Introduction
Background
Keratoacanthoma (KA) is a relatively common low-grade malignancy that originates in the pilosebaceous glands and closely and pathologically resembles squamous cell carcinoma (SCC). In fact, strong arguments support classifying keratoacanthoma as a variant of invasive SCC.1 In most pathology/biopsy reports, dermatopathologists refer to the lesion as "squamous cell carcinoma, keratoacanthoma-type."
Keratoacanthoma is characterized by rapid growth over a few weeks to months, followed by spontaneous resolution over 4-6 months in most cases. Keratoacanthoma reportedly progresses, although rarely, to invasive or metastatic carcinoma; therefore, aggressive surgical treatment often is advocated. Whether these cases were SCC or keratoacanthoma, the reports highlight the difficulty of distinctly classifying individual cases.2,3,4,5
Pathophysiology
Both sunlight and chemical carcinogens have been implicated as pathologic factors in growth of the tumor. Trauma, human papilloma virus, genetic factors, and immunocompromised status also have been implicated as etiologic factors.
Keratoacanthoma (KA) and conventional SCC share very similar epidemiological features, which suggests a possible common pathogenesis, such as actinic damage.6 Interestingly, in Palm Springs, California, this author has seen more patients with SCC/keratoacanthoma than straightforward SCC. In population-based studies in Kauai, Hawaii, keratoacanthoma and SCC had a comparable incidence (106 cases per 100,000 population for keratoacanthoma and 118.2 cases per 100,000 population for SCC).6,7 Most keratoacanthomas and SCCs developed on head/neck and limbs (keratoacanthoma, 78%; SCC, 85%). The incidence of keratoacanthoma and SCC increased significantly after age 64 years. The average age of patients was 67 years in keratoacanthoma and 66 years in SCC. Male-to-female ratios for both conditions were similar, at 2:1.6
Frequency
United States
The sole published study on keratoacanthoma in a white US population took place in Hawaii and estimated the incidence at 104 cases per 100,000. This study reported keratoacanthoma incidence equal to SCC and challenged the commonly reported incidence ratio of keratoacanthoma to SCC of 1:3. Peak incidence occurs in the seventh decade or beyond. Keratoacanthoma is uncommon in darker-skinned patients.6,8,9,10
International
Based on the Hawaiian data, the incidence of keratoacanthoma in ethnic Japanese, Filipino, and Hawaiian populations has been estimated to be 22, 7, and 6 cases per 100,000 population, respectively, approximately one fifth to one sixteenth of the incidence rate found in American whites. In other studies, the ratio of keratoacanthoma to SCC has ranged from 1:0.6 to 1:5 in different geographic locations.6,8,9,10
Mortality/Morbidity
Keratoacanthoma is believed to have a good prognosis; however, it has been reclassified as SCC-KA type to reflect the difficulty in histologic differentiation, as well as the uncommon but potentially aggressive nature of keratoacanthoma. Keratoacanthoma infrequently presents as multiple tumors and may enlarge (5-15 cm), become aggressive locally, or rarely, metastasize.11,12
Race
Keratoacanthoma is less common in darker-skinned individuals.
Sex
The male-to-female ratio for keratoacanthoma is 2:1.
Age
Keratoacanthoma has been reported in all age groups, but incidence increases with age. Keratoacanthoma is rare in persons younger than 20 years.
Clinical
History
Keratoacanthoma (KA) typically grows rapidly, attaining 1-2 cm within weeks, followed by a slow involution period lasting up to 1 year and leaving a residual scar if not excised preemptively. Since expedient therapy almost always is instituted, the true natural course of the tumor cannot be confirmed with certainty.
Physical
Pertinent physical findings in keratoacanthoma (KA) are limited to the skin. Lesions typically are solitary and begin as firm, roundish, skin-colored or reddish papules that rapidly progress to dome-shaped nodules with a smooth shiny surface and a central crateriform ulceration or keratin plug that may project like a horn. Most keratoacanthomas occur on sun-exposed areas. The face, neck, and dorsum of the upper extremities are common sites. Truncal lesions are rare. Lesions usually are skin-colored to pinkish-red. Unaffected skin retains its normal appearance. Note the images below.
Keratoacanthoma (squamous cell carcinoma-keratoacanthoma or SCC-KA type) on inner canthus.
Keratoacanthoma of the left forehead.
Close-up view of the keratoacanthoma.
Keratoacanthoma lesion (squamous cell carcinoma-keratoacanthoma or SCC-KA type).
Causes
The definitive cause of keratoacanthoma (KA) remains unclear; however, several potentiating factors should be considered. Epidemiologic data of keratoacanthoma is notably similar to SCC and Bowen disease (SCC in situ) concerning age, sex, and the anatomic site of lesions. These data strongly support a common etiology among keratoacanthoma, SCC, and Bowen disease. Epidemiologic data support sunlight as an important etiologic factor.
Industrial workers exposed to pitch and tar have been well established as having a higher incidence of keratoacanthoma, as well as SCC.13 Additionally, a 2006 study suggested a strong association between cigarette smoking and the development of keratoacanthoma.14
Trauma, human papillomavirus (specifically types 9, 11, 13, 16, 18, 24, 25, 33, 37, and 57),15,16 genetic factors, and immunocompromised status also have been implicated as etiologic factors.
Finally, research has identified that up to one third of keratoacanthomas harbor chromosomal aberrations. Recurrent aberrations include gains on 8q, 1p, and 9q with deletions on 3p, 9p, 19p, and 19q. One other report identified a 46,XY,t(2;8)(p13;p23) chromosomal aberration.17,18
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| References |
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References
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Further Reading
Keywords
keratoacanthoma, KA, squamous cell carcinoma, SCC, invasive SCC, invasive squamous cell carcinoma, squamous cell cancer, skin cancer, skin malignancy, pilosebaceous gland cancer, pilosebaceous glands
