What is melanoma skin cancer?

Melanoma is a cancer that starts in a certain type of skin cell. To understand melanoma, it helps to know a little about the skin.

Normal skin

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

• Covers and helps protect the organs inside the body
• Helps to keep out germs
• Helps keep in 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, the epidermis, is very thin and protects the deeper layers of skin and the organs. The bottom part of the epidermis is made up of basal cells. These cells divide to form keratinocytes, which make a protein called keratin. This protein helps the skin protect the body.

The outermost part of the epidermis is called the stratum corneum. It is made of 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 gives the skin its tan or brown color and protects the deeper layers of the skin from some of the harmful effects of the sun. Melanocytes are the cells that can become melanoma.

A layer called the basement membrane separates the epidermis from the deeper layers of skin. It is important because when a skin cancer becomes more advanced it grows through this barrier and into the deeper layers.

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 deepest layer of the skin is called the subcutis. It keeps in heat and has a shock-absorbing effect that helps protect the body’s organs from injury.

Skin tumors that are not cancer

Most skin tumors are not cancer (they are benign). These rarely, if ever, turn into cancer. Some of them include:

• Moles (also called nevi) – benign skin tumors that start from melanocytes
• Spitz nevi – skin tumors that sometimes looks a lot like melanoma
• Seborrheic keratoses – tan, brown, or black raised spots with a “waxy” texture
• Hemangiomas – benign blood vessel growths often called strawberry spots or port wine stains
• Lipomas – soft growths made up of fat cells
• Warts – rough-surfaced growths caused by a virus

Melanoma skin cancers

Melanoma is a cancer that begins in the melanocytes. Because most of these cells still make melanin, melanoma tumors are often brown or black. But this is not always the case, and melanomas can also appear pink, tan, or even white.

Melanoma most often starts on the trunk (chest or back) in men and on the legs of women, but it can start in other places, too. Having dark skin lowers the risk of melanoma, but a person with dark skin can still get melanoma.

Melanoma can almost always be cured in its early stages. But it is likely to spread to other parts of the body if it is not caught early. Melanoma is much less common than basal cell and squamous cell skin cancers (described below), but it is far more dangerous.

Other skin cancers

Skin cancers that are not melanoma are sometimes grouped together as non-melanoma skin cancers because they start in skin cells other than melanocytes. These cancers include basal cell and squamous cell cancers. They are much more common than melanoma. Because they rarely spread to other parts of the body, basal cell and squamous cell skin cancers are less worrisome and are treated differently than melanoma. They are discussed in our document called Skin Cancer: Basal and Squamous Cell.

Still other types of non-melanoma skin cancers are discussed in our documents called Kaposi Sarcoma and Lymphoma of the Skin.