In order to understand basal and squamous cell skin cancers, it helps to know a little about the skin.

About normal skin

The skin is the largest organ in the body. It covers and protects the organs inside the body. It also protects the body against germs and prevents the loss of too much water and other fluids. The skin sends messages to the brain about heat, cold, touch, and pain.

The skin has 3 layers. From the outside in, they are:

• epidermis
• dermis
• subcutis

Epidermis

The top layer of the skin is the epidermis. It is very thin and protects the deeper layers of skin and the organs. The epidermis itself has 3 layers. The bottom layer is made up of basal cells. The basal cells divide to form keratinocytes, which are also called squamous cells. These cells make a substance called keratin which helps protect the body.

Another type of cell, the melanocyte, is also found in the epidermis. These cells make the brown pigment called melanin. Melanin is what makes the skin tan or brown. It helps protect the deeper layers of the skin from the harmful effects of the sun.

A layer called the basement membrane separates the epidermis from the deeper layers of the skin. The basement membrane is important because when a cancer becomes more advanced, it may grow through this barrier.

Dermis

The middle layer of the skin is called the dermis. The dermis is much thicker than the epidermis. It contains hair shafts, sweat glands, blood vessels, and nerves.

Subcutis

The last and deepest layer of the skin is called the subcutis. The subcutis keeps in heat and has a shock-absorbing effect that helps protect the body's organs from injury.

Types of skin cancer

Because they behave differently, skin cancers are divided into 2 major groups.

Cancers that start from the pigment-making cells of the skin (the melanocytes) are called melanomas. Melanocytes can also form growths that are not cancer called moles. Melanoma and moles are discussed in a separate American Cancer Society document, Melanoma Skin Cancer.

The second main type is called keratinocyte carcinomas or keratinocyte cancers because their cells look a lot like keratinocytes (the cells found most often in normal skin). Carcinoma is a medical word for a cancer that starts in a lining layer of cells such as the skin or the lining cells of the digestive system. There are many types of keratinocyte cancer, but 2 types are most common -- basal cell carcinoma and squamous cell carcinoma.

Basal cell cancer

Basal cell cancer begins in the lowest layer of the epidermis, the basal cell layer. About 8 out of 10 skin cancers are basal cell carcinomas. They usually begin on areas exposed to the sun, such as the head and neck. Basal cell carcinoma was once found mostly in middle-aged or older people. But now it is also being seen in younger people. This may be because people are spending more time in the sun without protecting their skin.

Basal cell carcinoma tends to grow slowly. It is very rare for a basal cell cancer to spread to distant parts of the body (metastasize). But if it is not treated, it can grow into nearby areas and spread into the bone or other tissues beneath the skin.

After treatment, basal cell carcinoma can come back (recur) in the same place on the skin. New basal cell cancers can also start in other places on the skin. As many as half of the people who have one basal cell cancer will get a new skin cancer within 5 years.

Squamous cell cancer

This type of cancer begins in the upper part of the epidermis. It accounts for about 2 out of 10 skin cancers. It most often appears on places of the body that are exposed to the sun, like the face, ears, neck, lips, and backs of the hands. It can also start within scars or skin ulcers other places on the body. Less often, it forms in the skin of the genital area.

Squamous cell carcinomas are more likely to spread into fatty tissues just beneath the skin. They are also more likely to spread to lymph nodes (the bean-shaped collections of immune system cells) or to distant parts of the body than are basal cell carcinomas, but this is not common.

Less common types of skin cancer

There are also some other types of skin cancers that are not melanomas. These are not very common and account for less than 1% of non-melanoma skin cancers.

Skin tumors that are not cancer

Most skin tumors are benign (not cancer). These rarely, if ever, turn into cancers. These tumors include:

• most types of moles (see the American Cancer Society document, Melanoma Skin Cancer for information on moles)
• seborrheic keratoses -- tan, brown, or black raised spots with a "waxy" texture, or rough surface
• hemangiomas -- benign blood vessel growths often called strawberry spots or port wine stains
• lipomas -- soft growths of benign fat cells
• warts -- rough-surfaced growths caused by a virus

Other skin changes that may become cancer:

Actinic keratosis

Actinic keratosis is also known as solar keratosis. It is a pre-cancerous problem caused by too much time in the sun. It causes small, rough spots that may be pink-red or flesh-colored. They often appear on the face, ears, back of the hands, and arms of middle-aged or older people with fair skin.

Actinic keratosis is slow growing. It does not usually cause any symptoms other than the patches on the skin. It can turn into cancer, but this does not happen very often. Still, it is a sign that your skin has been damaged by the sun. Some actinic keratoses and other skin changes that could become cancers may have to be removed. Your doctor should regularly check any that are not removed for changes that could mean cancer.

Squamous cell carcinoma in situ (Bowen disease)

Squamous cell carcinoma in situ is also called Bowen disease. It is the earliest form of squamous cell skin cancer. In situ means that the cancer is only in the epidermis where it began. Bowen disease looks like scaly, reddish patches that may be crusted. The major risk factor for Bowen disease is too much sun exposure. Bowen disease in the anal and genital skin is often linked to the virus that causes genital warts.