As the body's largest organ, your skin performs a variety of functions. It protects against infection, stores water, helps regulate body temperature and converts sunlight into vitamin D.
To the naked eye, the skin appears as a thin sheath. It weighs six pounds on average. Yet, the skin is a highly complex organ comprised of two main layers: the inner dermis and outer epidermis. Each layer contains several different kinds of cells.
When skin cells begin to grow and divide uncontrollably, the result is skin cancer. There are several forms of skin cancer, and they are classified under two broad categories: non-melanoma skin cancer (NMSC) and melanoma. The vast majority of skin cancers are NMSC and have a high cure rate.
The two major forms of NMSC are basal cell carcinoma and squamous cell carcinoma. ("Carcinoma" is a medical term for cancer). Both arise from cells in the epidermis and are rarely life-threatening.
The most dangerous form of skin cancer is malignant melanoma, which arises from pigment-producing cells (melanocytes) in the dermis, and will kill about 7,910 Americans in 2006.
Basal cell carcinoma
This most common form of skin cancer accounts for 75 percent to 80 percent of skin cancer cases. Basal cell carcinoma forms in the deepest layer (basal cell layer) of the epidermis and grows very slowly, usually on chronically sun-exposed areas of skin, such as the nose, cheeks, neck and forearms.
According to the American Cancer Society (ACS), basal cell carcinoma used to occur almost exclusively in middle-aged or older patients. Today younger people are being diagnosed with the disease, probably owing to increased sun exposure.
Squamous cell carcinoma
Sixteen percent to 20 percent of skin cancers are squamous cell carcinomas, which arise in higher levels of the epidermis. Like basal cell carcinomas, squamous cell cancers typically appear on sun-exposed areas: the face, ears, neck, lips or back of the hands. Squamous cell carcinomas also can grow within scars or skin ulcers.
Squamous cell carcinomas are more likely than basal cell carcinomas to spread to tissues beneath the skin and to distant parts of the body. Fortunately, it is very rare for a squamous cell or basal cell carcinoma to spread to lymph nodes or other organs. They nevertheless should be removed as soon as possible after they are discovered; treatment of advanced NMSCs can leave disfiguring scars. About 1,200 Americans die of squamous cell carcinoma each year.
Melanoma
Only 4 percent of skin cancer cases are melanomas, yet this aggressive disease is responsible for almost 79 percent of all skin-cancer deaths, according to the ACS. The Centers for Disease Control and Prevention ranks malignant melanoma as the most rapidly increasing form of cancer in the United States. A melanoma can spread to other organs, usually the lungs, liver and brain, unless it is diagnosed and properly treated at an early stage.
There are several less common forms of non-melanoma skin cancer, including Kaposi's sarcoma (usually associated with AIDS), Merkel cell carcinoma, and cutaneous lymphoma. Collectively, all these account for less than 1 percent of non-melanoma skin cancers diagnosed in the United States, according to ACS.
This article was reviewed and updated June 2007.
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 Squamous cell carcinoma and Basal Cell Carcinoma arise from the epidermis, and are rarely life-threatening. <br> <br> B) Melanoma, a dangerous form of cancer, arises from pigment-producing cells just below the epidermis'))
