FAPI PET Versus FDG PET/CT in Gastrointestinal Cancers: An Overview

Fibroblast activation protein (FAP) is a type II transmembrane serine protease that is highly expressed in cancer-associated fibroblasts (CAFs) but absent in quiescent fibroblasts. Its overexpression is associated with poor prognosis in various cancers and contributes to treatment resistance. In recent years, radiolabeled FAP inhibitors (FAPI) for PET imaging have shown promising clinical value across a range of cancers. Gastrointestinal (GI) malignancies, which often exhibit a desmoplastic reaction with a high density of FAP-expressing CAFs, are particularly well-suited for FAPI PET. Given the limitations of [¹⁸F]FDG PET in GI cancers, such as low sensitivity in certain histological subtypes and high physiological background uptake, FAPI PET is expected to serve as a complementary method, potentially enhancing both diagnostic accuracy and treatment guidance. This review provides a comprehensive comparison of the clinical applications of FAPI PET and [¹⁸F]FDG PET in various GI cancers, including their value in diagnosis, staging, and treatment guidance. Additionally, this review summarizes studies on the expanding role of FAPI PET, including its use in assessing treatment response and predicting prognosis, aiming to provide insights into its potential contribution to the improved management of GI malignancies.