Cancer starts when cells in a part of the body start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about how cancer starts and spreads, go to What Is Cancer?
To understand basal and squamous cell skin cancers, it helps to know a little about the skin.
The skin has 3 layers. From the outside in, they are:
- Epidermis: This top layer of the skin is very thin. The flat cells at the top of this layer are called squamous cells, and below this are cells called basal cells. These are the cells that can become squamous cell or basal cell skin cancers. The epidermis also contains cells called melanocytes, which make the brown pigment melanin. These are the cells that can become melanoma.
- Dermis: This middle layer of the skin is much thicker than the epidermis. It contains hair shafts, sweat glands, blood vessels, and nerves.
- Subcutis: This deepest layer of the skin contains proteins and fat, which help keep in body heat and act as a shock absorber to help protect the body’s organs from injury.
Types of skin cancer
There are many types of skin cancer, but by far the 2 most common types are basal cell cancer (also known as basal cell carcinoma) and squamous cell cancer (also known as squamous cell carcinoma).
Basal cell cancer
About 8 of 10 skin cancers are basal cell cancers (BCCs). This is not only the most common type of skin cancer, but the most common type of cancer. BCC begins in the lowest layer of the epidermis, the basal cell layer.
BCC usually begins on skin exposed to the sun, such as the head and neck. It 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 out in the sun.
BCC tends to grow slowly. It is very rare for BCC to spread to other 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, BCC 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 BCC will get a new skin cancer within 5 years.
Squamous cell cancer
Squamous cell cancer (SCC) starts in the squamous cells in the upper part of the epidermis. It accounts for about 2 in 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 in scars or chronic skin sores elsewhere. Less often, it forms in the skin of the genital area.
SCC is more likely than BCC to spread into deeper layers of the skin. It is also more likely to spread to other parts of the body, but this is not common.
Keratoacanthomas are dome-shaped tumors 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 growing, and a few may even spread to other parts of the body. Many cancer experts think of them as a type of squamous cell skin cancer and treat them as such.
These cancers start from the pigment-making cells of the skin (melanocytes). Melanocytes can also form growths called moles that are not cancer. Melanomas are much less common than basal and squamous cell cancers, but they are more likely to grow and spread if left untreated. Melanoma and moles are discussed in Melanoma Skin Cancer.
Less common types of skin cancer
Other types of skin cancers account for less than 1% of all skin cancers. They include:
- Merkel cell carcinoma
- Kaposi sarcoma
- Lymphoma of the skin
- Skin adnexal tumors (tumors that start in hair follicles or skin glands)
- Sarcomas (soft tissue cancers)
Pre-cancerous and pre-invasive skin conditions
These conditions may develop into skin cancer over time, or they may be very early stages of skin cancer.
Actinic keratosis is also known as solar keratosis. It is a pre-cancer caused by too much time in the sun. It appears as a small, rough or scaly spot that may be pink-red or flesh-colored. It is most often seen on the face, ears, back of the hands, and arms of middle-aged or older people with fair skin. People who have them often get more than one.
Actinic keratoses are slow growing and do not usually cause any problems, although some might be itchy or sore. But they may turn into squamous cell cancers. This does not happen very often, but it can be hard sometimes for doctors to tell these apart from true skin cancers, so doctors often advise treating them. If they are not treated, your doctor should check them regularly 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. Like actinic keratosis, it often doesn’t cause any symptoms, although it might be itchy or sore.
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).
Bowen disease can sometimes become invasive squamous cell skin cancer, so doctors usually recommend treating it.
Skin tumors that are not cancer
Most skin tumors are benign (not cancer). These tumors include:
- Most types of moles (see Melanoma Skin Cancer for more about 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
- Lipomas: soft tumors made up of fat cells
- Warts: rough-surfaced growths caused by some types of human papilloma virus (HPV)
Last Revised: 02/01/2016