= rare benign histiocytosis

Etiology: reaction to infectious agents / autoimmune process

Histo: polymorphous infiltrate of lymphoplasmacytic cells and histiocytes of varying size embedded in fibrous stroma; S-100 + CD68 positive; CD1a negativity excludes Langerhans histiocytosis; CHARACTERISTIC emperipolesis (= phagocytosis of lymphocytes) in 70%

[em, Greek = inside, peri = around, polemai = wander about]

1. EXTRACRANIAL
   Age: primarily in children + young adults; M >F
   • painless bilateral cervical adenopathy + involvement of nasal cavity, bone, orbit
2. INTRACRANIAL (40%)
   Age: 4^th–5^th decades; M÷F=3÷1
   Location: cerebral convexities, parasagittal, petroclival, suprasellar regions
   • headache, visual change, seizures, numbness, paraplegia
   • well-circumscribed dural-based iso- to hyperattenuating single / multiple masses
   • isointense on T1WI + iso- to hypointense on T2WI:
     • central low SI on T2WI ← release of free radicals by inflammatory macrophages
   • marked enhancement + dural tail (common)
   • edema within adjacent brain parenchyma (frequent)
     DDx: meningioma (iso- to hyperintense on T2WI)