Verrucous lesion on the thumb

Over a nine-month period, a 79-year-old man developed on his left thumb a progressively enlarging verrucous plaque associated with infiltrated nodules and multiple keratin-filled crevices (Figure 1). The plaque had appeared as a small papule and was initially diagnosed as a wart. Subsequent cultures taken from the surface grew Gram positive bacteria, but the lesion expanded despite antibiotic therapy. A skin biopsy showed deeply invaginated epidermal-lined canals and cysts extending into the deep dermis. The canals had irregular lobular sheets of keratinocytes that projected into the surrounding dermis and were lined by atypical keratinocytes (Figure 2). Close examination of the surface epithelium showed focal hypergranulosis and vacuolated keratinocytes (Figure 3).