Merck Manual

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Gregory L. Wells

, MD, Idaho College of Osteopathic Medicine

Last full review/revision Dec 2020| Content last modified Dec 2020
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Keratoacanthomas are round, firm, usually flesh-colored nodules with sharply sloping borders and a characteristic central crater containing keratinous material; they usually resolve spontaneously, but some may be a well-differentiated form of squamous cell carcinoma.

Etiology of keratoacanthoma is unknown. Most experts consider these lesions to be well-differentiated squamous cell carcinomas with a tendency to involute.

Development is rapid. Usually the lesion reaches its full size, typically 1 to 3 cm but sometimes > 5 cm, within 1 or 2 months. Common sites are sun-exposed areas, the face, the forearms, and the dorsum of the hands. Spontaneous involution may start within a few months, but involution is not guaranteed.


  • Biopsy or excision

Because this lesion cannot be relied on to involute, biopsy or excision is recommended.


  • Surgery or injections of methotrexate or 5-fluorouracil

Spontaneous involution may leave substantial scarring; surgery or intralesional injections with methotrexate or 5-fluorouracil usually yield better cosmetic results, and excision allows histologic confirmation of the diagnosis.


It is unclear whether keratoacanthoma risk increases with increasing ultraviolet (UV) exposure. Because it may, a number of measures are often recommended to limit exposure.

  • Sun avoidance: Seeking shade, minimizing outdoor activities between 10 AM and 4 PM (when sun's rays are strongest), and avoiding sunbathing and the use of tanning beds

  • Use of protective clothing: Long-sleeved shirts, pants, and broad-brimmed hats

  • Use of sunscreen: At least sun protection factor (SPF) 30 with broad-spectrum UVA/UVB protection, used as directed (ie, reapplied every 2 hours and after swimming or sweating); should not be used to prolong sun exposure

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