Keratoacanthoma

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Keratoacanthoma
Classification and external resources
Skin keratoacanthoma whole slide
ICD-9	238.2
DiseasesDB	29383
eMedicine	derm/206
MeSH	[1]

Keratoacanthoma is a relatively common low-grade malignancy that originates in the pilosebaceous glands and closely resembles squamous cell carcinoma (SCC). In fact, strong arguments support classifying KA as a variant of invasive SCC. The pathologist often labels KA as "well- differentiated squamous cell carcinoma, keratoacanthoma variant. KA is characterized by rapid growth over a few weeks to months, followed by spontaneous resolution over 4–6 months in most cases. KA reportedly progresses, although rarely, to invasive or metastatic carcinoma; therefore, aggressive surgical treatment often is advocated. Whether these cases were SCC or KA, the reports highlight the difficulty of distinctly classifying individual cases.^[1]

Classification

Keratoacanthomas may be divided into the following types:^{[2]:763-764[3]:643-646}

• Giant keratoacanthoma
• Keratoacanthoma centrifugum marginatum
• Multiple keratoacanthomas (Ferguson Smith type of multiple self-healing keratoacanthomas, Multiple keratoacanthomas of the Ferguson-Smith type)
• Generalized eruptive keratoacanthoma (Generalized eruptive keratoacanthoma of Grzybowski)
• Subungual keratoacanthoma

Etiology

The etiology is unknown. No human papillomavirus-DNA sequences were detected in lesions by polymerase chain reaction. It is a disease of the elderly (mean age, 64 years) with an annual incidence rate of 104 per 100,000. It is not associated with internal malignancy. There may be a seasonal presentation of keratoacanthoma that suggests that ultraviolet radiation has an acute effect on the development of KA. KAs may develop in sites of previous trauma. Most cases are the “crateriform” type, which grow rapidly, then undergo spontaneous regression. Less than 2% belong to the rare destructive variants with no regression and persistent invasive growth. These are referred to as keratoacanthoma marginatum centrifugum and mutilating keratoacanthomas and can lead to severe defects.

KA begins as a smooth, dome-shaped, red papule that resembles molluscum contagiosum. In a few weeks the tumor may rapidly expand to 1 or 2 cm and develop a central keratin-filled crater that is frequently filled with crust. The growth retains its smooth surface, unlike a squamous cell carcinoma. Untreated, growth stops in approximately 6 weeks, and the tumor remains unchanged for an indefinite period. In the majority of cases it then regresses slowly over 2 to 12 months and frequently heals with scarring. The limbs, particularly the sun-exposed hands and arms, are the most common site; the trunk is the second most common site, but KA may occur on any skin surface, including the anal area. On occasion, multiple KAs appear, or a single lesion extends over several centimeters. These variants resist treatment and are unlikely to undergo spontaneous remission.

According to a review of literature by Robert A. Schwartz, KA was once considered a benign neoplasm that resembled a highly malignant one (pseudomalignancy), but it is now viewed in an opposite light as a cancer that resembles a benign neoplasm (pseudobenignity). KA is an abortive malignancy that rarely progresses into an invasive SCC. The KA may serve as a marker for the important autosomal dominant familial cancer syndrome, the Muir-Torre syndrome, as a result of a defective DNA mismatch repair gene.^[4]

Diagnosis

Diagnosis is best done with the clinical exam and history. It presents as a fleshy, elevated and nodular lesion with an irregular crater shape and a characteristic central hyperkeratotic core. Usually the patient will notice a rapid growing dome-shaped tumor on sun-exposed skin.

A skin biopsy must be performed to confirm the diagnosis. Unfortunately, a shave biopsy will often reveal only keratin fragments. A deep punch biopsy will often reveal a well differentiated, mildly atypical, squamous cell suggestive of an actinic keratosis or a squamous cell carcinoma. Only when the pathologist has access to the entire lesion (not practical in many circumstances) can a correct diagnosis be rendered. From a practical standpoint (insurance reimbursement), the correct diagnosis should be dictated as "well differentiated squamous cell carcinoma, keratoacanthoma variant". This is especially important for facial and nasal KA's, as it allows the surgeon to treat the tumor with the proper respect it deserves, with margin-controlled surgery like Mohs surgery. Correct diagnosis often requires communication between the surgeon and the pathologist. Many pathologists are still under the impression that keratoacanthomas are benign growths that do not require surgery. Unfortunately, when the lesion appears on the nose, face, or hands it often grows to marble or golf ball size before necrosing, and leaving a sizable crater in the process. From a practical standpoint, the correct diagnosis of low grade squamous cell carcinoma should be rendered after communication between the surgeon and pathologist, as the tumor is often much more aggressive and the morbidity is much higher than the relatively benign basal cell carcinoma.

Treatment

On the trunk, arms, and legs, electrodesiccation and curettage often suffice. Excision of the entire lesion is often required if one wants to confirm the clinical diagnosis of keratoacanthoma. On the nose and face, Mohs surgery allows for good margin control with minimal tissue removal; unfortunately, many insurance companies require the correct diagnosis of a malignancy before allowing such procedure. Recurrence after electrodesiccation and curettage is frequent, and usually can be identified and treated promptly with either further curettage or surgical excision. Allowing the KA to grow and necrose spontaneously is not acceptable in today's standard of care.

See also

• List of cutaneous conditions

References