Merkel cell carcinoma is a rare, fast-growing cancer that usually appears as a flesh-colored or bluish-red nodule on your skin, often on your face, head or neck. Merkel cell carcinoma tends to spread quickly (metastasize) to other parts of your body. Like other types of skin cancers, the incidence of Merkel cell carcinoma is increasing worldwide.

Merkel cell carcinoma, also called neuroendocrine carcinoma of the skin, most often develops in older people. Long-term sun exposure or having a weak immune system may increase your risk of developing Merkel cell carcinoma.

The type of treatment you'll undergo depends on whether the cancer has spread beyond your skin by the time it's diagnosed.

The first sign of Merkel cell carcinoma is a fast-growing, painless nodule (tumor) on your skin. The shiny nodule may be skin colored or may appear in shades of red, blue or purple. Nearly half of Merkel cell carcinomas appear on the face, head or neck, but they can develop anywhere on your body, even on areas not exposed to sunlight.

If the skin cancer has spread to other parts of your body, you may experience swollen lymph nodes, pain or fatigue.

Merkel cell carcinoma is named after Merkel cells, which are found at the base of the outermost layer of your skin (epidermis). Merkel cells may be associated with the sense of touch.

It's not certain what causes Merkel cell carcinoma to develop. One theory is that Merkel cell carcinoma may arise from the Merkel cell. Or it may be that certain immature cells (skin stem cells) become cancerous and develop features that make them look like Merkel cells.

Researchers have detected a number of genetic abnormalities in Merkel cell carcinoma cells. However, it's not known how these genetic changes affect cancer development.

While there's no definite cause of Merkel cell carcinoma, a number of factors may increase your risk of developing this cancer:

• Natural or artificial sunlight. Being exposed to natural sunlight or artificial light, including tanning beds, increases your risk. The majority of Merkel cell carcinomas appear on skin surfaces frequently exposed to sun.
• A weakened immune system. People with weakened immune systems — including those with HIV infection or those taking drugs that suppress the immune response — are more likely to develop Merkel cell carcinoma.
• History of other skin cancers. Merkel cell carcinoma is associated with the development of other skin cancers, such as basal cell or squamous cell carcinoma.
• Age. Older people are at an increased risk of developing Merkel cell carcinoma, especially those older than age 75. Younger people who develop this cancer usually have weakened immune systems.
• Race. Merkel cell carcinoma usually arises in people who have light-colored skin. Whites are much more likely to be affected by this skin cancer than are blacks.

If you notice a mole, freckle or bump that is changing in size, shape or color, growing rapidly or bleeding easily after minor trauma, such as washing your skin or shaving, talk to your doctor. Most skin lesions never become cancer, but if you see any changes, it's important to get checked out.

Diagnosis of Merkel cell carcinoma involves the following:

• Physical exam. Examining unusual moles, freckles, pigmented spots and other growths on your skin is the first step your doctor will likely take in making a diagnosis. A careful physical exam is important, because it's difficult to distinguish Merkel cell carcinoma from other skin cancers, including basal cell carcinoma. The nodule on your skin may also resemble other skin growths or even harmless cysts.
• Biopsy. After removing the tumor or a sample of the tumor from your skin, your doctor treats the cells with a special stain for viewing under the microscope.

Determining the extent
Your doctor may use the following tests to help determine whether the cancer has spread beyond your skin:

• Sentinel node biopsy. When cancer cells spread, they often travel first to your lymph nodes. Lymph nodes are small, rounded structures that filter foreign particles from lymph, a tissue-cleansing fluid in your body. This procedure involves injecting a dye near the skin tumor. The dye then flows through the lymphatic system to your lymph nodes. The first lymph node that receives the dye is called the sentinel node. Your doctor removes this lymph node and looks for cancerous cells under a microscope.
• Other diagnostic tests. Imaging tests, such as the X-ray or computerized tomography (CT) scan, provide pictures of your body organs and structures. You'll likely have a chest X-ray and a CT scan of your chest and abdomen to help your doctor determine whether the cancer has spread to other organs.

