Merkel cell carcinoma


Merkel cell carcinoma (MCC) is a rare and aggressive form of skin cancer. A quick diagnosis and appropriate treatment are essential to cure this disease. 

Seattle Cancer Care Alliance (SCCA) offers comprehensive treatment from a team of experts who specialize in MCC.

What is Merkel cell carcinoma?

MCC is a rare disease in which cancerous cells are found on or just beneath the skin. 

Merkel cells are cells normally in the bottom layer of your epidermis — the outermost layer of the two main layers of your skin (the dermis is the deeper layer). They function mainly as touch receptors and relay touch-related information, such as texture and pressure, to your brain. 

Scientists studying these cells believe that MCC doesn’t actually arise from normal Merkel cells. Rather, it seems more likely that the precursors of normal Merkel cells (stem cells in the epidermis) give rise both to normal Merkel cells and separately to Merkel cell carcinoma.

  • MCC usually appears as a lump that has grown rapidly on sun-exposed areas of the head, neck, arms or legs, but it can also appear in areas typically protected from the sun.
  • It often metastasizes (spreads) to other parts of the body. Even relatively small tumors may metastasize.
  • When the disease spreads, it tends to spread to nearby lymph nodes and may also spread to the liver, bone, lungs and brain.
What causes it?

Merkel cell carcinoma is usually caused in part by an extremely common — and typically harmless — virus that was discovered in 2008, the Merkel cell polyomavirus. 

Another very important cause is extensive exposure to sunlight, possibly many years earlier, especially in white people with fairer skin. 

About 80% of MCC cases are caused by the virus, and about 20% are caused by extensive sun damage. 

People who have significant, prolonged suppression of their immune system (such as after a kidney or heart transplant or due to infection with human immunodeficiency virus, HIV) are at high risk of MCC, but over 90% of MCC patients have no known problem with their immune system.

MCC is more common after age 65.

How common is it?

MCC is relatively rare, with about 2,500 people newly diagnosed each year in the United States. The incidence (rate of new cases per year) has tripled in the past 15 years, mainly because the baby boomers are aging.


MCC usually causes lumps or tumors on the skin that are:

  • Firm, painless, shiny
  • Skin colored or red or purple
  • As small as a quarter of an inch to more than two inches

It often resembles a benign (noncancerous) lesion, such as a cyst.


MCCs are diagnosed with a skin biopsy, which means samples of cells are taken from the tumor and examined under a microscope by a pathologist. 

Common types of biopsy include a punch biopsy (a small cylinder of tissue is taken) or a shave biopsy (part of the top of the  abnormal tissue is removed with a scalpel).

Special stains are used to distinguish this cancer from other forms of cancer, such as small cell lung cancer, lymphoma, melanoma or other sun-induced skin cancers, and from benign cysts.

If you have already been diagnosed and are coming to SCCA for a consultation, we will ask to have your pathology slides sent to us in advance to confirm your diagnosis.

An important aspect of proper care for MCC is often a sentinel lymph node biopsy — removing and checking the first lymph node to which the cancer may have spread — before surgery to remove the tumor itself.


Your team will recommend treatment based on the stage of your cancer. 

The stage depends on the following:

  • The size of your primary tumor 
  • The extent of disease in your lymph nodes and elsewhere in your body

MCCs are grouped into stages I through IV, with stage I being the least advanced and stage IV being the most advanced. Learn more about disease stages on the dedicated Merkel cell carcinoma website created by SCCA doctors and researchers.

Learn more

We have created an extensive resource of educational materials where you can learn more about the disease.