Room: Karl Dean Ballroom B1
Purpose: In-vivo monitoring of the range is a key enabler for precise particle therapy. One promising approach is PET imaging of positron emitters generated through fragmentation reaction, however, correction of biological washout is essential. For in-beam PET, 15O is the prevalent contributor, due to its short half-life of 2 min. To provide accurate modelling of the biological washout, which can be applied in a clinical setting, we observed diffusion process of implanted 15O ions in rabbits.
Methods: The brain of an anaesthetized rabbit was irradiated with a 15O ion beam which is generated as a secondary beam in the Heavy Ion Medical Accelerator in Chiba (HIMAC). PET imaging was started simultaneously with the start of irradiation using our original large-bore PET system. Region of interests (ROIs) were set in a 3D-PET image and time activity curves (TACs) of the ROIs were acquired. The washout rate was obtained based on the multiple component model analysis.
Results: PET image revealed that implanted 15O ion beam in brain diffused out in entire body in few seconds after irradiation due to washout effect. The PET intensity was concentrated in the regions that had high blood volume. The ratio of the ROIs values of heart, lung, liver and intestine to that of brain (target position) were 2.4%, 1.8%, 1.6% and 1.2%, respectively (15 s after the irradiation). The fit of experimental TACs of heart and lung showed there are two-components in the washout process, while single exponential decay was observed in liver, intestine and skeletal muscle.
Conclusion: These results suggested that implanted 15O ion beam washed out and circulated in entire body of rabbit. The speed and ratio of washout vary by each organ. This study provided important data to investigate the biological washout mechanism and establish an accurate washout correction model.