Bowen's disease. Light micrograph of a section through human skin with Bowen's disease, a pre- invasive form of squamous cell skin cancer, also called squamous cell carcinoma-in-situ. The cancer cells are confined to the epidermis, the outer layer of the skin (purple, centre). The cornified surface layer of the skin (dark pink, top) displays parakeratosis, thickening of the skin, which shows as red scaly plaques. Bowen's disease is caused by exposure to sunlight, some chemicals and a strain of the human papilloma virus. The plaques are most common on the trunks, arms, legs and genitals. Treatment is with removal of the plaques by freezing, chemotherapy or surgery. Magnification: x570 when printed at 10cm wide.