Keratoacanthoma

A keratoacanthoma is a false skin cancer that looks like a little volcano.

A keratoacanthoma often starts at the site of a minor injury to sun damaged skin. At first it may appear as a small pimple or boil and may be squeezed but is found to have a solid core. It then grows rapidly and by the time it is brought to the attention of the doctor may be up to 2cm in diameter.

Keratoacanthoma

©R Suhonen

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What causes keratoacanthoma?

Past sun exposure certainly plays a role. It appears that keratoacanthomas arise from a single hair follicle as they are only seen on hair-bearing skin, not on the palms, for instance. A minor injury seems to be required to trigger off a keratoacanthoma but this is often either not apparent or unremembered by the patient. Cells start multiplying in the hair follicle and the cell mass grows into a keratoacanthoma.

Some keratoacanthomas appear to be related to infection with human papilloma virus, the cause of warts.

Untreated, a true keratoacanthoma will go on growing for several months, reach a maximum size then self-destruct over several more months.

Unfortunately a keratoacanthoma can look exactly like a true skin cancer, a squamous cell carcinoma (SCC), or less commonly like a basal cell carcinoma (BCC).

Multiple keratoacanthomas

There are some rare conditions in which multiple keratoacanthomas appear. These are:

• Ferguson-Smith familial keratoacanthoma More common in men, there are large and sometimes ulcerated self-healing lesions.
• Grzybowski eruptive keratoacanthomas Thousands of very itchy keratoacanthomas appear on the skin and mucosal surfaces and can result in significant deformity.

Management requires oral medications such as acitretin, methotrexate or cyclophosphamide.

Treatment

If you have a keratoacanthoma, seek the advice of your doctor, dermatologist or plastic surgeon.

Keratoacanthomas should be treated for several reasons. Firstly, it is not always possible to be sure the lesion is a keratoacanthoma and not a true skin cancer. A pathologist may report squamous cell carcinoma when the dermatologist has been fairly sure that the lesion is a keratoacanthoma. Secondly, the patient wishes to be rid of what is usually an unsightly, often tender and worrisome lesion as soon as possible. Finally, the scar which results from treating a keratoacanthoma is often better than if it is left to resolve spontaneously.

• Freezing If a keratoacanthoma is small it may be treated by freezing with liquid nitrogen with a spray or on a cotton wool swab. Following this the treated site will swell, may or may not blister, then dries out to form a scab which takes about 2 weeks to come off, longer on the limbs.
• Curettage and cautery Curettage and cautery is sometimes used for thicker lesions. A little anaesthetic is injected around the base of the lesion and it is then scraped out with a sharp spoon. The base of the keratoacanthoma is cauterized with an instrument similar to a soldering iron. Following this healing is usually rapid and the scab comes off in about 3 weeks to leave a slightly depressed, pink to purple scar. This scar then pales down and remodels to eventually leave a usually very acceptable cosmetic result. Healing takes longer with larger lesions and on the lower legs where it can take up to 2 months.
• Excision Excision is another common method of removing a keratoacanthomas. After injecting local anaesthetic, the affected area is cut out in an ellipse ensuring complete removal. The resulting defect is then stitched up. The stitches are removed a week or so later, leaving a linear scar. Rarely, Mohs microscopically controlled surgery may be required for larger keratoacanthomas, especially if they have recurred.
• Radiotherapy - Sometimes a large keratoacanthoma is treated by radiotherapy. Several visits over a period of days are usually required. The treatment is quite painless. A scab then forms and drops off after several weeks.

Follow-up

Normally there will be no further problem with a keratoacanthoma after treatment. Rarely, a recurrence will form, usually on the edge of the scar. In this case the lesion can be readily re-treated, usually by the same method.

Patients with keratoacanthomas are at risk of further similar lesions and other skin cancers; seek your doctor's help promptly if you develop any growing lumps or sores which fail to heal.