[68Ga]Ga-FAPI Versus [18F]FDG PET/CT in Signet Ring Cell Carcinoma with Linitis Plastica: Diagnostic Performance and Pitfalls.

The diagnosis and staging of primary gastric carcinoma, particularly linitis plastica morphology, is challenging. Fibroblast activation protein, abundantly expressed in cancer-associated fibroblasts and absent in normal gastrointestinal mucosa, enables high tumor-to-background contrast. We report 2 cases of signet ring cell gastric adenocarcinoma with linitis plastica morphology, demonstrating minimal uptake on [18F]FDG PET/CT but intense uptake on [68Ga]Ga-FAPI-04 PET/CT. These findings highlight the superior diagnostic performance of fibroblast activation protein inhibitor imaging in this challenging subset of gastric cancers.