Fast neutron scatter imager for proton range monitoring in hadron therapy: First experimental results

Accurate in-vivo beam range verification is crucial for both maximization of therapeutic benefits and minimization of normal-tissue complications in hadron therapy. This study presents the first experimental proof-of-concept for proton beam range monitoring utilizing a prototype fast neutron scatter imager. The imaging system consists of 4×4 arrays of stilbene and EJ-276 organic scintillators coupled to silicon photomultipliers (SiPMs) with a multiplexed readout. At the National Cancer Center in the Republic of Korea, a clinical proton beam with four discrete energy levels (102.71, 152.09, 202.36, and 221.86 MeV) irradiated a tissue-equivalent PMMA phantom. By applying a list-mode maximum likelihood expectation maximization (LM-MLEM) algorithm, the spatial distributions of secondary neutrons were successfully reconstructed from double-scatter events. The reconstructed images clearly visualized the macroscopic secondary-neutron distribution and its distinct distal fall-off, close to the proton's Bragg peak. Notably, the energy-dependent spatial shifts of the neutron production position were accurately captured, which results strongly aligned with Geant4 Monte Carlo simulations. Although the current geometric efficiency and temporal processing bottlenecks limited the overall event yield, the present study's qualitative localization of the secondary-neutron distribution relative to the proton Bragg peak successfully demonstrates, for the first time, the fundamental feasibility of fast neutron scatter imaging. With planned hardware upgrades, this approach will pave the way for a robust, real-time in-vivo range verification system applicable to advanced heavier ion treatments such as carbon ion therapy.