The short answer
Merkel cell carcinoma is a rare, fast-growing skin cancer linked to a virus, UV exposure and weakened immunity. Checkpoint immunotherapy has changed treatment for advanced disease.
Merkel cell carcinoma usually appears as a painless, firm, rapidly growing red or purple lump on sun-exposed skin, most often on the head, neck or arms.
It grows and spreads faster than most skin cancers, so a lump that changes over weeks rather than years deserves prompt evaluation.
The main risk factors are ultraviolet exposure, a weakened immune system, older age, and Merkel cell polyomavirus, which is found in many but not all tumors.
Sentinel lymph node biopsy is commonly done at diagnosis because spread to nodes can be present without being felt.
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The full explanation.
What Merkel cell carcinoma is
Merkel cell carcinoma is a rare skin cancer that begins in neuroendocrine cells in the outer layer of the skin. These cells sit near nerve endings and are involved in the sense of touch.
It is uncommon, but it is aggressive. It tends to grow quickly and to spread early, and it is the second most common cause of death from skin cancer after melanoma. That combination, rare and fast-moving, is why care at a center that sees this cancer regularly is worth asking about.
What it looks like
Most Merkel cell carcinomas appear as a single firm, dome-shaped lump on sun-exposed skin, usually 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 in a fair-skinned person
A painless lump is easy to ignore. Rapid growth is the feature that should send you to a dermatologist for a biopsy.
What raises the risk
Three factors stand out.
Ultraviolet light. Rates are higher in sunnier regions and in people with fair skin that burns easily. Tanning beds and PUVA light therapy also raise risk.
A weakened immune system. People taking anti-rejection medicines after an organ transplant, people living with HIV, and people with blood cancers such as chronic lymphocytic leukemia are diagnosed more often and can have more aggressive disease.
Merkel cell polyomavirus. This virus is found in many Merkel cell tumors. It is also present harmlessly on the skin of most healthy adults, so infection alone does not cause cancer, and the disease is not contagious. Research suggests two routes to this cancer: one driven by the virus, and one driven mainly by accumulated sun damage. Which route applies can affect how the tumor behaves and how it is monitored.
Age matters as well. Most people are diagnosed after 50, with a median age around 65, and it is slightly more common in men.
Diagnosis and staging
Diagnosis starts with a skin biopsy. Because Merkel cell carcinoma can look like other cancers under the microscope, pathologists use special stains to confirm it.
Staging usually involves imaging, often a PET-CT, and frequently a sentinel lymph node biopsy. 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.
Treatment
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, because 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, which are treatable when reported early.
Chemotherapy can shrink tumors and is still used in some situations, though responses tend to be shorter than with immunotherapy.
If you take immune-suppressing medicine, that has to be part of the conversation, because it affects both risk of recurrence and whether checkpoint immunotherapy is safe for you.
Follow-up
Recurrence risk is highest in the first two to three years, so visits are frequent at first, usually every three to six months, with skin and lymph node examinations and imaging based on your stage. Some centers use a blood test for antibodies to Merkel cell polyomavirus to help watch for return of disease.
Between visits, report any new lump, any change in a scar, or any swollen node rather than waiting.
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Common questions
What does Merkel cell carcinoma look like?
Most often a single firm, dome-shaped lump that is red, pink or violet, painless, and growing noticeably over weeks. It is usually on sun-exposed skin such as the head, neck, arms or legs. Because it does not hurt and does not look alarming, it is often dismissed at first. Rapid growth is the feature that should prompt a skin biopsy.
If a virus causes it, is it contagious?
No. Merkel cell polyomavirus is common in the general population and lives harmlessly on the skin of most adults. In rare cases it integrates into a cell's DNA and contributes to cancer. You cannot catch Merkel cell carcinoma from another person, and you cannot give it to your family.
Why does immune suppression matter so much?
People taking anti-rejection medicines after a transplant, living with HIV, or with blood cancers such as chronic lymphocytic leukemia have higher rates of this cancer and often more aggressive disease. If this applies to you, your oncologist and the doctor managing your immune system usually need to coordinate, especially if checkpoint immunotherapy is being considered.
Does immunotherapy always work?
It does not work for everyone, but when it works the response can last a long time. Avelumab, pembrolizumab and retifanlimab are all approved for advanced Merkel cell carcinoma. These drugs can cause immune-related side effects affecting the thyroid, bowel, skin, liver or lungs, which are treatable when caught early. Report new diarrhea, rash, cough or unusual fatigue promptly.
What follow-up will I need?
Follow-up is usually more frequent than for other skin cancers, often every three to six months in the first years, with skin and lymph node examinations and imaging depending on your stage. A blood test for antibodies to Merkel cell polyomavirus is used in some centers to help monitor for recurrence.
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Written by: Cancer Explained Editorial TeamSources last checked: 2026-07-30Last updated: 2026-07-30Next planned review: 2028-07-29
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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