Merkel Cell Carcinoma: Rare Skin Cancer Guide

Merkel cell carcinoma is a rare skin cancer. It begins in neuroendocrine cells in the outer layer of the skin. These cells sit near nerve endings and take part in the sense of touch.

It is uncommon, but it is aggressive. It tends to grow quickly and to spread early. It is the second most common cause of death from skin cancer after melanoma. That combination — rare and fast-moving — is why it is worth asking about care at a center that sees this cancer regularly.

Most Merkel cell carcinomas appear as a single firm, dome-shaped lump on sun-exposed skin. Usually that is the head, neck, arms or legs. The lump is typically red, pink or violet, and it does not hurt.

Clinicians often use a checklist to remember the warning signs:

• Asymptomatic, meaning painless and not tender
• Expanding rapidly over weeks
• Immune suppression
• Older than 50
• UV-exposed skin

A painless lump is easy to ignore. Rapid growth is the feature that should send you to a dermatologist for a biopsy.

Three factors stand out.

Ultraviolet light. Rates are higher in regions with more sunlight. Tanning beds and PUVA light therapy raise risk too.

A weakened immune system. Some people are diagnosed more often and can have more aggressive disease. That includes people taking anti-rejection medicines after an organ transplant, people living with HIV, and people with blood cancers such as chronic lymphocytic leukemia.

Merkel cell polyomavirus. This virus is found in many Merkel cell tumors, but not all. It has also been found in normal skin of healthy people. So infection alone does not cause cancer, and the disease is not catching.

Age matters as well. Most people are diagnosed after 50, with a median age around 65. It is slightly more common in men and much more common in White people.

Diagnosis starts with a skin biopsy, which means taking a small sample of skin to examine. A pathologist checks the sample under a microscope.

Staging may involve imaging, such as a CT or PET scan. It frequently involves a sentinel lymph node biopsy as well, which samples the first node the cancer would drain to. That last step matters here. This cancer can already be in the lymph nodes without any node being large enough to feel. Knowing whether nodes are involved changes both treatment and follow-up. Stages run from 0 through IV, with stage IV meaning spread to distant sites.

Surgery to remove the tumor with a margin of normal skin is the usual first step for disease that has not spread.

Radiation therapy is often given to the surgical site, and sometimes to the lymph node area. This cancer is sensitive to radiation and tends to come back locally.

Immunotherapy has changed the outlook for advanced disease. Three checkpoint inhibitors are approved: avelumab, pembrolizumab and retifanlimab. They work by releasing a brake on the immune system so it can recognize the tumor. Not everyone responds, but responses can last a long time. These drugs can cause immune-related side effects in the thyroid, bowel, skin, liver or lungs. Those are treatable when reported early.

Chemotherapy is still used in some situations. NCI notes its side effects can be serious, especially in older people.

If you take immune-suppressing medicine, that has to be part of the conversation about your treatment.

In one NCI-cited study, 91% of recurrences happened within 2 years of diagnosis. So visits are closest at first. They include exams of the first site and the nearby lymph nodes. Imaging may be used too. Ask your team what schedule fits your stage.

Between visits, report any new lump, any change in a scar, or any swollen node rather than waiting.

• Call 911 or go to an emergency department if you suddenly cannot get enough air, or your chest hurts, or your heart is racing. NCI says checkpoint drugs can inflame the lungs or the heart muscle. Do not drive yourself.
• Ring your oncology team straight away if your temperature hits 100.4°F (38°C) or higher during treatment. CDC counts a fever in someone on cancer treatment as an emergency, since it can be the only warning of a fast-moving infection. Any hour is the right hour to call. If they cannot be reached quickly, go to an emergency department and mention your treatment.
• Call your care team right away if you are passing several more watery stools a day than usual, or there is blood in them. Checkpoint drugs can inflame the bowel. The same applies to belly pain, a rash that is spreading or blistering, new muscle weakness, deep tiredness, or unusual thirst. These can mean inflammation in other organs, such as the thyroid, pituitary or pancreas.
• Call your care team within a day or two if you notice a new lump, a firm change in an old scar, or a swollen gland — and do not wait for the next scheduled visit.

• CDC — Watch Out for Fever (Preventing Infections in People With Cancer)
• NCI: Immune Checkpoint Inhibitors
• NCI: Infection and Neutropenia during Cancer Treatment
• NCI: Merkel Cell Carcinoma Treatment (PDQ®) Health Professional Version
• NCI drug information: avelumab
• NCI drug information: retifanlimab-dlwr
• NCI drug information: pembrolizumab
• NCI: Immunotherapy to Treat Cancer