A novel approach for predicting the positron emitter distributions produced during therapeutic ion irradiation

In-beam PET is a valuable method for a beam-delivery independent dose monitoring in radiation therapy with ion beams. The clinical feasibility of in-beam PET has been demonstrated for carbon and proton beams up to now. From radiobiological point of view it is highly desirable to perform tumor irradiation also with other ions. To extend the application of in-beam PET also to these ions, extensive knowledge about positron emitter production via nuclear fragmentation reactions during ion irradiation is necessary. To model the positron emitter production correctly, cross sections for all possible nuclear reactions occurring in the tissue during irradiation and leading to positron emitters are required. Since for many ions of therapeutic interest these cross sections are not available in the required energy range, a novel approach for estimating the positron emitter production from experimental data is introduced. The prediction of positron emitter distributions is based on depth dependent thick target yields, which are deduced by linear super-position of measured yields in water, graphite and polyethylene as reference materials. First results on the prediction of positron emitter distributions in polymethyl methacrylate (PMMA) targets induced by Li and C irradiation are presented. By comparison with data deduced from experiments, it is shown that a rather accurate prediction of positron emitter distribution in PMMA is possible with this method.