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.
The skin is the largest organ in your body. It does many different things, such as:
- Covering the internal organs and helps protect them from injury
- Serving as a barrier to germs such as bacteria
- Preventing the loss of too much water and other fluids
- Helping control body temperature
- Protecting the rest of the body from ultraviolet (UV) rays
- Helping the body make vitamin D
The skin has 3 layers: the epidermis, the dermis, and the subcutis (see picture).
The top layer of skin is the epidermis. The epidermis is very thin, averaging only about 1/100 of an inch thick. 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 outer part of the epidermis is composed of flat keratinocytes called squamous cells that are constantly shed as new ones form. The lowest part of the epidermis is called the basal layer, and the keratinocytes here are called basal cells. These cells constantly divide to form new keratinocytes to replace the ones 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, which 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. For most people, 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.
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, which gives the skin its elasticity and strength.
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
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 look like early forms of keratinocytes, the most common type of skin cell. 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). When seen under a microscope, the cells in these cancers look like cells in the lowest layer of the epidermis, called the basal cell layer.
These cancers 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 in the sun.
These cancers tend to grow slowly. It’s very rare for a basal cell cancer to spread to other 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 look like abnormal versions of 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 cancers are more likely to grow into deeper layers of skin and spread to other parts of the body than basal cell cancers, 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, so many skin cancer experts consider them a type of squamous cell skin cancer and treat them as such.
These cancers develop from melanocytes, the pigment-making cells of the skin. Melanocytes can also form benign (non-cancerous) growths called moles. Melanoma and moles are discussed in our document Melanoma Skin Cancer.
Less common types of skin cancer
These less common types of skin cancer are different from keratinocyte cancers and melanomas and are treated differently. They include:
- Merkel cell carcinoma
- Kaposi sarcoma
- Cutaneous (skin) lymphoma
- Skin adnexal tumors (tumors that start in hair follicles or skin glands)
- Various types of sarcomas
Together, these types account for less than 1% of all skin cancers.
Pre-cancerous and pre-invasive skin conditions
These conditions may develop into skin cancer or may be very early stages 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 start on the face, ears, backs of the hands, and arms of middle-aged or older people with fair skin, although they can occur on other sun-exposed areas. People who have them usually develop more than one.
Actinic keratoses tend to grow slowly and usually do not cause any symptoms (although some might be itchy or sore). They sometimes go away on their own, but they may come back.
In some cases, actinic keratoses may turn into squamous cell cancers. Most actinic keratoses do not become cancers, but it can be hard sometimes for doctors to tell these apart from true skin cancers, so doctors often recommend treating them. If they are not treated, you and your doctor should check them regularly for changes that might be signs of skin 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 actinic keratosis, it usually does not cause any symptoms, although it might be itchy or sore.
Like most other skin cancers (and actinic keratoses), 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.
Bowen disease can sometimes progress to an invasive squamous cell skin cancer, so doctors usually recommend treating them. People who have these are also at higher risk for other skin cancers, so close follow-up with a doctor is important.
Benign skin tumors
Most tumors of the skin are benign (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
Last Medical Review: 10/21/2013
Last Revised: 02/20/2014