In-beam PET measurements of beta+ radioactivity induced by proton beams.

Our first in-beam PET measurements of the beta+ activation induced by proton irradiation are presented. Monoenergetic proton beams in the energy and intensity range suited for the treatment of deep-seated tumours were delivered by the synchrotron of the Gesellschaft für Schwerionenforschung Darmstadt (GSI). They were stopped in PMMA blocks placed in the centre of the field of view of the positron camera that is installed in the heavy ion tumour treatment facility at GSI. The beta+ activity signal was found to be three times larger than that produced by carbon ions at the same range and applied physical dose. The reconstructed spatial beta+ activity distributions were analysed and compared with the production of positron emitters predicted by a calculation based on experimental cross-sections and on the proton flux given by the FLUKA Monte Carlo code. The shape of the depth-activity profiles was well reproduced by the model and the correlation with the proton range and the depth-dose distributions was carefully investigated. Despite the non-trivial range determination from the beta+ activity distribution in the proton case, our experimental investigation supports the feasibility of an in situ proton therapy monitoring by means of in-beam PET, as already clinically implemented for the monitoring of carbon ion therapy at GSI Darmstadt.