Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer cells. To learn more about how cancers start and spread, see What Is Cancer?
Skin cancer begins when cells in the skin start to grow uncontrollably.
Types of skin cells
There are 3 main types of cells in the top layer of the skin (called the epidermis):
- Squamous cells: These are flat cells in the outer part of the epidermis that are constantly shed as new ones form.
- Basal cells: These cells are in the lower part of the epidermis, called the basal cell layer. These cells constantly divide to form new cells to replace the squamous cells that wear off the skin’s surface. As these cells move up in the epidermis, they get flatter, eventually becoming squamous cells.
- Melanocytes: These cells make the brown pigment called melanin, which gives the skin its tan or brown color. Melanin acts as the body’s natural sunscreen, protecting 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. When a skin cancer becomes more advanced, it generally grows through this barrier and into the deeper layers.
Types of skin cancer
Basal cell carcinoma
This the most common type of skin cancer. 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. 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.
If not removed completely, basal cell carcinoma can recur (come back) in the same place on the skin. People who have had basal cell skin cancers are also more likely to get new ones in other places.
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. 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 cancer are much less common and are treated differently. These 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 (AK), also known as solar keratosis, is a pre-cancerous skin condition caused by too much exposure to the sun. AKs 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.
AKs 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.
Some AKs may turn into squamous cell skin cancers. Most AKs do not become cancer, but it can be hard sometimes to tell them 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 (the upper layer of the skin) and have not invaded into deeper layers.
Bowen disease appears as reddish patches. Compared with AKs, Bowen disease patches tend to be larger (sometimes over ½ inch across), redder, scalier, and sometimes crusted. Like AK, Bowen disease usually doesn’t cause symptoms, although it might be itchy or sore.
Like most other skin cancers (and AKs), these patches most often appear in sun-exposed areas. Bowen disease can also occur in the skin of the anal and genital areas (where it is known as erythroplasia of Queyrat or Bowenoid papulosis). 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 it. 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 skin tumors 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 Melanoma Skin Cancer for more about moles)
- Seborrheic keratoses: tan, brown, or black raised spots with a waxy texture or occasionally a slightly rough and crumbly surface when they are on the legs (also known as stucco keratosis)
- 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)