Experimental feasibility study of proton therapy range verification by a digital tracking calorimeter prototype

Objective.Uncertainties in the proton range remain an important challenge in proton therapy. In-situ range verification techniques can be used to ensure the Bragg peak depth in the patient matches the treatment plan. Various such methods are under investigation, utilizing the signal from secondary radiation caused by proton interactions with the patient. This study experimentally investigates the feasibility of using a silicon-pixel-based sampling calorimeter for range verification through the detection of secondary photons and neutrons. Approach.The proton range verification capability of a sampling calorimeter based on monolithic active pixel sensors was tested both theoretically and experimentally. We theoretically investigate the plausibility of the planned sampling calorimeter to pick up a signal from the neutral secondary particles distal to the patient. Then, a smaller prototype pixel telescope was constructed to verify our theoretical findings experimentally by placing it behind a water tank irradiated by a medical proton beam with various energies in the range of 100-200 MeV at the Danish Centre for Particle Therapy, Aarhus, Denmark. Main results.We find that the aluminum used as absorbers between sensitive layers in the calorimeter serves as a converter for prompt gammas and neutrons. The combined detection efficiency ranges between 5.8·10^-5and 2.7·10^-3detector hits per primary proton for beam energies between 50 and 200 MeV. Irradiation of the water phantom leads to a signal in our detector, which is well-correlated with the nominal beam energy. A simulation-based model using only the number of hits per proton can predict the beam energy, acting as a proxy for proton range in a homogeneous target, down to a root mean squared error of 1.34 MeV. Significance.This study marks a promising step towards utilizing a tracking calorimeter designed for proton computed tomography for the secondary application of proton therapy range verification without additional hardware costs.