Actinic keratosis occurs on sun-exposed areas and is considered a precursor of squamous cell' carcinoma. When the atypical basal cells occupy the entire thickness of the epidermis, the lesion transforms into a carcinoma in situ. The presence of a hyperkeratotic layer is characteristic. Occasionally, so much keratin is produced that a "cutaneous horn" is formed. A verruca vulgaris may also be a raised lesion, but it is usually more pebbly, and there is no squamous atypia. Superficial epidermal cells show vacuolation or koilocytosis. A keratoacanthoma is a dome-shaped nodule with a central keratin-filled crater, that is surrounded by epithelial cells. Microscopically, it may mimic a squamous cell carcinoma. A dysplastic nevus is typically a flat, pigmented lesion. A seborrheic keratosis can be raised, but it usually appears as a coinlike plaque