Non-melanoma skin cancer is the most common form of cancer to occur in the United States. More than 1 million cases of non-melanoma skin cancer are diagnosed each year, according to the American Cancer Society (ACS). It is difficult to determine the exact number of individuals who develop the condition as many cases are not reported. About half of all Americans who live to age 65 will be diagnosed with skin cancer.

Very few people die from non-melanoma skin cancer. The ACS estimates that about 2,700 people in the United States will die from this type of cancer in 2007. Most of these individuals are elderly and did not receive treatment in the early stages of the disease. People with compromised immune systems also have a higher risk of mortality.

Another form of skin cancer, melanoma, is responsible for more than 75 percent of skin cancer-related deaths. It is important to note, however, that this type of skin cancer accounts for less than 4 percent of skin cancer diagnoses, according to the ACS. 

The two most common forms of non-melanoma skin cancer are basal cell carcinoma and squamous cell carcinoma. To understand how these forms differ, it is helpful to know a little about the skin's anatomy.

The skin is made up of three layers – the epidermis, the dermis and the subcutaneous (also called the subcutis) layer. Non-melanoma skin cancers originate in the epidermal layer. The epidermis itself is divided into several sub-layers, including the stratum corneum. Dead cells are pushed to the top of the stratum corneum, which forms the top of the epidermis. As they move upward, these cells (keratinocytes) flatten out. Eventually, they are shed and replaced.

These cells are considered squamous cells, because of their fish scale-like shape. Just below this sublayer of dead cells are live keratinocytes, which produce a substance called keratin. Keratin is a sturdy protein that forms the upper layers of the skin, nails and hair.

The innermost layer of the epidermis is the basal layer, made up of basal cells. As these basal cells divide, they form new keratinocytes as older ones are shed from the skin’s surface. A basement membrane further separates the dermis and the subcutaneous layer from the epidermis.

Like all cancers, non-melanoma skin cancer occurs when DNA, the genetic blueprint that controls how cells grow and behave, becomes damaged and is not repaired. This damage causes the cells to multiply continuously until they form a growth or tumor (a mass of excess tissue).

More than 75 percent of all non-melanoma skin cancers diagnosed in the United States are basal cell carcinomas, according to the ACS. Basal cell carcinoma develops and grows slowly over time, and rarely spreads (metastasizes) to other parts of the body. It tends to develop on the sun-exposed areas of the body (e.g., face, ears neck, scalp, shoulders, back).  

Squamous cell carcinoma, the second most common skin cancer, is also slow-growing, but can penetrate deeper layers of the skin if left untreated. In rare cases, it can be fatal if it metastasizes to other organs. Squamous cell carcinoma can develop on all areas, but it most commonly occurs on sun-exposed areas of the body. The rim of the ears and lower lip are especially vulnerable.

Squamous cell carcinoma can occur without warning on apparently healthy, undamaged skin. Scientists are unsure why the condition develops but hereditary may play a role.