The potential of in-beam positron-emission-tomography for proton therapy monitoring: first phantom experiments

Since 1997 in-beam PET is successfully used to monitor the precision of dose application in highly conformal carbon ion tumour therapy at the experimental facility at the Gesellschaft fur Schwerionenforschung Darmstadt (GSI), Germany. The potential of the method for the monitoring of proton therapy is currently under investigation. In our first experiment, three monoenergetic proton beams were stopped in targets of organic plastics placed in the centre of the field of view of the in-beam PET scanner at GSL The β⁺-activity was found to be three times higher than that induced by carbon ions at the same range and applied dose. The reconstructed β⁺-activity distributions were rather well reproduced by model calculations. The possible extraction of valuable clinical information from the comparison between measured and calculated 3´-activity distributions is discussed. Despite the weaker spatial correlation between β⁺-activity and dose depth-profiles in the proton case, the presented analysis strongly supports the feasibility and clinical usefulness of in-beam PET for the monitoring of proton therapy.