Idiopathic Multicentric Castleman Disease Diagnosed after Lower Extremity Venous Thrombosis Mimicking Immunoglobulin G4-related Disease

It is difficult to distinguish idiopathic multicentric Castleman disease (iMCD) from immunoglobulin G4-related disease (IgG4-RD). A 47-year-old man was diagnosed with venous thrombosis in the right lower extremity. Multiple lymphadenopathies and splenomegaly were incidentally detected. His serum IgG4 levels were high, and the biopsied lymph nodes showed high plasma cell infiltration with many IgG4-positive cells between the follicles. He was initially diagnosed with IgG4-RD and was administered prednisolone 30 mg/day; however, inflammation and IgG4 persisted. The patient was rediagnosed with iMCD and treated with tocilizumab, which led to an improvement of his condition. When diagnosing IgG4-RD, it is therefore important to consider iMCD in the differential diagnosis.