Epidermal inclusion cyst

Epidermal inclusion cysts or epidermal cysts are common cutaneous lesions that represent proliferation of squamous epithelium within a confined space in the dermis or subdermis.

These are occasionally termed sebaceous cysts although, technically, this is a different entity. The term sebaceous cyst implies that the lesion originates in the sebaceous glands, which is not correct and, as such, the term epidermal inclusion cyst is preferred.

Epidermal cysts are either found incidentally or present as a firm non-tender lump. If they rupture a local inflammatory response to the necrotic debris released can mimic infection. Although they can be found anywhere, they are typically located on the scalp, face, neck, trunk, and back 1. Rarely they can be seen within bones representing an intraosseous epidermoid cyst 2.

Rarely epidermal cysts can undergo malignant degeneration with squamous cell carcinoma 1.

They are thought to occur as a result of 1,2:

  • traumatic/surgical implantation
  • occlusion of the pilosebaceous unit
  • congenital rests of cells
  • human papillomavirus type 57 or 60 infections implicated palmoplantar epidermoid cysts 1

They are closely related to cholesteatomas, and should not be confused with epidermoid cysts of the CNS.

On all modalities, they appear as well-circumscribed masses arising in or just deep to the skin.

Well-circumscribed predominantly hypoechoic mass. Typical shapes include 6:

  • ovoid to spherical: ~70%
  • lobulated: ~20%
  • tubular: ~8%.

If small, it can mimic a typical anechoic cyst. Using subcutaneous fat as a reference, lesions tend to be hypoechoic. Larger lesions can be a little heterogeneous due to the presence of mucoid, fat, calcification or pus. Usually no associated vascularity. They, however, can have a variable appearance if ruptured, occasionally associated with vascularity and lobulated contours 6. As a useful tool to distinguish between an epidermal inclusion cyst and a dermoid cyst, if the lesion is not covered by periosteum, it is more likely to represent a dermoid cyst. On color Doppler, it may show twinkling artefact 8.

The density of epidermal inclusion cysts is similar to that of water. The margins are usually slender and sclerotic.

Imaging on MRI is similar to that of CNS epidermoid cysts or cholesteatomas, namely the content of the cyst is similar to CSF/water. In unruptured cases, typical signal characteristics include:

  • T1: low/intermediate signal
  • T2: high signal
  • DWI
  • T1 C+ (Gd)
    • no enhancement centrally
    • may have thin peripheral enhancement

With ruptured cases, they may have septa, show thick and irregular rim enhancement, and can be accompanied by a fuzzy enhancement in surrounding subcutaneous tissues 1

They are benign and generally do not require treatment. If infected they may require incision and drainage. If they continue to grow they may require excision.

  • superimposed infection
  • rupture
  • concurrent occurrence of tumors within them, e.g. melanoma (very rare) 4

General imaging differential considerations include:

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Article information

rID: 13731
Synonyms or Alternate Spellings:
  • Superficial epidermal cyst
  • Sebaceous cyst
  • Sebaceous cysts
  • Epidermal cyst
  • Epidermal inclusion cysts
  • Epidermal cysts

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Cases and figures

  • Case 1: T2
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  • Case 1: T1 C+
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  • DIFFUSION: Epider...
    Case 2: DWI
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  • Case 2: CT
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  • Sebaceous cyst
    Case 3: mammogram
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  • Sebaceous cyst
    Case 3: ultrasound
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  • Case 4: MRI
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  • Case 5: in submandibular region
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  • Case 6: frontal epidermal inclusion cyst
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  • Case 7: ruptured cyst - anterior abdominal wall
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  • Case 8: knee
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  • Case 9: leak from cyst in neck
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  • Case 10: abdominal wall
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  • Case 11: intraosseous epidermoid cyst - probable
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  • Case 12: anterior knee
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  • Case 13: neck
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  • Case 14: sole of foot
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