Skin - Nonmelanocytic tumors
Adnexal tumors - hair follicles

Author: Christopher Hale, M.D. (see Authors page)

Revised: 6 October 2016, last major update June 2012

Copyright: (c) 2002-2016,, Inc.

PubMed Search: Keratoacanthoma [title]

Cite this page: Keratoacanthoma. website. Accessed October 24th, 2016.
Definition / General
  • Considered by some to be squamous cell carcinoma, acanthomatous type
  • May represent proliferation of infundibular portion of hair follicle (since keratinization occurs without a granular cell layer), or a subtype of well differentiated squamous cell carcinoma
Clinical Features
  • 80% males, usually sun exposed skin of face
  • Also associated with inflammatory dermatoses, congenital lesions, genetic diseases, scars
  • Younger age group than squamous cell carcinoma of skin
  • Familial cases may be multiple
  • Usually arises from normal skin, grows rapidly for 4 - 8 weeks, then regresses over 6 months to leave a depressed, annular scar
  • Rarely metastasizes, usually in immunosuppressed patients
  • Gryzbowski type: numerous eruptive lesions
  • Ferguson-Smith type: multiple ulcerating tumors with atypical distribution
  • Subungual keratoacanthoma: may arise from nail matrix; rapidly growing mass in tip of finger or toe; associated with lytic, cup shaped defect of distal digit
Case Reports
Clinical Images

Images hosted on other servers:

Before and after topical treatment
with 5% potassium dobesilate cream

Gross Description
  • Flesh colored, dome shaped lesion with central, keratin-filled crater
Micro Description
Early (evolving) phase:
  • Composed of well circumscribed solid lobules of large, pale squamous cells with little keratinization
  • Distorted follicular infundibulum
  • Mild atypia

Stable phase:
  • Has central crater filled with keratin but no granular layer
  • Larger more irregular infiltrating squamous nests and islands, accompanied by marked inflammatory infiltrate with lichenoid features and eosinophils but no plasma cells
  • May be deeply infiltrative, with microabscesses of neutrophils and eosinophils approaching surface
  • Often marked atypia, mitotic figures, atypical mitotic figures at periphery, perineural invasion, rarely vascular invasion

Regressing (resolving) phase:
  • Has keratin filled crater, mature epithelium without atypia
  • Flattening of cup-shape, horizontal fibrosis in dermis
  • Reduction of inflammation, transdermal elimination of elastic fibers
  • Note: overhanging edges, keratin-filled crater and hemispheric shape are most important features in differentiating from squamous cell carcinoma

  • Actinic-arises from actinic keratosis and has marked atypia
  • Follicular-plaque with numerous vertical strands of squamous epithelium resembling keratoacanthoma
  • Giant-10-15 cm, may cover most of a member
Micro Images

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Negative Stains
  • p53 (usually)
Molecular / Cytogenetics Description
Differential Diagnosis