Experimental study of neutron yield in synchrotron-based carbon ion therapy: implications for neutron capture enhanced particle therapy

Objective. To investigate the feasibility of Neutron Capture Enhanced Particle Therapy (NCEPT) using synchrotron-accelerated carbon ion beams by evaluating the production and characteristics of thermal neutrons (with energies below 0.5 eV), which are optimal for neutron capture reactions. Approach. The fluence of thermal neutrons was measured via gold detector activation in a PMMA phantom irradiated with scanning carbon ion beams. Monte Carlo simulations using MCNP 6.2 were concurrently conducted to verify the experimental results and assess the potential for NCEPT dose enhancement under spread-out Bragg peak (SOBP) beam conditions. Main Results. The experimental measurement of thermal neutron fluence within the SOBP region was consistent with the Monte Carlo simulations. The simulations further revealed that the maximum neutron fluence appeared within the SOBP region. However, a quantitative comparison showed that the neutron fluence generated by the carbon ion beam is orders of magnitude lower than the minimum requirements for conventional BNCT. Consequently, the observed physical dose enhancement was not clinically significant. Significance. This study provides the first experimental evidence confirming the generation of thermal neutrons by synchrotron-accelerated scanning carbon ion beams. While the current neutron yield limits clinical utility, the spatial congruence between the maximum neutron fluence and the SOBP region remains a promising feature, serving as the basis for future research focusing on optimizing parameters of NCEPT.