<unk>  <unk>  is a relatively common, benign, epithelial tumor that was previously considered to be a variant of squamous cell carcinoma (SCC). 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<unk>  that suggests that ultraviolet radiation has an acute effect on the development of<unk> .<unk> s 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<unk>  marginatum centrifugum and mutilating<unk> s and can lead to severe defects.<unk>  begins as a smooth, dome-shaped, red papule that resembles molluscum contagiosum. In a few weeks the tumor may rapidly expand to 1 or 2cm 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<unk>  may occur on any skin surface, including the anal area. On occasion, multiple<unk> s appear, or a single lesion extends over several centimeters. These variants resist treatment and are unlikely to undergo spontaneous emission.
According to a review of literature by Robert A. Schwartz,<unk>  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). <unk>  is an abortive malignancy that rarely progresses into an invasive SCC. The<unk>  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.
Keratoacanthoma
Inverted follicular keratosis
White sponge nevus
Eccrine nevus
Nevus comedonicus
Xanthoma
Angiokeratoma
Trichoepithelioma