Potential Role of 18F-FAPI-74 PET/CT in Proton Beam Therapy for Liver Metastasis: Clinical Insights and Limitations

We present 18F-FAPI-74 PET/CT findings before and after proton beam therapy (PBT) in a 71-year-old woman with liver metastasis following pancreatic cancer resection. Before PBT, no uptake was observed on FDG-PET, whereas the lesion was visualized on FAPI-PET, consistent with contrast-enhanced MRI. Follow-up FAPI-PET at 3 and 6 months post-PBT revealed mild, newly developed uptake confined to the high-dose irradiated area. However, contrast-enhanced MRI demonstrated complete tumor disappearance and normalization of serum tumor marker levels. These findings suggest that post-irradiation 18F-FAPI-74 uptake is more likely attributable to reactive changes, such as radiation-induced hepatic fibrosis, rather than residual or recurrent tumor.