More than one million new cases of skin cancer are diagnosed each year in the United States, making it the most commonly diagnosed type of cancer.
The skin is the largest organ in the body. It protects against germs, covers internal organs, and helps regulate the body’s temperature. The two main layers of the skin are the epidermis and the dermis. The epidermis forms the top, outer layer of the skin. The dermis is a thicker layer beneath the epidermis.
Skin cancer generally develops in the epidermis. The three main types of cells in the epidermis are squamous cells, basal cells, and melanocytes. Squamous cells form a flat layer of cells at the top of the epidermis. Basal cells are round cells found beneath the squamous cells. Melanocytes are pigment-producing cells that are generally found in the lower part of the epidermis.
Skin cancer is often categorized as melanoma or non-melanoma. Melanoma is a cancer that begins in melanocytes. It is less common than non-melanoma skin cancer, but tends to be more aggressive. In 2006 an estimated 62,000 individuals in the U.S. will be diagnosed with melanoma, and close to 8,000 will die of the disease.
The most common type of non-melanoma skin cancer is basal cell carcinoma. This type of cancer rarely spreads to distant sites in the body, but it can be disfiguring and may invade nearby tissues.
The second most common type of non-melanoma skin cancer is squamous cell carcinoma. Although this type of cancer is more likely to metastasize (spread to lymph nodes or other sites in the body) than basal cell carcinoma, metastasis is still rare. Both basal cell carcinoma and squamous cell carcinoma most commonly develop on sun-exposed parts of the skin, but can develop on other parts of the skin as well.
An alarming trend in both melanoma and non-melanoma skin cancers is that the frequency of these cancers in children and young adults appears to be increasing. This highlights the importance of prevention at all ages.
Because of their very different characteristics and treatment, melanoma and non-melanoma skin cancer are discussed further in separate sections.
Go to the Melanoma Information Center
Go to the Non-Melanoma Information Center
 Christenson LJ, Borrowman TA, Vachon CM et al. Incidence of Basal Cell and Squamous Cell Carcinomas in a Population Younger Than 40 Years. JAMA. 2005;294:681-690.