Merkel cell carcinoma is a rare, fast-growing type of skin cancer that forms on the skin or in the cells just below it. The cells involved in this cancer, the Merkel cells, are hormone-making cells found in the top layer of the skin. These cells are believed to be involved in the sense of touch. Merkel cell carcinoma develops when Merkel cells start to grow in an uncontrolled way.
Merkel cell carcinoma is a very unusual type of cancer. Approximately 1200 new cases are diagnosed in the United States each year. Most patients are over age 50 at the time of diagnosis. The disease is more common among white people than those in other ethnic groups, although some cases have also been reported in people of Japanese heritage.
Risk factors for Merkel cell carcinoma include extensive exposure to sunlight; exposure to artificial sunlight (used in the treatment of psoriasis); having a weakened immune system because of diseases such as AIDS or because of the use of immunosuppressive drugs; having a history of other types of skin cancer; and being older than age 70, male, or white.
Merkel cell carcinoma usually appears as a single lump on the skin on a part of the body exposed to the sun. The lump is usually fast-growing, painless, firm, raised, and red or purple. It may look like other types of skin cancer or other skin problems. Diagnosing Merkel cell carcinoma requires a biopsy (a procedure in which a sample of abnormal tissue is removed and sent to a laboratory for examination under a microscope). To diagnose this particular type of cancer, special procedures must be used to examine the biopsy specimen. The cells must be treated with a special stain and examined under an electron microscope.
Merkel cell carcinoma is usually treated by surgery. If the cancer has spread, radiation therapy and chemotherapy may also be used.