The most common form of skin cancer. Its main cause is long-term exposure to ultraviolet light, usually from the sun; most tumours develop on exposed sites, chiefly the face and neck. It grows very slowly, often enlarging with a raised, pearly edge, and the centre may ulcerate (rodent ulcer). It does not metastasise (that is, spread to distant sites) and can be cured by surgical excision or RADIOTHERAPY. Small lesions can also be successfully treated by curettage and cauterisation (see CURETTE, ELECTROCAUTERY), LASER treatment or CRYOSURGERY. If the diagnosis is uncertain, a biopsy and histological examination should be done.