The feasibility of in-beam PET for accurate monitoring of proton therapy: results of a comprehensive experimental study

Positron emission tomography is currently the only feasible method for in-situ and non-invasive 3D monitoring of the precision of the treatment in highly conformal ion therapy. Its positive clinical impact has been proven for fractionated carbon ion therapy of head and neck tumours at the experimental facility at the Gesellschaft fur Schwerionenforschung Darmstadt (GSI), Germany. Following previous promising experiments, the possible extension of the method to the monitoring of proton therapy has been investigated further in extensive in-beam measurements at GSI. Millimetre accuracy for verification of the lateral field position and for the most challenging issue of range monitoring has been demonstrated in mono-energetic and SOBP proton irradiation of PMMA targets. The irradiation of an inhomogeneous phantom with tissue equivalent inserts in combination with further dynamic analysis has supported the extension of such millimetre precision to real clinical cases, at least in regions of interest for low perfused tissues. All the experimental investigations have been reproduced by the developed modeling rather well. This indicates the possible extraction of valuable clinical information as particle range in-vivo, irradiation field position and even local deviations from the dose prescription on the basis of the comparison between measured and predicted activity distributions. Hence, the clinical feasibility of in-beam PET for proton therapy monitoring is strongly supported.