Gamma electron vertex imaging for in-vivo beam-range measurement in proton therapy: Experimental results

Proton therapy, thanks to the dose characteristics of the Bragg peak, according to which most of the radiation energy is delivered at the end of the beam with a very high dose gradient at the distal edge, can deliver a highly conformal radiation dose to the treatment volume. Currently, however, the benefit of this high dose gradient is not fully utilized in clinical practice due mainly to the dose-distribution uncertainty in the beam direction (i.e., the uncertainty of the beam range in the patient). In this paper, we present an imaging system based on gamma electron vertex imaging (GEVI), which is suitable for high-energy (1-30 MeV) gammas, and test its performance for therapeutic proton beams. GEVI images prompt gamma vertices, which are closely correlated with the dose distribution at the distal edge, by converting prompt gammas to electrons via Compton scattering and then tracking the recoiled electrons. Our experimental results show that the GEVI system can image the 2D vertices of the prompt gammas and, thus, can be utilized for the measurement of proton-beam ranges in patients. We believe, indeed, that GEVI makes possible real-time monitoring of in-vivo proton-beam ranges, whose utility significantly improves treatment effectiveness and enhances patient safety. We also expect that the GEVI system will find applications in other fields (e.g., gamma-ray astronomy, nuclear engineering, and high-energy physics) requiring high-energy-gamma (1-30 MeV) imaging. Published by AIP Publishing.