Study of a novel inverted collimator concept for prompt-gamma-based range verification in proton therapy using a hybrid simulation approach.

Objective Prompt-gamma imaging is a promising technique for in-situ range verification during proton therapy, with the potential to reduce range uncertainties. Current prompt-gamma cameras typically rely on mechanical collimators, among which the knife-edge slit (KES) and multi-parallel slit (MPS) collimators are the two most widely studied concepts. In this work, we propose a novel inverted (INV) collimator concept that uses a single bar of high-density material to cast a gamma-ray shadow onto the central region of a pixelated scintillation detector. In addition to advantageous properties such as a high efficiency and a large field of view, the INV collimator also features a significantly reduced weight compared to the KES and MPS collimators. Approach We optimized and compared the different collimator designs with respect to their simulated statistical range retrieval precision. To overcome the computational time limitations associated with full Monte Carlo simulations, we developed a new hybrid simulation tool based on a dedicated fixed-ray calculation of the collimator combined with precalculated Monte Carlo simulations of prompt-gamma emission and detection. This approach enables rapid parameter optimization while incorporating all experimentally relevant effects, such as the detector spectral response and an empirical neutron background. Main results We present optimized collimator geometries for the MPS, KES, and the novel INV collimator. The INV collimator achieves a statistical range retrieval precision (2σ) of 2.2 mm at a proton beam energy of 100 MeV and 2.4 mm at 150 MeV for 108protons irradiating a homogeneous tissue-equivalent phantom, demonstrating performance comparable to the optimized KES design. In addition, we present a first experimental validation of the INV collimator concept at the CNAO hadron therapy facility in Pavia, Italy, demonstrating agreement between the hybrid simulation and measurements. Significance These results suggest that the INV collimator is a promising concept for future prompt-gamma cameras, offering competitive performance and facilitating clinical implementation due to its reduced device weight.