The promptly emitted photons during cancer ion beam treatment offer an additional possibility of dose monitoring that potentially overcomes some of the draw-backs of positron emission tomography as ion therapy monitoring modality. With a Compton camera it might be possible to use these γ-rays for imaging. Our group has developed a small evaluation detector system to investigate this further. The performance allows the resolution of radioactive point sources of 5.9 mm full width at half maximum at 7 cm distance. The system appears to be viable to study the feasibility of in-beam dose monitoring by imaging prompt photons in practice.