Keratoacanthoma (molluscum sebaceum)|
Keratoacanthoma (KA) is a common benign skin tumour that is believed to arise from cells that produce keratin, a special protein, on the skin - 'keratinizing epidermal cells'. These distinct skin lesions occur most often as solitary tumours in sun-exposed areas in elderly, fair-skinned patients2. There is considerable debate as to whether keratoacanthoma is a distinct lesion or merely a variant of squamous cell carcinoma. These two forms of skin cancer do show some different characteristics but there is considerable overlap. Keratoacanthoma may mimic well-differentiated squamous cell carcinoma, but it may heal spontaneously, without treatment3.
What causes keratoacanthoma?
Keratoacanthomas are rare in African-Americans and Bantus. They also occur more frequently on the skin of the face and hands than on unexposed skin1. This lends evidence to the suggestion that sunlight or UV radiation is a possible cause. Chemical carcinogens are believed to be responsible for others because the tumour is observed in patients from both sunny locations and from smoky industrialised towns1. KA shows an increased incidence in pitch and tar workers, especially those who smoke4.
Many reports describe KA following various types of mechanical trauma or secondary to other skin lesions such as eczema, psoriasis and herpes zoster1. It may be that skin which is susceptible to tumour formation (due to exposure to sunlight or other carcinogens) will develop KA if subjected to injuries. It is suggested that genetic factors may be involved in the production of certain types of multiple keratoacanthoma, in which several family members are affected, this may equally well be due to sharing of a common environmental hazard1.
A viral cause for KA has been suggested but remains unproven4.
These tumours commonly appear and then seem to disappear within a matter of months2. The classical keratoacanthoma has a raised margin and a central keratin-filled crater (so do some squamous cell carcinomas)5. Keratoacanthomas appear clinically as flesh-colored, dome-shaped nodules with a central, keratin-fllled plug, making it look very crater-like. Lesions range in size from 1 cm to several centimeters across and have a higher distribution in facial skin including the cheeks, nose, and ears and the dorsa of the hands3.
Most KA's occur on exposed hairy skin. About 74% occur on the face and neck; about 17 percent occur on the backs of hands or forearms, the rest on various arts of the body. KA or KA-like lesions have been described in the oral cavity, conjunctiva, and subungual (under a fingernail or toenail) region1. At times the skin may be stretched over the lesion so that it has a shiny translucent appearance, and fine blood vessels may be seen just below the surface.
KA have three distinct clinical stages: Proliferation, maturation, and involution.
- Proliferation- KA shows rapid growth over 2 to 4 weeks, achieving a size of 2cm or greater (rarely 9-20cm).
- Maturation-lasts a few months, few new epithelial cells are formed.
- Involution-the tumour is resorbed by the body, and the central keratin-filled core is expelled.
The process from onset to resolution generally takes 4 to 6 months4.There is growing belief that keratoacanthomas may represent a form of squamous cell carcinoma that regresses as a consequence of interactions with host tissue that fail to support inexorable growth3. The majority of keratoacanthomas have detectable p53 oncoprotein, and occasional tumors show a point mutation in the p53 gene6. p53 is a tumour supressor gene.
KA is relatively common, and in white races tends to occur with about one third of the frequency of squamous ce