The purpose of this clinical study is to investigate the clinical feasibility and effectiveness of off-line Positron-Emission-Tomography (PET) quality assurance for promoting the accuracy of proton and carbon ion beam therapy. One main clinical advantage of ion therapy over conventional radiation therapy is the excellent conformation of the delivered dose to the tumour volume while well sparing the surrounding healthy tissue. However, clinical exploitation of this potential to the maximum extent requires in-vivo validation of the actual treatment delivery and, in particular, of the ion beam range within the patient. Since the primary ions are completely stopped in the target volume as opposed to photon radiation, no conventional quality assurance techniques like transmission electronic portal imaging can be applied to monitor ion beam therapy. Hence, ion treatment planning currently relies on models and experimental data accurately validated in tissue-equivalent targets, but no direct verification of the actual treatment delivery and of the ion beam range within the patient is possible in standard clinical practice.