Biological washout modelling for in-beam PET: rabbit brain irradiation by ¹¹C and ¹⁵O ion beams.

Positron emission tomography (PET) has been used for dose verification in charged particle therapy. The causes of washout of positron emitters by physiological functions should be clarified for accurate dose verification. In this study, we visualized the distribution of irradiated radioactive beams, C-11 and O-15 beams, in the rabbit whole-body using our original depth-of-interaction (DOI)-PET prototype to add basic data for biological washout effect correction. Time activity curves of the irradiated field and organs were measured immediately after the irradiations. All data were corrected for physical decay before further analysis. We also collected expired gas of the rabbit during beam irradiation and the energy spectrum was measured with a germanium detector. Irradiated radioactive beams into the brain were distributed to the whole body due to the biological washout process, and the implanted C-11 and O-15 ions were concentrated in the regions which had high blood volume. The C-11-labelled (CO2)-C-11 was detected in expired gas under the C-11 beam irradiation, while no significant signal was detected under the O-15 beam irradiation as a form of (CO2)-O-15. Results suggested that the implanted C-11 ions form molecules that diffuse out to the whole body by undergoing perfusion, then, they are incorporated into the blood-gas exchange in the respiratory system. This study provides basic data for modelling of the biological washout effect.