragg peak for low-energy beams. The purpose of this study is to accurately determine the ionization quenching correction factors and to characterize a novel 3D scintillator detector for scanned proton beams. The 3D scintillator detector consisted of a liquid scintillator filled tank imaged by three identical sCMOS cameras. The system exhibited a high spatial (0.20 mm) and temporal resolution (10 ms). It was capable of capturing and verifying the range of all the 94 beam energies delivered by the synchrotron with sub-millimeter accuracy. The use of multiple orthogonally positioned cameras allows for detecting the precise locations of delivered beams in 3D. The beam images captured by the detector were synchronized with synchrotron beam delivery trigger signals. The developed image acquisition technique demonstrates the capability of the detector to capture single spots with a reproducible accuracy of 2%. Ionization quenching correction factors were used to correct the response of scintillators for dose linearity. The EDSE scintillation model was explored which relates the scintillation light emission to the energy deposition by secondary electrons. This project explored key improvements necessary for volumetric scintillator-based detector and demonstrated the capabilities of a novel 3D scintillator detector as a potential comprehensive quality assurance tool and for patient treatment verification detector for spot scanning proton therapy