At present, PET offers the unique possibility to monitor the precision of ion irradiation in-vivo and non-invasively. The method is based on the detection of the b+-activity which is formed as a by-product of the irradiation, i.e. without administration of radio-tracers to the patient. A positive clinical impact of in-beam (i.e. during the irradiation) PET monitoring has been demonstrated for carbon ion therapy in the pilot project at GSI Darmstadt, Germany, and promising clinical data of post-radiation PET/CT imaging have been recently reported for passive proton beam delivery in USA and Japan. Therefore, a pilot clinical study is hereby proposed at the Heidelberg Ion Therapy Center in order to 1) assess the applicability of post-radiation PET imaging to scanned ion beam delivery, 2) identify the patient population which may benefit from it and 3) extract population-based information on the reliability of the beam range in different tumour locations for all the ion species clinically available at HIT.