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Intradiploic Epidermoid

Summary

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  • Rare, benign, slow-growing lesion arising from ectodermal inclusions within the skull
  • Typically asymptomatic until large enough to cause mass effect or erosion
  • Characteristic imaging findings include a lytic lesion with scalloped margins and no contrast enhancement

Pathophysiology

  • Originates from trapped ectodermal cells during neural tube closure in embryonic development
  • Slow growth rate due to accumulation of desquamated epithelial cells and keratin debris
  • May cause bone remodeling and expansion over time

Demographics

  • Accounts for <1% of all intracranial tumours
  • No gender predilection
  • Most commonly diagnosed in adults between 20-50 years of age
  • Rare in children

Diagnosis

  • Often incidental finding on imaging studies
  • Clinical presentation:
    • Asymptomatic in early stages
    • Headache, focal neurological deficits, or seizures when large enough to cause mass effect
  • Differential diagnosis:
    • Dermoid cyst
    • Arachnoid cyst
    • Hemangioma
    • Fibrous dysplasia

Imaging

  • Plain radiographs:
    • Lytic lesion with sclerotic margins
    • "Geographic skull" appearance in advanced cases
  • CT:
    • Well-defined, hypodense lesion
    • Scalloped margins with sclerotic borders
    • No contrast enhancement
    • May show calcifications in 10-25% of cases
  • MRI:
    • T1: Hypointense to isointense
    • T2: Hyperintense
    • FLAIR: Hyperintense
    • DWI: Restricted diffusion
    • No contrast enhancement
    • Chemical shift artefact may be present due to lipid content

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  • An incidental lesion in the left occipital lobe had a narrow zone of transition on CT, low values on ADC and did not enhance on post-gadolinium imaging.

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  • 50-year-old patient with neck pain had a lesion identified on a plain radiograph.
  • A large lesion centred on the left occiptial bone projected into the posterior fossa. The smooth margins, "dirty" FLAIR signal, and diffusion-restriction are indicate an epidermoid cyst.

Treatment

  • Asymptomatic lesions: Observation with regular follow-up imaging
  • Symptomatic or enlarging lesions:
    • Surgical excision is the treatment of choice
    • Complete resection recommended to prevent recurrence
    • Partial resection may be considered in cases where complete removal carries high surgical risk
  • Recurrence rate:
    • 8.3-25% if incompletely resected
    • <1% with complete resection

Differential diagnosis

Differential Diagnosis Differentiating Feature
Dermoid cyst Contains dermal appendages like hair follicles or sebaceous glands
Intraosseous hemangioma Sunburst pattern on CT, high signal on T1-weighted MRI
Fibrous dysplasia Ground-glass appearance on CT, low signal on T1 and T2 MRI
Eosinophilic granuloma Beveled edge appearance, more aggressive bone destruction
Arachnoid cyst No restricted diffusion on DWI, follows CSF signal on all sequences
Meningioma Homogeneous enhancement, dural tail sign
Metastasis Multiple lesions; irregular margins; destructive bone pattern; no restricted DWI; no smooth scalloped edges
Giant cell tumour Soap bubble appearance on CT; no fat or restricted DWI
Aneurysmal bone cyst Fluid-fluid levels, septations, blood products on MRI
Cholesterol granuloma Hyperintense on T1-weighted images due to cholesterol content