It arises most frequently on chronically sun-exposed glabrous skin of the head and neck or lower legs. It typically presents as single or multiple well-demarcated scaly erythematous patches, nodules or plaques which histologically show extensive keratinocytic atypia. It may develop from preexisting actinic keratosis (Actinic intraepidermal squamous cell carcinoma). Although it is most commonly associated with exposure to ultraviolet radiation, other carcinogens such as arsenic and tar may be implicated. Human papilloma virus may represent an additional risk factor in immunosuppressed patients.