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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 a mere 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 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 lifethreatening.

The most dangerous form of skin cancer is malignant melanoma. It arises from pigment-producing cells (melanocytes) in the dermis and kills thousands of Americans every year.

Basal cell carcinoma

Basal cell carcinoma is the most common form of skin cancer. It accounts for about 8 out of 10 cases of skin cancer. Basal cell carcinoma forms in the bottom part (basal cell layer) of the epidermis (top layer of skin). It grows very slowly, usually on chronically sun-exposed areas of skin, such as the nose, cheeks, neck and forearms. It rarely spreads to other parts of the body.

According to the American Cancer Society, 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

About 2 out of 10 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. These 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.

Melanoma

Less than 5 percent of skin cancer cases are melanomas, yet this aggressive disease causes most skin cancer deaths, according to the American Cancer Society. Melanoma is becoming more common every year, especially in the United States, where the percentage of people who develop melanoma has more than doubled in the past 30 years.

A melanoma can spread to other organs, usually the lungs and liver, if it is not diagnosed and treated at an early stage.

Sources: American Cancer Society. What is melanoma skin cancer? Accessed May 28, 2008. National Cancer Institute. What you need to know about skin cancer. Accessed May 28, 2008. National Cancer Institute. What you need to know about melanoma. Accessed May 28, 2008.

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