• Description: a rare disorder caused by clonal proliferation of Langerhans cells (antigen-presenting cells)
• Epidemiology: peak incidence between 5 and 10 years
• Clinical features
  • Osseous lesions (90%): pain and/or swelling, pathological fractures
    • Localization: skull (most common site), femur, vertebra, mandible, ilium, ribs
      • Can cause recurrent otitis media if the mastoid bone is involved
    • Unifocal or multifocal
  • Systemic manifestations (less common)
    • Skin rash: brown to purple papules and/or eczematous rash
    • Hepatomegaly and/or splenomegaly
    • Lymphadenopathy
    • Bone marrow infiltration
    • Hypopituitarism that typically manifests as central diabetes insipidus
• Diagnostics
  • Normal calcium levels
  • X-ray: osteolytic lesions with or without marginal sclerosis
  • Biopsy (confirmatory test): proliferation of Langerhans cells (polygonal cells with coffee bean-shaped nuclei and eosinophilic cytoplasm)
    • Electron microscopy: Birbeck granules (tennis racket-shaped or rod-like structures) in the cytoplasm
    • Immunohistochemistry: cells are positive for S100, CD1a, and/or CD207