Bone & joints

Other nonneoplastic

Epidermoid inclusion cyst


Resident / Fellow Advisory Board: Josephine K. Dermawan, M.D., Ph.D.
Deputy Editor-in-Chief: Borislav A. Alexiev, M.D.
Alnoor Akber, M.D.
Nasir Ud Din, M.B.B.S.

Last author update: 3 May 2022
Last staff update: 3 May 2022

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PubMed search: Epidermoid inclusion cyst

Alnoor Akber, M.D.
Nasir Ud Din, M.B.B.S.
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Cite this page: Akber A, Ud Din N. Epidermoid inclusion cyst. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/boneepidermoidcyst.html. Accessed August 17th, 2022.
Definition / general
  • Benign epithelial inclusion cyst in the bone lined with stratified squamous epithelium, identical to cutaneous counterpart
Essential features
  • Benign cystic lesion
  • Post traumatic squamous epithelium embedded in bone
  • Cyst wall lined by squamous epithelium, including granular layer
  • Cyst contents contain laminated keratin
Terminology
  • Epidermoid inclusion cyst, epidermal inclusion cyst, intraosseous epidermoid cyst
ICD coding
  • ICD-10: L72.0 - epidermal cyst
Epidemiology
  • Rare incidence
  • Young to middle aged
Sites
Pathophysiology
Clinical features
  • Asymptomatic / painless lump
  • Can become tender due to inflammation
Diagnosis
  • Clinical, radiological and pathological correlation is adequate for diagnosis
Radiology description
  • Xray
    • Round osteolytic lesion with sharply demarcated sclerotic borders and thin cortex
    • Expansion of bone
    • Pathologic fracture is uncommon
  • MRI (Clin Neurol Neurosurg 2021;200:106381):
    • Hypointense on T1
    • Hyperintense on T2
Radiology images

Contributed by Nasir Ud Din, M.B.B.S
Distal phalanx of thumb Distal phalanx of thumb

Distal phalanx of thumb

Distal phalanx of fourth finger

Distal phalanx of fourth finger

Skull bone

Skull bone

Prognostic factors
Case reports
Treatment
  • Simple curettage or excision
Gross description
  • Unilocular cyst filled with white to pale yellow, malodorous, cheesy material
Frozen section description
  • Seldom required
Microscopic (histologic) description
  • Cyst is lined by squamous epithelium, including a granular layer
  • Cyst wall is devoid of skin adnexal structures
  • Cyst contents contain laminated keratin flakes
  • Acute inflammation and foreign body type giant cell reaction may be present in the ruptured cyst
  • Reference: Eur J Orthop Surg Traumatol 2019;29:1355
Microscopic (histologic) images

Contributed by Nasir Ud Din, M.B.B.S.
Absence of adnexal structures

Absence of adnexal structures

Keratin flakes in cystic cavity

Keratin flakes in cystic cavity

Lamellated keratin

Lamellated keratin

Squamous epithelium with granular layer

Squamous epithelium with granular layer

Reactive bone around the cyst

Reactive bone around the cyst

Chronic inflammation

Chronic inflammation

Positive stains
  • H&E diagnosis
Sample pathology report
  • Bone, right fifth distal phalanx, excision:
    • Epidermoid inclusion cyst
Differential diagnosis
  • Dermoid cyst:
    • Presence of skin appendages in the cyst wall
    • Compact keratin
  • Enchondroma:
    • Included in the radiological differential diagnosis
    • Histologically composed of hypocellular cartilaginous nodules
  • Glomus tumor:
    • Rare in bone
    • Included in the radiological differential diagnosis
    • Histologically uniform small round cells with eosinophilic cytoplasm, distinct cell borders
  • Osteomyelitis:
    • Clinically mimics epidermal inclusion cyst
    • Infiltration of bone by inflammatory cells including neutrophils, lymphocytes, and plasma cells
    • Bone erosion and necrosis
    • Reactive bone formation
  • Squamous cell carcinoma:
    • Associated precursor lesions, such as actinic keratosis or squamous cell carcinoma in situ are often present
    • Invasion of dermis by tumor
    • Tumor is composed of dysplastic squamous cells and may show lack of normal maturation
    • Moderate and poorly differentiated carcinoma show focal or no keratinization
Board review style question #1

Which of the following is one of the morphological features of intraosseous epidermoid inclusion cyst?

  1. Cyst contents contain compact keratin
  2. Cyst wall lined by benign squamous epithelium including a granular layer
  3. It is a malignant neoplasm
  4. Presence of skin adnexal structures in the cyst wall
Board review style answer #1
B. Cyst wall lined by benign squamous epithelium including a granular layer. The presence of a granular layer in the squamous epithelium and lamellated keratin are the key features that differentiate the epidermoid inclusion cyst from the trichilemmal (pilar) cyst.

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Reference: Epidermoid inclusion cyst
Board review style question #2

Which of the following morphological features differentiates dermoid cyst from epidermoid inclusion cyst?

  1. Benign squamous epithelium
  2. Chronic inflammation and fibrosis
  3. Overlying skin with dysplasia
  4. Presence of skin adnexal structures in the cyst wall
Board review style answer #2
D. Presence of skin adnexal structures in the cyst wall. Dermoid cyst differs from epidermoid inclusion cyst by the presence of skin appendages.

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Reference: Epidermoid inclusion cyst
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