• Benign cystic developmental lesion
• Less than 0.5% of primary intracranial tumors
• Results from a failure of separation or abnormal sequestration of superficial ectodermal components from the deeper developing neural structures during embryogenesis
• Very common lesion of the head and neck, with extracranial lesions often occurring in the orbits

• Usually unilocular but with a thicker rim than epidermoid cyst
• Inclusion elements of lipid material from apocrine or sebaceous material
• Fat may float on fluid component
• Grow slowly over months to years due to secretions and desquamation

• Signs and Symptoms
  • Often discovered incidentally on imaging
  • Sinus tract may be visible on the skin
    • Can become infected through this communication
  • May rupture spontaneously due to trauma or in the iatrogenic setting, causing symptoms of severe chemical meningitis
• Age and Sex
  • Slight male predilection
  • Usually manifests in children
    • May manifest younger with higher-risk areas of the brain
      • Due to hydrocephalus
      • More tightly enclosed space of the posterior fossa
• Prognosis
  • Usually excellent due to benignity of lesion and resectability
  • Usually only resected if symptomatic or potentially obstructive

• General
  • Lesion containing or entirely made up of liquid fat
  • Usually extra-axial when intracranial
  • Often in the sellar/parasellar regions
  • Often present in other head/neck locations, most commonly in and around the orbits
  • Slow growing over months to years
  • Can cause obstructive hydrocephalus
  • Disseminated fatty sebaceous material layering nondependently in the cerebrospinal fluid (CSF) spaces with rupture
  • Rim may calcify
• Modality specific
  • CT
    • Predominantly hypodense due to fatty components
      • Hounsfield Units usually between −50 and −300
    • Hyperdense calcifications may be present in the rim
  • MRI
    • T1WI
      • Hyperintense fatty components in the cyst, usually somewhat heterogeneous in pattern
      • Hyperintensity in the anterior aspects of CSF spaces (if patient is supine in the scanner), layering nondependently due to lower molecular weight than CSF
    • T2WI
      • Dermoid components will follow fat signal
      • Bright or dark on T2WI
    • DWI
      • May be bright on DWI, but usually no true restricted diffusion (ADC not dark)
    • GRE/SWI
      • Black susceptibility signal of the fatty components
    • T1+contrast
      • Bright components on contrasted images are usually not true enhancement but actually intrinsic T1 hyperintensity of fatty components
      • Leptomeningeal or ependymal enhancement may be present with chemical meningitis
      • If fat-saturated contrast sequence is obtained, dermoid elements will become dark

• MRI to include fat-saturated and non-fat-saturated sequences

• Hematoma or hemorrhagic mass may have similar T1 hyperintensity but should not follow fat signal on all sequences
• Epidermoid cyst usually not bright on T1WI, demonstrates striking restricted diffusion
• Lipoma is usually more homogeneous in T1-bright signal intensity
• Other tumors, location dependent, do not contain fat
• Pneumocephalus has a similar appearance to ruptured dermoid on CT, and adjusting the window/level settings will demonstrate fat density that is less dark than air

Contributor: Aaron Kamer, MD

DOI: https://doi.org/10.18791/nsatlas.v1.03.01.10