Keratoacanthoma

Keratoacanthoma is a relatively common low-grade malignancy that originates in the pilosebaceous glands. It clinically and pathologically resembles squamous cell carcinoma.¹ Keratoacanthoma rarely progresses into an invasive squamous cell carcinoma. Keratoacanthoma may serve as a marker for the important autosomal dominant familial cancer syndrome, the Muir-Torre syndrome.¹

• Incidence is estimated at 1 in 1000.
• Peak incidence occurs in those aged over 60 years. Is rare in young adults.
• Uncommon in darker-skinned patients.
• Males are twice as often affected as females.
• Epidemiological data is similar to squamous cell carcinoma and Bowen disease in terms of age, sex and the site of lesions.

Risk factors

• Sunlight and chemical carcinogens have been implicated.
• Trauma, genetic factors and immunocompromised status have also been associated.²
• Industrial workers exposed to pitch and tar have a higher incidence of both keratoacanthoma and squamous cell carcinoma.

• Typically rapid growth over a few weeks to months, followed by a slow spontaneous resolution over 4-6 months (but may take up to 1 year).
• Most occur on sun-exposed areas, e.g. face, neck, and dorsum of hands and forearms.
• Usually solitary and begin as firm, round, skin-coloured or reddish papules that rapidly progress to dome-shaped nodules with a smooth shiny surface. A central crater of ulceration may develop, or a keratin plug that may project like a horn.
• Leaves a residual scar if not excised.
• Occasionally presents as multiple tumours and may enlarge to up to 15 cm, become locally aggressive and metastasise.

• Squamous cell carcinoma

• Shave biopsy of keratoacanthoma is indistinguishable from invasive squamous cell carcinoma. Therefore, excisional or deep incisional biopsy is required.

• The Muir-Torre syndrome includes the presence of sebaceous gland tumours, with or without keratoacanthomas, and associated with visceral malignancies, especially colorectal. It has autosomal dominant inheritance.^3,4

• Complete excision is the treatment of choice for all skin neoplasms thought to be keratoacanthoma.⁵
• Medical treatment is reserved for when surgical intervention is not possible, e.g. multiple lesions not amenable to surgery because of size or location.
• Treatments that have produced some success include systemic retinoids, e.g. isotretinoin, intralesional methotrexate, 5-fluorouracil, bleomycin and steroids and topical imiquimod and 5-fluorouracil.

Radiotherapy

• Keratoacanthomas are radiosensitive and respond well to low doses of radiation.
• Radiation therapy may be useful in selected patients with large tumours when resection will result in cosmetic deformity, or for tumours that have recurred following attempted excision.
• Both laser therapy and cryotherapy have been used successfully in small keratoacanthomas.

• Reportedly progresses, although rarely, to invasive or metastatic carcinoma.

• Prognosis is excellent following excisional surgery.
• Without surgery, prognosis is usually good but has recently been reclassified as squamous cell carcinoma-keratoacanthoma type to reflect the difficulty in histologic differentiation, as well as the uncommon but potentially aggressive nature of keratoacanthoma.
• Increased risk for developing subsequent non-melanoma skin cancer.⁶

• Patient education in sun-protection techniques