Skin tumor - Nonmelanocytic

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Skin - Nonmelanocytic tumors

Adnexal tumors-hair follicles

Keratoacanthoma

Reviewer: Christopher Hale, M.D. (see Reviewers page)
Revised: 28 September 2012, last major update June 2012
Copyright: (c) 2001-2012, PathologyOutlines.com, Inc.

General
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● 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
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● 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
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● 70 year old woman with rapidly growing lesion on cheek (Case of the Week #253)
● 76 year old woman with facial lesion and cutaneous horn (Ann Dermatol 2011;23:89)

Clinical images
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Various images

Before and after topical treatment with 5% potassium dobesilate cream

Gross description
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● Flesh colored, dome shaped lesion with central, keratin-filled crater

Micro description
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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

Variants:
● 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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Various images

Negative stains
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● p53 (usually)

Molecular
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● Appears to be different from squamous cell carcinoma based on different telomerase, p53 and COX2 activity (Mod Pathol 2004;17:468)
● Distinct chromosomal aberrations between keratoacanthoma and SCC (J Invest Dermatol 2006;126:2308)

Differential diagnosis
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● Well differentiated squamous cell carcinoma

End of Skin - Nonmelanocytic tumors > Adnexal tumors-hair follicles > Keratoacanthoma

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