Purpose: Real-time in vivo control of the ion range in a patient during protontherapy is a major challenge for Quality Assurance of treatments. A few years ago, prompt gamma rays have been investigated for beam range verification with proton1 and carbon ion beams2. Since then, several teams in the world have been progressing toward the construction of first clinical prototypes3,4,5. The imaging concept is usually designed and optimized with the help of Monte Carlo simulations (MC), which have become the gold standard for physical calculations especially for simulations of prompt gammas emitted by proton inelastic interaction in complex geometries. It remains however hindered by its slow statistical convergence.