What are basal and squamous cell skin cancers?

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

Normal skin

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

• Covers the internal organs and helps protect 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
• Protects the rest of the body from ultraviolet (UV) rays
• Helps the body make vitamin D

The skin has 3 layers: the epidermis, the dermis, and the subcutis (see picture).

Epidermis

The top layer of skin is the epidermis. The epidermis is thin, averaging only 0.2 millimeters 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 that helps the skin protect the rest of the body.

The outermost part of the epidermis is called the stratum corneum. 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 cells 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 found in the epidermis. These skin cells make a brown pigment called melanin. Melanin gives the skin its tan or brown color. It protects the deeper layers of the skin from some of the harmful effects of the sun. When skin is exposed to the sun, melanocytes make more of the pigment, causing the skin to tan or darken.

The epidermis is separated from the deeper layers of skin by the basement membrane. This is an important structure because when a skin cancer becomes more advanced, it generally grows through this barrier and into the deeper layers.

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 elasticity and strength.

Subcutis

The 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 helps the body conserve 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 our 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

These are by far the most common skin cancers. They are called keratinocyte carcinomas or keratinocyte cancers because when seen under a microscope, their cells share some features of keratinocytes, the most common cell type of normal skin. Most keratinocyte cancers are basal cell carcinomas or squamous cell carcinomas.

Basal cell carcinoma

This is not only the most common type of skin cancer, but the most common type of cancer in humans. About 8 out of 10 skin cancers are basal cell carcinomas (also called basal cell cancers). They usually develop on sun-exposed areas, especially the head and neck. Basal cell carcinoma was once found almost entirely in middle-aged or older people. Now it is also being seen in younger people, probably because they are spending more time out in the sun.

When seen under a microscope, basal cell carcinomas share features with the cells in the lowest layer of the epidermis, called the basal cell layer. These cancers tend to grow slowly. It is very rare for a basal cell cancer to spread 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

About 2 out of 10 skin cancers are squamous cell carcinomas (also called squamous cell cancers). The cells in these cancers share features with the squamous cells seen in the outer layers of the skin.

These cancers commonly appear on sun-exposed areas of the body such as the face, ears, neck, lips, and backs of the hands. They can also develop in scars or chronic skin sores 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 grow and spread more 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 dome-shaped tumors that are found on sun-exposed skin. They may start out growing quickly, but 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. Their growth is often hard to predict, and many skin cancer experts consider them a type of squamous cell skin cancer and treat them as such.

Less common types of skin cancer

Along with melanoma and keratinocyte cancers, there are some other much less common types of skin cancer. These cancers are also non-melanoma skin cancers, but they are quite different from keratinocyte cancers and are treated differently. They 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 uncommon type of skin cancer develops from neuroendocrine cells (hormone-making cells that resemble nerve cells in some ways) in the skin. They are most often found on the head, neck, and arms but can start anywhere.

These cancers are thought to be caused in part by sun exposure and in part by Merkel cell polyomavirus (MCV). About 8 out of 10 Merkel cell carcinomas are thought to be related to MCV infection. MCV is a common virus. Many people are infected with MCV, but it usually causes no symptoms. In a small portion of people with this infection, changes in the virus' DNA can lead to this form of cancer.

Unlike basal cell and squamous cell carcinomas, Merkel cell carcinomas often spread to nearby lymph nodes and internal organs. They also tend to come back after treatment. Treatment of Merkel cell carcinoma is described in the section, “Treating Merkel cell carcinoma.”

Kaposi sarcoma

This cancer usually starts within the dermis but can also form in internal organs. It is related to infection with Kaposi sarcoma herpesvirus (KSHV), also known as human herpesvirus 8 (HHV8). 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 our document, Kaposi Sarcoma.

Skin lymphomas

Lymphomas are cancers that start in lymphocytes, a type of immune system cell found throughout the body, including in the skin.

Most lymphomas start in lymph nodes (bean-sized collections of immune system cells) or internal organs, but some types of lymphoma begin mostly or entirely in the skin. Primary cutaneous lymphoma is the medical term for lymphomas that start 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 our document, Lymphoma of the Skin.

Adnexal tumors

These tumors start in the hair follicles or glands (such as sweat glands) of the skin. Benign (non-cancerous) adnexal tumors are common, but malignant (cancerous) ones, such as sebaceous adenocarcinoma and sweat gland adenocarcinoma, are rare.

Sarcomas

Sarcomas are cancers that develop from connective tissue cells, usually in tissues deep beneath the skin. Much less often they may start in the skin’s dermis and subcutis. Several types of sarcoma can start in the skin, including dermatofibrosarcoma protuberans (DFSP) and angiosarcoma (a blood vessel cancer). Sarcomas are discussed in our document, Sarcoma – Adult Soft Tissue Cancer.

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 too much exposure to the sun. Actinic keratoses are usually small (less than 1/4 inch across), rough or scaly spots that may be pink-red or flesh-colored. Usually they develop on the face, ears, backs 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 tend to grow slowly. They usually do not cause any symptoms. They often go away on their own, but they may come back. In some cases actinic keratoses may turn into squamous cell cancers.

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 only in 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 1/2 inch across), redder, scalier, and sometimes crusted.

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

Benign skin tumors

Most tumors of the skin are not cancerous and rarely if ever turn into cancers. There are many kinds of benign skin tumors, including:

• Most types of moles (see our 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 tumors made up of fat cells
• Warts: rough-surfaced growths caused by a virus