Merkel cell carcinoma is a rare type of skin cancer that tends to grow quickly and can spread to areas beyond the skin. Johns Hopkins dermatologic surgeon Jeffrey Scott, M.D., M.H.S., tells you what you should know about this type of skin cancer.

Merkel Cell Carcinoma Causes and Risk Factors

The cause of Merkel cell carcinoma is not yet known. However, known risk factors for Merkel cell carcinoma include:

• Exposure to UV rays. Like many other types of skin cancer, Merkel cell carcinoma is more likely to occur in people who have been exposed to a lot of ultraviolet (UV) rays from the sun or from other sources such as tanning beds. UV therapy for psoriasis increases the risk of developing Merkel cell cancer.
• Weakened immune system. Those with weakened immune systems due to chronic lymphocytic leukemia, organ transplant or other cause are at increased risk for this cancer.
• Light-colored skin. People with lighter skin are at higher risk.
• Older age. Merkel cell cancer is more common in people older than 50.
• Other cancers. Merkel cell carcinoma is more common in people who have blood cancers such as multiple myeloma or leukemia, and in those with melanoma, a form of skin cancer.
• Gender. Men are more likely than women to get Merkel cell carcinoma.

Merkel Cell Polyomavirus

Researchers have found that people with Merkel cell carcinoma almost always show evidence of a prior infection with a virus known as Merkel cell polyomavirus. It is not known how the virus may contribute to the growth of this cancer. Most people are infected with this virus at some point in their lives, but very few develop this cancer.

Merkel Cell Carcinoma Symptoms

Merkel cell carcinoma tumors are most often found on frequently sun-exposed areas of skin, such as the face, head, neck, shoulders and arms, but they can start anywhere on the body. Look for these signs, and see a doctor if you notice:

• A firm, painless skin lump.
• A lump that is red, pink or blue and may have a shiny appearance.
• A bump, spot or growth on your skin that seems to be getting bigger very quickly.

Merkel Cell Cancer Diagnosis

Diagnosis and treatment of early-stage Merkel cell carcinoma is important to prevent the cancer from spreading. Confirming Merkel cell carcinoma can be challenging, since the cancer is rare and can look like other, more common skin bumps.

If a person has a rapidly growing, painless lump on the skin along with other risk factors, the doctor may recommend a biopsy, a sample of tissue taken and tested in a lab. Your doctor removes a small piece of the tumor with a scalpel. Looking at the sample under a microscope can determine if cancer cells are present.

Other Tests Your Doctor May Recommend

If your biopsy is positive for Merkel cell carcinoma, your doctor may order tests that can help determine how much the cancer has advanced.

• A positron emission tomography (PET) scan or a CT scan can detect spread of Merkel cell carcinoma to areas in the chest, abdomen and pelvis.
• A brain MRI may be recommended for patients experiencing symptoms that could mean a spread of the cancer to the brain.
• A sentinel node biopsy, in which the doctor takes a sample of tissue from a lymph node close to the skin bump to see if the cancer cells have invaded the node.

Merkel Cell Carcinoma Treatment

Treatment often involves more than one method. The doctor will develop a therapeutic plan that may include:

• Surgery to remove the tumor. Depending on where the cancer is located, your doctor may recommend different approaches of skin cancer surgery.
  • Wide excision may include a border of 1 or 2 centimeters of healthy tissue. 
  • Mohs surgery removes an area of skin, one layer at a time, until the layers show no microscopic evidence of cancer cells. Since Merkel cell carcinoma is likely to recur, Mohs surgery may be followed by radiation therapy.

• Radiation therapy. A therapy called definitive radiation therapy uses targeted X-rays to kill cancer cells and shrink tumors. Definitive radiation therapy may be used after surgery or as the main treatment if surgery is not an option, such as in the case of a patient whose tumor is located in an area where wide excision would affect blood vessels, nerves or other essential structures.

• Immunotherapy. In patients with Merkel cell carcinoma that has spread to different parts of the body, therapy with drugs called monoclonal antibodies can help the immune system fight the cancer.

• Chemotherapy. Less commonly, people with Merkel cell carcinoma may be treated with medicines that help destroy cancer cells.

Can Merkel cell carcinoma be prevented?

Not all risk factors for Merkel cell carcinoma can be addressed, such as your age or having had cancer in the past, but you can lower your risk by practicing sun safety and protecting your skin from sun exposure and other forms of UV light.

If you are at risk for Merkel cell carcinoma or any other skin cancer, it is important to check your skin regularly. Be aware of any lumps, growths, moles or other abnormal areas on your skin and new spots or areas that are growing or changing, as well as sore areas that bleed, crust or itch and do not heal.

Merkel Cell Carcinoma Prognosis

The outcome of Merkel cell cancer treatment depends on how advanced the cancer is when it is diagnosed. Merkel cell carcinoma can spread (metastasize) quickly into adjacent tissue or throughout other areas of the body by lymph or by blood. When this happens, the cancer can become challenging to control.

The following factors may be associated with a better chance of controlling Merkel cell carcinoma:

• A small lesion that has not penetrated into adjacent tissue
• A primary lesion that is not on the head, face or neck
• Female sex
• Younger age
• A healthy immune system
• No other major health problems