Unfortunately, Merkel cell carcinoma is an aggressive cancer and complications are not uncommon:

• Metastasis. Even with treatment, up to half the people with Merkel cell carcinoma eventually develop cancer that spreads beyond the skin. Merkel cell carcinoma tends to travel first to nearby lymph nodes and then may spread to your liver, bone, lungs or brain, where it can interfere with the functioning of these organs. Cancer that has metastasized is more difficult to treat and can be fatal.
• Treatment side effects. Although radiation therapy and chemotherapy may be effective in halting cancer growth, these therapies can also kill healthy cells and cause complications.

The type of treatment you'll receive depends on whether the cancer has spread beyond your skin by the time it's diagnosed. You might have just one type of treatment or a combination of treatments:

• Surgery. During surgery, your doctor removes the tumor along with a border of normal skin surrounding the tumor. Removing a thin border of normal skin helps catch any remaining cancer cells. If there is evidence that the cancer has spread to lymph nodes in the area of the skin tumor, those lymph nodes are removed (lymph node dissection).

  Occasionally, your doctor may use a procedure called Mohs' micrographic surgery. After removing the visible tumor, your doctor creates "slices" of the tumor tissue and examines each slice under a microscope. The process is repeated until cancer cells are no longer visible in the tissue slice. This type of surgery takes out as little normal tissue as possible — thereby reducing scarring — and is often used to remove skin cancer from the face.

• Radiation therapy. Radiation therapy involves directing high-energy X-rays at cancer cells. Radiation damages the genetic material in growing cells. Cancer cells are particularly susceptible to the effects of radiation because they divide more rapidly than normal cells do.

  In cases of Merkel cell carcinoma, radiation therapy is usually given to kill any cancer cells that remain after the tumor is removed. It may also be used to shrink Merkel cell carcinoma before it's removed or as the sole treatment in people who choose not to undergo surgery.

  General side effects from radiation therapy include red, irritated skin, hair loss and fatigue. If you receive radiation therapy to your head or neck, you may notice that you have a dry mouth, difficulty swallowing and other symptoms.

• Chemotherapy. Chemotherapy uses drugs to kill the cancer cells or stop them from dividing. Depending on the type of drug, chemotherapy may be injected, swallowed or applied to your skin.

  Chemotherapy is generally used when Merkel cell carcinoma has spread to other organs in your body or when it has recurred. However, whether chemotherapy improves the survival rate in people with Merkel cell carcinoma is unclear.

  Chemotherapy often causes significant side effects, most of which are temporary. Common side effects include nausea, vomiting and diarrhea; hair loss; fatigue; and an increased risk of infection. Rarely, permanent damage to your body organs may occur.

During the course of treatment, you'll undergo physical exams, including blood tests, to monitor how you're responding to radiation therapy or chemotherapy. Afterward, you'll continue having frequent checkups with your doctor to watch for recurrences.

The outlook after treatment for Merkel cell carcinoma varies, depending on a number of factors. These include the stage of the tumor at diagnosis, your age, the tumor size and location, and whether you have other health conditions. Generally, otherwise healthy people with small tumors and no evidence of cancer spreading beyond the skin tend to have the best prognoses.

Merkel cell carcinoma can't be prevented. However, you may be able to reduce your risk of this cancer and other skin cancers by limiting your exposure to sunlight.

While sunlight isn't proved to cause Merkel cell carcinoma, it's a risk factor for this cancer, other skin cancers and premature aging. Here are a few ideas for maintaining healthy skin:

• Be 'sun smart.' Avoid sun exposure as much as possible from 10:00 a.m. to 3:00 p.m. — the most intense sunlight hours of the day. If you're outdoors, seek shade under an umbrella or tree.
• Shield your skin and eyes. Wear a wide-brimmed hat, tightly woven clothing and sunglasses with ultraviolet light (UV) protection. You'll look stylish and help prevent sun damage to your skin.
• Apply sunscreen liberally and often. Apply sunscreen with a sun protection factor (SPF) of 15 or greater when you head outdoors. Remember to reapply sunscreen after swimming or if you've been sweating.
• Watch for changes. If you notice a mole, freckle or bump that is changing in size, shape or color, talk to your doctor. Most skin lesions never become cancer, but catching cancer in its early stages increases the chances that treatment will be successful.