99m Tc- FAPI Uptake in Immunoglobulin G4-Related Orbital and Sinonasal Disease With Bone Involvement.

An 85-year-old man with right periorbital edema, bloody epiphora, and rhinorrhea underwent nasal endoscopy, revealing a polypoid lesion in the right maxillary sinus. The tissue biopsy revealed dense lymphoplasmacytic infiltration with elevated IgG4-positive plasma cells, consistent with IgG4-related disease. 99m Tc-FAPI-46 SPECT/CT was performed and demonstrated intense uptake in the maxillary sinus polyp and along its bony walls, as well as medial orbital and fronto-ethmoidal regions, correlating with CT findings. Fibroblast activation protein inhibitors (FAPI) imaging delineated active fibro-inflammatory disease beyond structural changes, highlighting its potential in detecting osseous involvement in IgG4-related disease.