What are basal and squamous cell skin cancers?

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

Normal skin

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

• Covers the internal organs and protects them from harm
• Keeps out germs
• 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. 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 bottom of the epidermis is made up of basal cells. The basal cells divide to form keratinocytes, which make a protein called keratin. Keratin helps the skin protect the body.

The outermost part of the epidermis is made of dead keratinocytes that are shed as new ones form. The cells in this layer are called squamous cells.

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 some of 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 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 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 called moles that are not cancer. Melanoma and moles are discussed our document, Melanoma Skin Cancer.

The second main type of skin cancer is called keratinocyte carcinomas or keratinocyte cancers because their cells look a lot like keratinocytes (the cells found most often in the epidermis). Carcinoma is a medical word for a cancer that starts in a lining layer of cells (like the skin or the lining cells of the digestive system). The 2 most common types are 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 skin 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. But if it is not treated, it can grow into nearby areas and spread into the bone or other tissues under 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 starts in the squamous cells in the upper part of the epidermis. It accounts for about 2 out of 10 skin cancers. It most often starts on skin that has been exposed to the sun, like the face, ears, neck, lips, and backs of the hands. It can also start within scars or skin ulcers elsewhere. Less often, it forms in the skin of the genital area.

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

Keratoacanthomas are growths that are found on sun-exposed skin. They may start out growing quickly, but their growth usually slows down. Many shrink or even go away on their own over time without any treatment. But some keep on growing, and a few may even spread to other parts of the body. Many doctors think of them as a type of squamous cell skin cancer.

Less common types of skin cancer

There are also some other types of skin cancers that are not melanomas or keratinocyte cancers. 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, that is, not cancer. These rarely, if ever, turn into cancers. These tumors include:

• 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 growths of benign fat cells
• Warts – rough-surfaced growths caused by a virus

But there are other skin changes that may turn into cancer over time.

Pre-cancerous and pre-invasive skin conditions

Actinic keratosis

Actinic keratosis is also known as solar keratosis. It is a pre-cancer change caused by too much time in the sun. It causes small, rough spots that may be pink-red or flesh-colored. They are most often seen 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 that can be seen on the skin. It can go away on its own or 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 actinic keratoses that are not removed to see if they have changes that could mean cancer.

Squamous cell carcinoma in situ (Bowen disease)

Squamous cell carcinoma in situ is also called Bowen disease. In situ means that the cancer is only in the epidermis where it began. This is the earliest form of squamous cell skin cancer. 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 (human papilloma virus or HPV).