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Causes
By Mayo Clinic staff
Your skin consists of three layers — the epidermis, dermis and subcutis. The epidermis, the topmost layer, is as thin as a pencil line and provides a protective layer of skin cells that your body continually sheds. Squamous cells lie just below the outer surface. Under a microscope, squamous cells in the deeper part of the epidermis resemble bricks; closer to the surface, they look like fish scales.
Basal cells, which produce new skin cells, are at the bottom of the epidermis. Squamous cell carcinomas develop from cells just above the basal layer. They form when cell death and renewal no longer occur as they should. Ordinarily, new cells push older cells toward your skin's surface, and the older cells die and are sloughed off — a process controlled by DNA, your body's genetic material. But if DNA is damaged, this orderly pattern is disrupted, causing cells to grow out of control.
The DNA-UV connection
Most of the damage to DNA in skin cells results from exposure to UV radiation from sunlight and commercial tanning lamps and beds. The damage is cumulative, so the more time you spend in the sun or in a tanning booth, the greater your chance of developing skin cancer. Your risk increases even more if most of your outdoor exposure occurs at times of the day when or in locations where the sun is strongest.
Although sun exposure causes most cases of squamous cell cancinoma, other factors also can lead to this type of cancer, including:
- Therapeutic radiation. Psoralen plus ultraviolet A (PUVA) treatments for psoriasis and X-rays to the head or neck increase your risk of squamous cell carcinoma as well as of melanoma, the most deadly form of skin cancer. It can take years for skin cancers to develop, and many radiation-induced carcinomas that occur later in life may have had their origins in radiation treatments for childhood acne or ringworm. The likelihood that therapeutic radiation will cause cancer depends on a number of factors, including the pigmentation in your skin, the total dose of radiation you receive, and your medical status.
- Chemical toxins. Arsenic, a toxic metal that's found widely in the environment, is a well-known cause of squamous cell carcinoma and other cancers. Though arsenic contaminates the soil, air and groundwater, most people get their greatest exposure in food, especially chicken, beef and fish, and in wine grapes sprayed with arsenic-containing toxins. The Department of Health and Human Services estimates that the average American ingests 11 to 14 milligrams of arsenic every day. Farmers, refinery workers, and people who drink contaminated well water or live near smelting plants are likely to ingest much higher levels.
- Human papillomavirus (HPV). This group of viruses has more than 100 strains, about a third of which are sexually transmitted. Some of the viruses cause genital warts; others can lead to cancer of the vagina, cervix or penis. Now, researchers think that infection with certain types of HPV may also play a role in the development of squamous cell skin cancers.
- Immunosupressant drugs. Up to 80 percent of people who take medications to prevent organ rejection after transplant surgery develop squamous cell carcinoma, though symptoms may not appear for years after surgery. People who have had heart transplants are at greatest risk because they tend to take more drugs at higher doses than do people who have other types of transplants.
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- Squamous cell carcinoma. American Academy of Dermatology. http://www.aad.org/public/Publications/pamphlets/SquamousCellCarcinoma.htm. Accessed Feb. 6, 2009.
- Squamous cell carcinoma of the skin. National Cancer Institute. Accessed Feb. 6, 2009.
- Skin cancer prevention. National Cancer Institute. http://www.cancer.gov/cancertopics/pdq/prevention/skin/Patient/page2. Accessed Feb. 6, 2009.
- Detailed guide: Skin cancer - basal and squamous cell. American Cancer Society. Accessed Feb. 6, 2009.
- Ferri F. Squamous cell carcinoma. In: Ferri FF. Ferri's Clinical Advisor 2009. St. Louis, Mo.: Mosby; 2008. http://www.mdconsult.com/das/book/body/119479231-4/801716560/1417/447.html#4-u1.0-B978-0-323-04836-1..50007-1--cesec44_620. Accessed Feb. 9, 2009.
- Urist MM, et al. Melanoma and cutaneous malignancies. In: Townsend CM, et al. Townsend: Sabiston Textbook of Surgery. 18th ed. Philadelphia, Pa.: W.B. Saunders; 2007. http://www.mdconsult.com/das/book/body/119479231-5/0/1565/1.html?tocnode=54736195&fromURL=1.html#4-u1.0-B978-1-4160-3675-3..X5001-1--TOP_1. Accessed Feb. 6, 2009.
- Sunscreen. The Skin Cancer Foundation. http://www.skincancer.org/the-scfs-guide-to-sunscreens.html. Accessed Feb. 6, 2009.
- Arsenic toxicity exposure pathways. U.S. Department of Health and Human Services. http://www.atsdr.cdc.gov/csem/arsenic/exposure_pathways.html. Accessed Feb. 6, 2009.
- Ridky TW. Nonmelanoma skin cancer. Journal of the American Academy of Dermatology. 2007;57:484.
- Neoplasia. In: Kumar V, et al. Robbins and Cotran's Pathologic Basis of Disease. 7th ed. Philadelphia, Pa.: Saunders Elsevier; 2005. http://www.mdconsult.com/das/book/body/119515236-4/801835407/1249/66.html#4-u1.0-B0-7216-0187-1..50011-0--cesec92_600. Accessed Feb. 6, 2009.
- Premalignant and malignant nonmelanoma skin tumors. In: Habif TP. Clinical Dermatology: A Color Guide to Diagnosis and Therapy. 4th ed. New York, N.Y.: Mosby; 2004. http://www.mdconsult.com/das/book/body/119515236-5/0/1195/157.html?tocnode=51442435&fromURL=157.html#4-u1.0-B0-323-01319-8..50023-6_2588. Accessed Feb. 6, 2009.
- Common questions about diet and cancer. American Cancer Society. Accessed Feb. 6, 2009.
- Food Groups to Encourage. In: U.S. Department of Health and Human Services and U.S. Department of Agriculture. Dietary Guidelines for Americans. 6th Edition. Washington, D.C.: U.S. Government Printing Office; 2005. http://www.health.gov/DietaryGuidelines/dga2005/document/html/chapter5.htm. Accessed Feb 6, 2009.
