Clinical Imaging Performance Evaluation of a Multi-Knife-Edge Slit Collimator-based Prompt Gamma Ray Imaging System

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Project Summary Proton therapy’s intrinsic advantage over conventional radiotherapy is the ability to selectively deposit large amounts of dose within a primary tumor site or region of interest while minimizing dose to healthy tissue. To fully leverage this advantage, proton range must be precisely controlled, monitored, and verified. Small errors in particle delivery can result in significant dose delivered to healthy tissue and more importantly minimal dose delivered to target volume. However, there is a lack of clinical solutions for proton range verification. Current approaches employ either positron emission tomography (PET)-based or prompt gamma imaging (PGI)-based techniques, which exploit the secondary emissions from charge particle tracks. PET is enabled for proton range verification due to positron emitting nuclei created in the charge particle tracks. Unfortunately, radionuclides of interest for this technique can have a half-life as long as 20 minutes, wherein significant biological washout can occur, degrading the correlation between annihilation photon origin and the original proton track location. PGI presents the most promising opportunity for real-time, in vivo range verification for charge particle therapies, as the secondary emissions in this approach are produced on very short time scales (picoseconds-to-milliseconds). To address the need for a clinical in vivo range verification system, Lawrence Berkeley National Laboratory (LBNL) has developed a prototype imaging system based on a novel multi-knife-edge slit collimator for high energy gamma-rays in combination with pixelated scintillation detector readout. This prototype provides two- dimensional imaging of PGI tracks using this collimator. Preliminary evaluations of the imager with 50 MeV proton beams showed exceptional promise for imaging proton tracks via their prompt gamma emissions, including sub- mm precision for proton range quantification with proton statistics commensurate to those present during therapy. We propose to evaluate this prototype PGI system in clinically relevant beam energies, beam currents, and scenarios to demonstrate its performance capabilities in a clinical setting. Data from these evaluations will fully characterize the prototype system’s capabilities, highlight clinical utility, and also guide the design of an advanced clinical imaging system for real-time, in vivo proton range verification in a small modular platform that can be positioned about the patient and gantry for any treatment regimen.