Skin nonmelanocytic tumor
Cysts
Trichilemmal (pilar) type


Topic Completed: 1 January 2015

Minor changes: 13 December 2019

Copyright: 2002-2019, PathologyOutlines.com, Inc.

PubMed Search: Trichilemmal cyst [title]

Ghassan A. Tranesh, M.D.
Hong Qu, M.D.
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Cite this page: Tranesh GA, Qu H. Trichilemmal (pilar) type. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/skintumornonmelanocytickeratinouscysttrichilemmal.html. Accessed August 10th, 2020.
Definition / general
  • Present as smooth, yellow, dome-shaped intradermal swellings
  • In contrast to epidermoid cysts, characteristically lack a punctum (sharp point or tip)
Terminology
  • Also called pilar cyst, isthmus-catagen cyst or a "wen"
  • The term "sebaceous cyst" favored by many clinicians is incorrect - these lesions are either epidermoid or trichilemmal cysts
Epidemiology
  • More common in females
Sites
Pathophysiology
  • Origin unknown, but may be produced by budding off from external root sheath as a genetically determined structural aberration
  • Familial occurrence seen in 75%, in pattern suggesting autosomal dominant inheritance
  • The hair follicle's outer root sheath is recapitulated at the level of the follicular isthmus in the cyst wall
Clinical features
  • Typically, encapsulated cyst, and uncomplicated lesions are easily "shelled out" at surgery
  • Acute inflammation is uncommon
    • When present, is usually nonbacterial, may be associated with cyst rupture
  • Cholesterol clefts in up to 90%
  • Calcification in 25%, independent of patient age or size of cyst
  • Rarely other neoplasms, such as Merkel cell carcinoma, colonize or arise in trichilemmal cyst
Case reports
Treatment
  • Excision if clinically indicated
Clinical images

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Well defined, freely mobile

Multiple scalp cysts

Gross description
  • Cysts filled with solid, homogenous material
Gross images

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Eyelid cyst

Microscopic (histologic) images

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Acanthotic epidermis

Lining epithelium

Courtesy of Angel Fernandez-Flores, M.D., Ph.D.

Differential diagnosis
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