RADIATION therapy is an important treatment modality in cancer therapy and new radiation species, like protons and light ions have the potential of increasing tumor conformality of irradiation. Such high precision radiotherapy treatment requires efficient quality assurance techniques. Therefore, the objective of these investigations is the development of a real time invivo dosimetry system for proton and ion beams. Proton and ion beams offer advantages over conventional treatment modalities, such as photons. Because of the way these particles deposit their energy on their path through tissue they allow for an increased dose deposition in the tumor volume and reduce the damage of the surrounding healthy tissue. However, the parameters of the ion beams must be calculated from models. Small changes in the irradiated volume will lead to a mismatch of the deposited dose maximum and the tumor. This causes missing dose in the tumor volume and potential damage to healthy tissue. Therefore, a dose monitoring system is highly desirable. Until now, the only clinically applied in-vivo dosimetry method for ion beams is Positron Emission Tomography (PET) [1]. Between 1997 and 2008, the in-beam PET method was used at the GSI Helmholtzzentrum f ur Schwerionenforschung, Darmstadt, Germany, for monitoring the dose delivered by 12C beams. Due to inherent limitations of this method, a direct quantification of the delivered dose is not feasible. Therefore, another approach currently under investigation monitors the dose via the detection of prompt gamma rays. It has been shown by several groups [2], [3], [4] that monitoring of an ion beam treatment via photon measurement is possible. Because of the high energy of the produced gamma rays and the required spatial resolution, the favored technical solution is a Compton camera system. The project is aimed to design and construct such a camera, and evaluate if it could lead to clinical applications.