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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 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 mo. 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.
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 h and after swimming or sweating); should not be used to prolong sun exposure
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