  In this work, we propose a novel technique for in-vivo proton therapy range
verification. This technique makes use of a small hadron tumour marker,
$^{92}$Mo, implanted at a short known distance from the clinical treatment
volume. Signals emitted from the marker during treatment can provide a direct
measurement of the proton beam energy at the marker's position.
Fusion-evaporation reactions between the proton beam and marker nucleus result
in the emission of delayed characteristic $\gamma$ rays, which are detected
off-beam for an improved signal-to-noise ratio. In order to determine the
viability of this technique and to establish an experimental setup for future
work, the Monte Carlo package GEANT4 was used in combination with ROOT to
simulate a treatment scenario with the new method outlined in this work. These
simulations show that analyzing the intensity of delayed $\gamma$ rays produced
from competing reactions yields a precise measurement of the range of the
proton beam relative to the marker, with sub-millimeter uncertainty.
