user: What is (are) Skin Cancer ?

assistant: Key Points
                    - Skin cancer is a disease in which malignant (cancer) cells form in the tissues of the skin.    - There are different types of cancer that start in the skin.     - Skin color and being exposed to sunlight can increase the risk of nonmelanoma skin cancer and actinic keratosis.     - Nonmelanoma skin cancer and actinic keratosis often appear as a change in the skin.    - Tests or procedures that examine the skin are used to detect (find) and diagnose nonmelanoma skin cancer and actinic keratosis.    - Certain factors affect prognosis (chance of recovery) and treatment options.
                
                
                    Skin cancer is a disease in which malignant (cancer) cells form in the tissues of the skin.
                    The skin is the bodys largest organ. It protects against heat, sunlight, injury, and infection. Skin also helps control body temperature and stores water, fat, and vitamin D. The skin has several layers, but the two main layers are the epidermis (upper or outer layer) and the dermis (lower or inner layer). Skin cancer begins in the epidermis, which is made up of three kinds of cells:         -  Squamous cells: Thin, flat cells that form the top layer of the epidermis.    -  Basal cells: Round cells under the squamous cells.    -  Melanocytes: Cells that make melanin and are found in the lower part of the epidermis. Melanin is the pigment that gives skin its natural color. When skin is exposed to the sun, melanocytes make more pigment and cause the skin to darken.          Skin cancer can occur anywhere on the body, but it is most common in skin that is often exposed to sunlight, such as the face, neck, hands, and arms.
                
                
                    There are different types of cancer that start in the skin.
                    The most common types are basal cell carcinoma and squamous cell carcinoma, which are nonmelanoma skin cancers. Nonmelanoma skin cancers rarely spread to other parts of the body. Melanoma is a much rarer type of skin cancer. It is more likely to invade nearby tissues and spread to other parts of the body. Actinic keratosis is a skin condition that sometimes becomes squamous cell carcinoma.   This summary is about nonmelanoma skin cancer and actinic keratosis. See the following PDQ summaries for information on melanoma and other kinds of cancer that affect the skin:         -  Melanoma Treatment     -  Mycosis Fungoides and the Szary Syndrome Treatment     -   Kaposi Sarcoma Treatment     -  Merkel Cell Carcinoma Treatment     -  Unusual Cancers of Childhood Treatment     -  Genetics of Skin Cancer
                
                
                    Nonmelanoma skin cancer and actinic keratosis often appear as a change in the skin.
                    Not all changes in the skin are a sign of nonmelanoma skin cancer or actinic keratosis. Check with your doctor if you notice any changes in your skin.    Signs of nonmelanoma skin cancer include the following:         - A sore that does not heal.    - Areas of the skin that are:               - Raised, smooth, shiny, and look pearly.       - Firm and look like a scar, and may be white, yellow, or waxy.      - Raised, and red or reddish-brown.       - Scaly, bleeding or crusty.