Stage I is divided into stages IA and IB. In stage IA, the tumor is not more than 1 millimeter thick, with no ulceration. The tumor is in the epidermis (outer layer of skin) and upper layer of the dermis (inner layer of skin). In stage IB, the tumor is either not more than 1 millimeter thick, with ulceration, and may have spread into the dermis or the tissue below the skin; or 1 to 2 millimeters thick, with no ulceration.
