A Case of Clonal (Nested) Variant of Seborrheic Keratosis

I. Background Seborrheic keratoses are common, benign, pigmented epidermal tumors.[1] Many terms such as senile wart, melanoacanthoma, basal cell papilloma, senile keratosis and seborrheic wart have been applied, but seborrheic keratosis is the most widely accepted term. These usually develop after the age of 50 years although occasionally, seen in young adulthood without any sexual predilection.[1] The common site of involvement includes the trunk, particularly the interscapular area, sides of the neck, the face and the arms. The tumors are not, however, seen on the mucous membranes.[2] Lesions appear as coin-like, sharply demarcated, exophytic lesions and are “stuck on the skin” with a verrucous, rough, dull or punched-out surface. Flat lesions often have a smooth surface and are scarcely elevated above the surface of the skin.[2] The etiology is not well-known, although heredity, sunlight and human papilloma virus (HPV) have been suggested as risk factors. Recent genetic studies have suggested that somatic mutations in Fibroblast Growth Factor Receptor 3 (FGFR3) gene are important in the development of these lesions.[3] Although seborrheic keratosis is a commonly encountered lesion by the dermatologists, the clonal variant is very rare and can be diagnosed only after histopathological examination.