In order to understand basal and squamous cell skin cancers, it helps to know about the normal structure and function of the skin.

About normal skin

The skin is the largest organ in your body. It does several different things:

• covers the internal organs and protects them from injury
• serves as a barrier to germs such as bacteria
• prevents the loss of too much water and other fluids
• helps control body temperature

The skin has 3 layers (see picture below):

• epidermis
• dermis
• subcutis

Epidermis

The top layer of skin is the epidermis. The epidermis is thin, averaging only 0.2 millimeters (mm) thick (about 1/100 of an inch). It protects the deeper layers of skin and the organs of the body from the environment.

Keratinocytes are the main cell type of the epidermis. These cells make an important protein called keratin. Keratin contributes to the skin's ability to protect the rest of the body.

The outermost part of the epidermis is called the stratum corneum, or horny layer. It is composed of dead keratinocytes that are continually shed as new ones form. The cells in this layer are called squamous cells because of their flat shape.

Living squamous cells are found just below the stratum corneum. These have moved here from the lowest part of the epidermis, the basal layer. The cells of the basal layer, called basal cells, continually divide to form new keratinocytes. These replace the older keratinocytes that wear off the skin's surface.

Cells called melanocytes are also present in the epidermis. These skin cells make the protective brown pigment called melanin. Melanin is what makes the skin tan or brown. It is formed to protect the deeper layers of the skin from the harmful effects of the sun.

The epidermis is separated from the deeper layers of skin by the basement membrane. The basement membrane is an important structure because when a cancer becomes more advanced, it generally grows 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 follicles, sweat glands, blood vessels, and nerves that are held in place by a protein called collagen. Collagen, made by cells called fibroblasts, gives the skin its resilience and strength.

Subcutis

The last and deepest layer of the skin is called the subcutis. The subcutis and the lowest part of the dermis form a network of collagen and fat cells. The subcutis conserves heat and has a shock-absorbing effect that helps protect the body's organs from injury.

Types of Skin Cancer

Melanomas

Cancers that develop from melanocytes, the pigment-making cells of the skin, are called melanomas. Melanocytes can also form benign growths called moles. Melanoma and moles are discussed in a separate American Cancer Society document, Melanoma Skin Cancer.

Skin cancers that are not melanoma are sometimes grouped together as non-melanoma skin cancers because they tend to act very differently from melanomas.

Keratinocyte Cancers

The second main type is called keratinocyte carcinomas or keratinocyte cancers because viewed under a microscope their cells share some features of keratinocytes, the most abundant cell type of normal skin. The most common types of keratinocyte cancer are basal cell carcinoma and squamous cell carcinoma.

Basal cell carcinoma: Viewed under a microscope, cells of these cancers share some features with the cells in the lowest layer of the epidermis, called the basal cell layer.

About 8 out of 10 skin cancers are basal cell carcinomas. They usually develop on sun-exposed areas, especially the head and neck. Basal cell carcinoma was once found almost exclusively in middle-aged or older people. Now it is also being seen in younger people, probably because they are spending more time in the sun with their skin exposed.

Basal cell carcinoma tends to be slow growing. It is very rare for a basal cell cancer to spread (metastasize) to nearby lymph nodes or to distant parts of the body. But if a basal cell cancer is left untreated, it can grow into nearby areas and invade the bone or other tissues beneath the skin.

After treatment, basal cell carcinoma can recur (come back) in the same place on the skin. People who have had basal cell cancers are also more likely to get new ones elsewhere on the skin. As many as half of the people who are diagnosed with one basal cell cancer will develop a new skin cancer within 5 years.

Squamous cell carcinoma: Squamous cell carcinomas account for about 2 out of 10 skin cancers. They commonly appear on sun-exposed areas of the body such as the face, ear, neck, lip, and back of the hands. They can also develop in scars or skin ulcers elsewhere. They sometimes start in actinic keratoses (described below). Less often, they form in the skin of the genital area.

Squamous cell carcinomas tend to be more aggressive than basal cell cancers. They are more likely to invade fatty tissues just beneath the skin, and are more likely to spread to lymph nodes and/or distant parts of the body, although this is still uncommon.

Keratoacanthomas are growths that are found on sun-exposed skin. Although they may start out growing quickly, their growth usually slows down. Many keratoacanthomas shrink or even go away on their own over time without any treatment. But some continue to grow, and a few may even spread to other parts of the body. Because their growth is often hard to predict, many skin specialists think it is safest to consider them as a form of squamous cell skin cancer.

