Skin cancer is the most common type of cancer. It forms in the epidermis, the outer layer of the skin, which is comprised of three types of cells. Squamous cells are oval to flat cells that make up the top layers; basal cells are round cells beneath the squamous cells; and melanocytes, residing alongside basal cells, produce melanin, the pigment that gives skin color. Squamous and basal cell cancers are common and, unlike melanoma, usually do not spread, which generally makes them easier to treat.

How Cancer Spreads

Cancer spreads when cells break away from the original tumor and travel to nearby tissues or distant organs. Melanoma develops the ability to spread once it reaches the inner layer of the skin, the dermis, where it can gain access to lymph or blood vessels that allow tumor cells to travel. Once melanoma spreads, it becomes difficult to treat.

Melanoma can metastasize (spread) to any organ in the body, although it generally follows a predictable path. It is most likely to first spread to the lungs and the area between the lungs; 70 to 87 percent of metastasized melanoma spreads to this area. After the lungs, melanoma is most likely to spread to other areas of the skin and to the liver.

According to the American Academy of Dermatology, oncologists recognize the spreading pattern of melanoma, but they're not sure why it's so predictable. They have three theories:

  1. Melanoma cells travel to body tissues indiscriminately, but multiply only in areas that have appropriate cellular growth factors.
  2. Cancer cells become "glued" to specific sites.
  3. Cancer cells are selectively attracted to specific sites by organ-specific molecules (chemokines), a process called chemoattraction. Scientists do know melanoma cells have many more chemokine receptors than cells not affected by cancer.

New Hope for Skin Cancer Treatment

Recently, researchers discovered a gene called mda-9/syntenin, which is elevated in major cancers. It helps melanoma spread by regulating proteins that promote angiogenesis, or the development of new blood vessels. These blood vessels bring critical nutrients to cancer cells and help them grow. Drugs that target this gene could potentially stop the spread of melanoma by removing the metastasis-promoting proteins.

The best way to prevent the spread of melanoma, however, is to be aware of the signs and detect it early—before it penetrates deeply into the dermis with its blood and lymph pathways. If you notice a change in shape, color, size, or feel of an existing mole, or the development of a new mole, seek prompt medical attention.

Craig Kraffert, MD, reviewed this article.


Sources: "How Deadly Skin Cancer Spreads Into Other Parts of the Body." Web. 3 January 2013.

National Cancer Institute. "What You Need to Know About Melanoma and Other Skin Cancers." Web. 11 January 2011.

Mayo Clinic. "How cancer spreads." Web. 3 August 2012.

The University of Chicago Medicine. "Skin Cancer Treatment (PDQ®)." Web.

American Academy of Dermatology. "Melanoma: How It Returns, Where It Spreads." Web.