The in-beam PET method is used to monitor treatment in C-12 radiotherapy at GSI (Darmstadt, Germany). The β+-activity arising from nuclear interactions between beam particles and tissue is measured. Since activity and dose are not connected in an easy way, in-beam PET depends on comparison between the measured activity and a simulation. In future ion therapy facilities like HIT (Heidelberg, Germany), beams of other ions like O-16 will be used for treatment. To enable in-beam PET for other ions, a new simulation approach is needed. We have performed the first in-beam PET measurements with O-16 beams and used the obtained data for benchmarking the FLUKA Monte Carlo code. We have developed new biasing methods to keep the execution times short. Overall good agreement between calculated and measured activity was found. We conclude that the FLUKA code can be a valuable choice for application of in-beam PET monitoring of ion therapy.

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