Epidermal inclusion cyst

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

Synonyms include "epidermoid cyst" and "epidermoid inclusion cyst". These are occasionally termed "sebaceous cyst", 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 (case 11) 2.

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

The 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.

Ultrasound

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. Larger lesions can be a little heterogeneous. Usually no associated vascularity. They, however, can have a variable appearance if ruptured with occasional associated vascularity and a lobulated contours 6.

CT

The density of epidermal inclusion cysts is similar to that of water.

MRI

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
  • T2: high
  • 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

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

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.

General imaging differential considerations include:

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Article Information

rID: 13731
Section: Pathology
Tag: skin
Synonyms or Alternate Spellings:
  • Sebaceous cyst
  • Sebaceous cysts
  • Epidermal cyst
  • Epidermal inclusion cysts
  • Epidermal cysts

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Cases and Figures

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    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
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    Case 14
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