Total-Body Dynamic PET/CT Imaging of Proton-Induced Activity and Biologic Washout After Proton Therapy

Off-line PET imaging after proton therapy is limited by transport delay and by the sensitivity and short axial field of view of conventional scanners, which hinder imaging at ultralow activity and characterization of whole-body biologic washout. We evaluated whether a near-room total-body PET/CT system could enable clinically practical imaging of proton-induced activity after treatment and its whole-body biologic washout. Methods: We conducted a series of phantom studies to evaluate the performance of total-body PET at ultralow activity with Monte Carlo (MC)-simulated activity distributions as references. Nineteen patients with solid tumors underwent off-line total-body dynamic PET/CT imaging shortly after proton therapy. Spatial correlation between PET and MC results was quantified using the Dice similarity coefficient (DSC) and normalized cross-correlation (NCC). Dynamic images were reconstructed, and time-activity curves were extracted from volumes of interest to analyze the dynamic behavior of proton-induced activity. Results: In phantom experiments, PET images showed close spatial correspondence to MC reference distributions under ultralow activity. In patients, the agreement between PET and MC was higher for relatively stationary targets, with a mean DSC/NCC of 0.81 ± 0.09/0.81 ± 0.09 for the brain and 0.81 ± 0.05/0.83 ± 0.08 for the breast. Dynamic total-body PET images revealed biologic washout and whole-body redistribution of isotope activity within and beyond the gross tumor volumes, showing enrichment within the cardiac blood pool, major blood vessels, and blood-rich organs, such as the spleen and liver. Conclusion: Near-room total-body PET/CT enabled interpretable imaging under ultralow activity after treatment. Dynamic total-body imaging additionally captured whole-body biologic washout of proton-induced activity through blood circulation, providing a foundation for future washout modeling and methodologic development for in vivo treatment assessment.