An Atlas of Gallium-68-fibroblast Activation Protein Inhibitor (FAPI) in Oncology and Nononcology Imaging

Fibroblast activation protein (FAP) is a quinoline-based membrane bound glycoprotein enzyme. It is not expressed in normal adult tissue except for the myometrium and sometimes in pancreas. The expression increases in inflammation and in cancer-associated fibroblasts. Gallium 68-FAP inhibitor (FAPI) is known to localize in various neoplasms by the virtue of their property of cancer-associated fibroblast activity. Fibroblasts are also known to be associated in inflammatory and infectious condition such as cholecystitis, pyelonephritis, and tuberculosis. Immunoglobulin G4 related disease (IgG4RD) is characterized by infiltration of various tissues with IgG4-rich lymphoplasmacytic fibrotic tissue and has been reported to express cancer-associated fibroblast. The time-tested molecule for cancer imaging F18-fluorode-oxyglucose requires 4-6 h of fasting state. Ga68-FAPI does not require fasting. This pictorial essay illustrates various entities, cancerous as well as noncancerous using Ga68-FAPI positron emission tomography computed tomography (PET CT) imaging.