IgG4 producing POEMS syndrome: A rarely recognized subtype.

Serum IgG4 is typically measured for Immunoglobulin G4-related Disease (IgG4-RD), a fibroinflammatory condition associated with polyclonal increase in serum IgG4. Yet, increased IgG4 may still be monoclonal, and little is known about IgG4 POEMS syndrome. We present a case of 40-year-old male with a mass lesion in the left sacral ala. The mass was composed of non-neoplastic fibrous tissue and dense infiltrate of mature plasmacytes with dense eosinophilic cytoplasm and eccentrically placed nuclei that express monoclonal Lambda free light chains and show diffuse positivity for IgG and IgG4. We discuss clinical manifestations and challenges encountered in the diagnosis and treatment of this rare coexistence.