Skin nonmelanocytic tumor
Cysts
Trichilemmal (pilar) type


Topic Completed: 6 May 2021

Minor changes: 6 May 2021

Copyright: 2002-2021, PathologyOutlines.com, Inc.

PubMed Search: Trichilemmal cyst [title] skin

Maryam Aghighi, M.D.
Jodi Speiser, M.D.
Page views in 2020: 18,778
Page views in 2021 to date: 9,634
Cite this page: Aghighi M, Speiser J. Trichilemmal (pilar) type. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/skintumornonmelanocytickeratinouscysttrichilemmal.html. Accessed May 8th, 2021.
Definition / general
  • Trichilemmal cyst, also known as a pilar cyst, is a keratin filled cyst that originates from the outer hair root sheath that is commonly located on the scalp
Essential features
  • 90% present on scalp
  • Circumscribed simple cyst with stratified squamous epithelium lining, which lacks a granular cell layer and contains dense eosinophilic keratin
  • Proliferating trichilemmal cyst and other neoplasms, such as Merkel cell carcinoma, can develop from or within a benign trichilemmal cyst
Terminology
  • Also called pilar cyst, isthmus catagen cyst or a wen
ICD coding
Epidemiology
Sites
  • 90% on scalp
  • Scrotum
  • Rarely on pulp of finger
Pathophysiology
  • Origin unknown but may arise from external root sheath as a genetically determined structural aberration
  • Mostly autosomal dominant inheritance (J Invest Dermatol 2019;139:2075)
  • Hair follicle's outer root sheath is recapitulated at the level of the follicular isthmus in the cyst wall
Clinical features
  • Most commonly found on the scalp (90%) and scrotum
  • When present as multiple lesions, autosomal dominance is common
  • Asymptomatic, firm, mobile, dermal or subcutaneous nodules measuring 0.5 to 5 cm in diameter
  • No central punctum is present
  • Typically, encapsulated cyst and uncomplicated lesions are easily shelled out at surgery
  • Acute inflammation 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 (J Ultrasound Med 2019;38:91)
Diagnosis
  • Clinical impression or excision
Prognostic factors
  • Benign but may be locally aggressive
  • Rarely malignant but may result in metastasis
  • Rarely other neoplasms, such as Merkel cell carcinoma, colonize or arise in trichilemmal cyst (An Bras Dermatol 2019;94:452)
  • Proliferating trichilemmal cysts can develop from a benign trichilemmal cyst; growth may be provoked by an unknown trigger, such as trauma, irritation or inflammation (Arch Craniofac Surg 2017;18:50)
Case reports
Treatment
  • No treatment necessary for asymptomatic lesions
  • Incision and drainage under local anesthesia
  • Enucleation of the cyst
  • Incision followed by expression of contents and removal of cyst wall
  • Surgical excision if clinically indicated
  • Reference: Int J Trichology 2013;5:115
Clinical images

Contributed by David Rosenfeld, M.D. and Ada Agidi, M.D.
Pilar cyst

Pilar cyst

Gross description
Microscopic (histologic) description
  • Well circumscribed subcutaneous or dermal simple cyst, lined by stratified squamous epithelium that has a palisaded outer layer and contains dense laminated eosinophilic keratin
  • Granular layer is absent
  • Calcification in up to 25%
  • Granulomatous response due to rupture
  • Sebaceous or apocrine glands may be seen (Requena: Cutaneous Adnexal Neoplasms, 1st Edition, 2017)
  • Hidrocystoma-like lining
Microscopic (histologic) images

Contributed by Aaron Muhlbauer, M.D. and Jodi Speiser, M.D.
Pilar cyst Pilar cyst

Pilar cyst

Virtual slides

Images hosted on other servers:

Pilar cyst

Molecular / cytogenetics description
Sample pathology report
  • Scalp, biopsy:
    • Trichilemmal (pilar) cyst (see comment)
    • Comment: The sections show a dermal cyst lined by stratified squamous epithelium with a palisaded outer layer, lack of granular layer and containing dense laminated eosinophilic keratin.
Differential diagnosis
  • Infundibular cyst:
    • Most common sites are face, neck and trunk
    • Central punctum is present
    • Origin is epithelium of hair follicle infundibulum
    • Cyst wall is delicate and prone to rupture
    • On histology, the granular cell layer is present
    • Contains laminated keratin unlike pilar cyst with dense, eosinophilic keratin material in the cyst lumen and no granular layer
  • Proliferating pilar cyst:
    • Can grow as large as 25 cm, may cause pressure necrosis (destruction) on underlying tissues, ulceration and foul smelling discharge
    • Well defined lobular proliferation of squamous cystic islands centered in the dermis
    • The lining epithelium is a stratified squamous epithelium exhibiting trichilemmal keratinization
    • Well circumscribed with foci of necrosis and dyskeratosis
    • Increased typical mitotic figures in basilar epithelium along with mild cytologic atypia
    • Squamous eddies are common
  • Hidrocystoma:
    • Uni or multi locular cyst in dermis
    • Consist of two or more layers of columnar-cuboidal epithelium with an outer layer of myoepithelial cells
  • Malignant pilar tumor:
    • Infiltrative border
    • Marked cytologic atypia with numerous atypical mitotic figures
    • Perineural or vascular invasion
    • Geographic necrosis
Additional references
Board review style question #1

A 68 year old man presents with a dermal nodule on his scalp. Which of the following statements is correct?

  1. Histology shows marked cytologic atypia with numerous atypical mitotic figures
  2. Histology shows simple cyst with squamous epithelium lining, lack of a granular cell layer and containing dense keratin
  3. Histology shows thin walled clear cystic spaces in dermis
  4. Histology shows well defined lobular proliferation of squamous cystic islands centered in the dermis
Board review style answer #1
B. Histology shows simple cyst with squamous epithelium lining, lack of a granular cell layer and containing dense keratin

Comment Here

Reference: Trichilemmal (pilar) cyst
Board review style question #2
A 50 year old woman presents with an encapsulated cyst on her scalp. The gross findings include a dermal cyst with a thick cyst wall filled with solid, homogenous material. Which of the following is the most likely neoplasm to arise within this cyst?

  1. Basal cell carcinoma
  2. Malignant proliferating pilar cyst
  3. Merkel cell carcinoma
  4. Proliferating pilar cyst
  5. Squamous cell carcinoma
Board review style answer #2
D. Proliferating pilar cyst

Comment Here

Reference: Trichilemmal (pilar) cyst
Back to top
Image 01 Image 02