IgG4-Related disease simulating paraneoplastic syndrome: Role of ¹⁸FDG PET/CT imaging

Immunoglobulin G4 (IgG4)-related systemic disease (IgG4-RSD) is a new systemic entity associated with autoimmune pancreatitis (AIP). Other organ involvements take the form of sclerosing cholangitis, sclerosing cholecystitis, sclerosing sialadenitis, retroperitoneal fibrosis, and interstitial nephritis. Recently, lung diseases related to IgG4 have been described to occur with or without other organ involvement. These diseases include interstitial lung disease (ILD), pulmonary inflammatory pseudotumor, and lymphomatoid granulomatosis. Most of these cases occur in combination with AIP, which also appears to have a general preponderance for males. The true incidence of IgG4-related ILD and the incidence of AIP are unknown. Here, we describe a case of a 53-year-old gentleman who presented with weight loss, fever, loose motions, altered sensorium, and persistent low hemoglobin, for which he was referred for 18-fluorodeoxyglucose positron emission tomography/computed tomography scan to diagnose probable underlying occult malignancy/paraneoplastic syndrome. It revealed features suggestive of IgG4 disease involving the pancreas and lungs, which was confirmed subsequently.