  We report on an hybrid in-beam PET and prompt-gamma Compton imaging system
aimed at quasi real-time ion-range verification in proton-therapy treatments.
Proof-of-concept experiments were carried out at the radiobiology beam line of
the CNA cyclotron facility using a set of two synchronous Compton imagers and
different target materials. The time structure of the 18~MeV proton beam was
shaped with a series of beam-on and beam-off intervals, thereby mimicking a
pulsed proton beam on a long time scale. During beam-on intervals, Compton
imaging was performed utilizing the high energy $\gamma$-rays promptly emitted
from the nuclear reactions occurring in the targets. In the course of the
beam-off intervals in-situ positron-emission tomography was accomplished with
the same imagers using the $\beta^{+}$ decay of activated nuclei. The targets
used were stacks of different materials covering also various proton ranges and
energies. A systematic study on the performance of these two complementary
imaging techniques is reported and the experimental results interpreted on the
basis of Monte Carlo calculations. The results demonstrate the possibility to
combine both imaging techniques in a concomitant way, where high-efficiency
prompt-gamma imaging is complemented with the high spatial accuracy of PET.
Empowered by these results we suggest that a pulsed beam with a suitable duty
cycle, in conjunction with in-situ Compton- and PET-imaging may help to attain
complementary information and quasi real-time range monitoring with high
accuracy.
