FDG-Avid But FAPI-Negative Intramucosal Carcinoma in a Colorectal Adenoma.

Fibroblast activation protein inhibitor (FAPI) PET targets cancer-associated fibroblasts and has demonstrated high uptake in advanced colorectal cancer. However, its performance in early lesions remains unclear. A man in his 70s underwent FDG PET/CT and 18 F FAPI-74 PET/CT for follow-up of hepatocellular carcinoma. FDG PET showed intense focal uptake in the sigmoid colon, whereas FAPI PET demonstrated no abnormal uptake. Endoscopic resection revealed carcinoma in adenoma confined to the lamina propria without desmoplastic reaction. FAPI uptake reflects stromal fibroblast activity and may therefore be limited in intramucosal adenoma-dominant lesions.