The skin is the largest organ in the body, weighing about six pounds. As the body's outer covering, it protects us against heat and light, injury and infection. It also regulates our body's temperature and stores water, fat and vitamin D.

Background

Skin cancer begins in the epidermis, or outer layer of skin. The epidermis is very thin, averaging only 0.2 mm (about one hundredth of an inch). It protects the deeper layers of the skin and the organs of the body from the environment. The epidermis is made up of flat cells called squamous cells. These cells form an important protein called keratin, which contributes to the skin's ability to protect the rest of the body.

Types of Skin Cancer

The three most common types of skin cancer are melanoma, basal cell carcinoma and squamous cell carcinoma. Basal cell carcinoma and squamous cell carcinoma, along with several rare forms of skin cancer, are referred to collectively as nonmelanoma skin cancer.

Melanoma

Melanoma skin cancer begins in the melanocytes, the cells that produce the skin coloring or pigment known as melanin. Other names for this cancer include malignant melanoma, melanoma skin cancer and cutaneous melanoma. Melanoma is much less common than nonmelanoma, but it is much more serious and more likely to spread to other parts of the body.

Melanoma most often appears on the trunk of fair-skinned men and on the lower legs of fair-skinned women. However, people with other skin types and other areas of the skin are commonly affected. Because most melanoma cells still produce melanin, melanoma tumors are often brown or black. Although it is rare, melanomas can form in parts of the body not covered by skin such as the eyes, mouth, vagina, large intestine and other internal organs.

Basal and squamous cell carcinomas are the most common types of nonmelanoma skin cancer. They are called nonmelanoma because they develop from skin cells other than melanocytes. The two most common types of nonmelanoma include:

Basal cell carcinoma--This is a slow-growing cancer that seldom spreads to other parts of the body. It looks like a pearly growth, sometimes with an area that won't heal. It can be translucent and grow gradually, or it can look like a sore that won't heal.

Squamous cell carcinoma--This form of nonmelanoma also rarely spreads, but it spreads more often than basal cell carcinoma. It looks like a crusty, scaly patch with a hard, callous-like surface.

Other nonmelanoma skin cancers include Kaposi's sarcoma, cutaneous lymphoma, skin adnexal tumors, various types of sarcomas (cancers that develop from connective tissue cells of the dermis or subcutis), and Merkel cell carcinoma. Together, these types account for less than one percent of nonmelanoma skin cancers.

There are also some precancerous skin conditions that can develop into skin cancer. These include actinic keratosis and squamous cell carcinoma in situ.

Actinic keratosis--Actinic keratosis, caused by overexposure to the sun, appears as rough red or brown, scaly patches. Like skin cancer, it usually appears on sun-exposed areas, but it can be found anywhere on the body. It grows slowly and usually does not cause any symptoms or signs other than patches on the skin. It is possible, but not common, for actinic keratoses to turn into squamous cell cancer. Many times the patches go away, but they may come back.

Squamous cell carcinoma in situ -- also called Bowen's disease, is the earliest form of squamous cell skin cancer. The cancer cells are only found in the outer layer of skin and appear as reddish-colored patches. Compared to actinic keratosis, the patches of Bowen's disease tend to be larger (often over 1/2 inch wide), redder, more scaly and crusted.

Atypical moles (dysplastic nevi) are moles that share many of the clinical features of melanoma. Atypical moles occasionally develop into malignant melanomas.

Statistics about Skin Cancer

Skin cancer is the most common type of cancer, accounting for about half of all cancers. Nonmelanoma (basal cell carcinoma or squamous cell carcinoma) is more common than melanoma, but melanoma is much more serious and more likely to spread. According to the American Cancer Society, melanoma accounts for about four percent of skin cancer cases, but causes about 79 percent of skin cancer deaths.

The number of new melanomas diagnosed in the United States is increasing. Since 1973, the incidence rate for melanoma (the number of new melanomas diagnosed per 100,000 people each year) has doubled from six to 13. The American Cancer Society estimates that about 47,700 new melanomas will be diagnosed in the United States during 2000. About 7,700 people in the United States are expected to die of melanomas during 2000. 

About 1.3 million cases of nonmelanoma skin cancer are diagnosed in this country each year. The American Cancer Society predicts that there will be about 1,900 deaths from nonmelanoma skin cancer during 2000.