General
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• Rare; crystalline material accumulates in cytoplasm of histiocytes; usually kappa light chain origin
  
• Typically associated with disorders that express monoclonal immunoglobulins such as myeloma, lymphoplasmacytic lymphoma, monoclonal gammopathy of undetermined significance

Case reports
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• 51 year old woman with upper lip and cheek tumor (Head Neck Pathol 2012;6:111)
  
• 73 year old woman with tongue mass, chronic rheumatoid arthritis, polyclonal hypergammaglobulinemia (Arch Pathol Lab Med 1998;122:920)

Micro description
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• Histiocytes filled with nonbirefringent eosinophilic crystals
  
• Lymphocytes and plasma cells also present
  

Micro images
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Eosinophilic histiocytes

CD68+

Immunoglobulin stains

Positive stains
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• Crystals are typically monoclonal, immunoreactive with kappa or lambda light chain and gamma heavy chain
  
• CD68+ histiocytes

Negative stains
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• Desmin, muscle-specific actin, myoglobin, S100, CD1a

Electron microscopy images
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Dense crystals

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