Relapsing Polychondritis Mimicking ANCA-Negative Granulomatosis with Polyangiitis: Diagnostic Value of ¹⁸F-FDG PET/CT

Relapsing polychondritis (RP) is a rare autoimmune disease of cartilaginous structures, often diagnosed late due to nonspecific presentations. Both RP and granulomatosis with polyangiitis (GPA) can cause diffuse tracheobronchial wall thickening on computed tomography (CT) and may be seronegative for anti-neutrophil cytoplasmic antibody (ANCA), creating a diagnostic impasse. We report a 46-year-old man with two months of fever, productive cough, and sternal pain. A saddle nose deformity was the only cartilaginous sign; serum ANCA was repeatedly negative. Neck CT showed diffuse tracheal and bilateral main bronchial wall thickening; the report listed amyloidosis and GPA as differential diagnoses, omitting RP. Despite laboratory, microbiological, and imaging workup, the fever fulfilled criteria for fever of unknown origin (FUO), prompting ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT. PET/CT demonstrated intense FDG uptake in the cartilaginous wall of the tracheobronchial tree, forming the classic inverted-Y sign, with bilateral costal cartilage hypermetabolism (a site not involved in GPA) and no uptake in the kidneys, sinuses, or orbits, collectively establishing a diagnosis of RP. Corticosteroid therapy elicited prompt clinical and biochemical response. This case demonstrates that ¹⁸F-FDG PET/CT can differentiate RP from GPA when CT and serology are uninformative.