Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?
Melanoma is a cancer that starts in a certain type of skin cell. To understand melanoma, 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 helping 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).
This top layer of skin 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.
The main types of cells in the epidermis include:
- 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 are the cells that can become melanoma. They make a brown pigment called melanin, which gives the skin its tan or brown color. Melanin 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. When a skin cancer becomes more advanced, it generally grows through this barrier and into the deeper layers.
This middle layer of the skin 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 (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.
Benign skin tumors
Many types of benign (non-cancerous) tumors can develop from different types of skin cells.
Benign tumors that start in melanocytes
A mole (nevus) is a benign skin tumor that develops from melanocytes. Almost everyone has some moles. Nearly all moles (nevi) are harmless, but having some types can raise your risk of melanoma. See the section “What are the risk factors for melanoma skin cancer?” for more information about moles.
A Spitz nevus is a kind of mole that sometimes looks like melanoma. It is more common in children and teens, but it can also be seen in adults. These tumors are generally benign and don’t spread. But sometimes doctors have trouble telling Spitz nevi from true melanomas, even when looking at them under a microscope. Therefore, they are often removed, just to be safe.
Benign tumors that develop from other types of skin cells
- Seborrheic keratoses: Tan, brown, or black raised spots with a “waxy” texture
- Hemangiomas: Benign blood vessel growths, often called strawberry spots
- Lipomas: Soft growths made up of fat cells
- Warts: Rough-surfaced growths caused by some types of human papilloma virus (HPV)
Most of these tumors rarely, if ever, turn into cancers. There are many other kinds of benign skin tumors, but most are not very common.
Melanoma skin cancers
Melanoma is a cancer that begins in the melanocytes. Other names for this cancer include malignant melanoma and cutaneous melanoma. Most melanoma cells still make melanin, so melanoma tumors are usually brown or black. But some melanomas do not make melanin and can appear pink, tan, or even white.
Melanomas can develop anywhere on the skin, but they are more likely to start on the trunk (chest and back) in men and on the legs in women. The neck and face are other common sites.
Having darkly pigmented skin lowers your risk of melanoma at these more common sites, but anyone can develop this cancer on the palms of the hands, soles of the feet, and under the nails. Melanomas in these areas account for more than half of all melanomas in African Americans but fewer than 1 in 10 melanomas in whites.
Melanomas can also form in other parts of your body such as the eyes, mouth, genitals, and anal area, but these are much less common than melanoma of the skin.
Melanoma is much less common than basal cell and squamous cell skin cancers, but it is far more dangerous. Like basal cell and squamous cell cancers, melanoma is almost always curable in its early stages. But it is much more likely than basal or squamous cell cancer to spread to other parts of the body if not caught early.
Other skin cancers
Skin cancers that are not melanomas are sometimes grouped as non-melanoma skin cancers because they develop from skin cells other than melanocytes. They tend to behave very differently from melanomas and are often treated with different methods.
Basal and squamous cell skin cancers
Most non-melanoma skin cancers are basal cell or squamous cell cancers. They are by far the most common skin cancers, and actually are more common than any other form of cancer. Because they rarely spread (metastasize) to other parts of the body, basal cell and squamous cell skin cancers are usually less concerning and are treated differently from melanoma. These cancers are discussed in Skin Cancer: Basal and Squamous Cell.
Less common skin cancers
Other types of non-melanoma skin cancer are much less common than basal and squamous cell cancers 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.
Last Revised: 02/01/2016