  Uncertainties in the proton range in tissue during proton therapy limit the
precision in treatment delivery. These uncertainties result in expanded
treatment margins, thereby increasing radiation dose to healthy tissue.
Real-time range verification techniques aim to reduce these uncertainties in
order to take full advantage of the finite range of the primary protons. In
this paper, we propose a novel concept for real-time range verification based
on detection of secondary neutrons produced in nuclear interactions during
proton therapy. The proposed detector concept is simple; consisting of a
hydrogen-rich converter material followed by two charged particle tracking
detectors, mimicking a proton recoil telescopic arrangement. Neutrons incident
on the converter material are converted into protons through elastic and
inelastic (n,p) interactions. The protons are subsequently detected in the
tracking detectors. The information on the direction and position of these
protons is then utilized in a new reconstruction algorithm to estimate the
depth distribution of neutron production by the proton beam, which in turn is
correlated with the primary proton range. In this paper, we present the results
of a Monte Carlo feasibility study and show that the proposed concept could be
used for real-time range verification with millimetric precision in proton
therapy.