Less common types of skin cancer

Along with melanoma and keratinocyte cancers, there are some other types of skin cancer that are much less common. While these cancers are also "non-melanoma skin cancers," they are quite different from keratinocyte cancers and are treated differently, so it is useful to consider them separately.

Other non-melanoma skin cancers include:

• Merkel cell carcinoma
• Kaposi sarcoma
• cutaneous (skin) lymphoma
• skin adnexal tumors
• various types of sarcomas

Together, these types account for less than 1% of non-melanoma skin cancers.

Merkel cell carcinoma: This rare type of skin cancer develops from neuroendocrine cells (hormone-making cells that resemble nerve cells in some ways) in the skin. These cancers are thought to be caused in part by sun exposure. They are most often found on the head, neck, and arms but can start anywhere.

Treatment of Merkel cell carcinoma is described in the section "How are basal and squamous cell skin cancers treated?" Unlike basal cell and squamous cell carcinomas, Merkel cell carcinomas often come back after treatment and spread to nearby lymph nodes. They can also spread to internal organs, something that is quite uncommon for squamous cell carcinomas and even less common for basal cell carcinomas.

Kaposi sarcoma: This cancer usually starts within the dermis but can also form in internal organs. Before the mid-1980s, this cancer was rare and found mostly in elderly people of Mediterranean descent. Kaposi sarcoma has become more common because it is more likely to develop in people with human immunodeficiency virus (HIV) infection and the acquired immunodeficiency syndrome (AIDS). It is discussed in the separate American Cancer Society document, Kaposi Sarcoma.

Skin lymphomas: Lymphomas are cancers that start in lymphocytes, a type of immune system cell found in the bone marrow (the soft inner part of some bones), lymph nodes (bean-sized collections of immune system cells), the bloodstream, and some internal organs. The skin also contains a significant number of lymphocytes.

Although most lymphomas start in lymph nodes or internal organs, there are certain types of lymphoma that appear to begin mostly or entirely in the skin. Primary cutaneous lymphoma is the medical term meaning "a lymphoma that starts in the skin." The most common type of primary cutaneous lymphoma is cutaneous T-cell lymphoma (most of these are called mycosis fungoides). Cutaneous lymphomas are discussed in a separate American Cancer Society document, Lymphoma of the Skin.

Adnexal tumors: These tumors start in the hair follicles or glands (such as sweat glands) of the skin. Malignant (cancerous) adnexal tumors are extremely rare, but benign (non-cancerous) ones are common.

Sarcomas: These cancers develop from connective tissue cells, usually in tissues deep beneath the skin. Much less often they may develop in the skin's dermis and subcutis. There are several types of sarcoma that can develop in the skin, including dermatofibrosarcoma protuberans (DFSP) and angiosarcoma (a blood vessel cancer).

Benign skin tumors

Most tumors of the skin are not cancerous and rarely if ever turn into cancers. These tumors include the following:

• 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

Pre-cancerous and pre-invasive skin conditions

These conditions may develop into skin cancer or may be very early stages in the development of skin cancer.

Actinic keratosis (solar keratosis)

Actinic keratosis, also known as solar keratosis, is a pre-cancerous skin condition caused by overexposure to the sun. Actinic keratoses are small (usually less than one-fourth inch across), rough spots that may be pink-red or flesh-colored. Usually they develop on the face, ears, back of the hands, and arms of middle-aged or older people with fair skin, although they can arise on other sun-exposed areas. People with one actinic keratosis usually develop many more.

Actinic keratoses are slow growing. They usually do not cause any symptoms. It is possible, but not common, for actinic keratoses to turn into squamous cell cancer. They often go away on their own, but they may come back.

Even though most actinic keratoses do not become cancers, they are a warning that your skin has suffered sun damage. Some actinic keratoses and other skin conditions that could become cancers may have to be removed. Your doctor should regularly check any that are not removed for changes that could indicate cancer.

Squamous cell carcinoma in situ (Bowen disease)

Squamous cell carcinoma in situ, also called Bowen disease, is the earliest form of squamous cell skin cancer. "In situ" means that the cells of these cancers are still entirely within the epidermis and have not invaded the dermis.

Bowen disease appears as reddish patches. Compared with actinic keratoses, Bowen disease patches tend to be larger (sometimes over one-half inch across), redder, scalier, and sometimes crusted.

Like invasive squamous cell skin cancers, the major risk factor is overexposure to the sun. Bowen disease of the anal and genital skin is often related to sexually transmitted infection with human papilloma viruses (HPVs), the viruses that can also cause genital warts.