Pediatric Dermoid

• Etiology:
  — Benign inclusion cyst of trapped surface ectoderm that originates from midline inclusion of surface ectoderm during third to fifth weeks of life at time of closure of neural tube
  — Epidermoids contain only ectoderm – are lined with squamous epithelium and contain desquamated keratinized debris
  — Dermoids contain ectoderm and mesoderm – are lined with squamous epithelium and contain dermal structures such as hair follicles and sebaceous and sweat gland elements
• Imaging:
  — Can be anywhere in body
  — Found at sites of suture closure and neural tube closure and hemispheric diverticulum
  — Can arise in scalp soft tissues or diploic space of calvaria or between bone and dura
  — Common locations are frontozygomatic suture, frontonasal suture, sellar and parasellar regions, posterior fossa and ventricles
• Imaging US:
  — Dermoid cyst and epidermoid cyst cannot be distinguished between on US
  — Dermoid: Classically solid appearing (mixed internal echogenicity)
  — Epidermoid: Classically cystic appearance with or without debris
  — Well-circumscribed and avascular and hypoechoic to subcutaneous fat
  — May be complex or with hyperechoic foci (calcification, fat, mucoid, and or purulent material)
  — Edge shadowing and posterior acoustic enhancement confirm cystic nature
  — May or may not have sinus tract extending to skin
  — If there is surrounding hyperemia it is most likely due to chemical inflammation from rupture rather than true infection
• Imaging CT:
  — In upper outer quadrant at fronto-zygomatic suture
  — Fat containing mass forming a fossa in adjacent bone
  — Bony erosion common
• Imaging MRI:
  — T1WI: Dermoids are bright due to presence of lipids and cholesterols, epidermoids have simple fluid and are dark
  — DWI: Epidermoids have restricted diffusion
• DDX:
• Complications: May rupture with spreading of fat droplets along cerebrospinal fluid containing spaces resulting in aseptic meningitis
• Treatment: Typically surgical resection as intracranial connection leads to CNS infection risk and for cosmetic reasons
• Clinical:
  — Mobile, non-tender, well-circumscribed, slow-growing mass
  — 10% of all pediatric head and neck masses
  — Most common orbital mass

Radiology Cases of Orbital Dermoid