What are keratoacanthomas?

Keratoacanthomas are relatively common epidermal tumors that almost invariably appear on sun-exposed skin. There is controversy as to whether these lesions should be considered as benign or malignant. Microscopically, the tumor is composed of well-differentiated but cytologically atypical keratinocytes that are difficult for pathologists to separate from squamous cell carcinoma. Clinically, however, keratoacanthomas can behave in a benign fashion and spontaneously regress over a period of weeks or months if left untreated. Clouding the issue is the fact that approximately 10% of lesions that clinically resemble keratoacanthomas develop into invasive SCC. There are also reports of keratoacanthomas that have metastasized. It is not clear whether such lesions are SCC from their inception, keratoacanthomas that have developed into SCC, or more aggressive keratoacanthomas. Currently, the standard of care is to treat keratoacanthomas as well- differentiated squamous cell carcinomas, and many pathologists will describe their distinctive histology as “squamous cell carcinoma, keratoacanthoma type.” Solitary keratoacanthomas are the most common variant, but the tumor is also associated with specific genetic syndromes. The site of origin of keratoacanthomas is uncertain, but experimental and epidemiologic studies implicate follicular epithelium.

Karaa A: Keratoacanthoma: a tumor in search of a classification, Int J Dermatol 46:671–678, 2007.

Mandrell JC, Santa Cruz D: Keratoacanthoma: hyperplasia, benign neoplasm, or a type of squamous cell carcinoma? Semin Diagn Pathol 26:150–163, 2009.