Five-Year Persistent 18F-FAPI-74 Uptake in Radiation-induced Hepatic Fibrosis Following Proton Beam Therapy for Hepatocellular Carcinoma.

A 79-year-old man underwent proton beam therapy (PBT) for hepatocellular carcinoma. During follow-up, MRI demonstrated stable T2-weighted hyperintensity in the irradiated region without interval enlargement or mass formation. Five years after PBT, a new hepatocellular carcinoma developed elsewhere in the liver. 18F-FDG PET showed no apparent abnormal uptake in the irradiated region, whereas 18F-fibroblast activation protein inhibitor-74 PET demonstrated persistent uptake. These findings may reflect chronic radiation-induced fibroinflammatory changes and suggest that fibroblast activation protein inhibitor-PET can visualize such changes even years after PBT.