Striking effectiveness of siltuximab-based treatment in refractory idiopathic multicentric Castleman disease resembling IgG4-related disease.

Castleman disease (CD) and immunoglobulin G4-related disease (IgG4-RD) are rare systemic immune-mediated disorders that share similar clinical manifestations and overlapping pathological features. We present a case of insidious idiopathic multicentric CD (iMCD) with elevated serum IgG4 levels, characterized by a persistent dry cough and widespread lymphadenopathy. A 38-year-old male patient exhibited persistent dry cough, generalized lymphadenopathy, and fatigue. An inguinal lymph node biopsy showed plasma cell-type MCD characteristics, such as atrophic lymphoid follicles and significant plasma cell infiltration. Immunohistochemical staining revealed positivity for IgG, CD38, and CD138, with over 50 IgG4-positive plasma cells per high-power field and an IgG4/IgG ratio of 40%, confirming the diagnosis of the idiopathic plasmacytic lymphadenopathy (IPL) subtype of multicentric Castleman disease (iMCD-IPL). After receiving a siltuximab-based regimen followed by radiotherapy, the patient achieved a notable partial remission. We present a case illustrating the significant efficacy of siltuximab-based therapy in iMCD-IPL, resembling IgG4-related disease.