Merkel cell carcinoma (MCC) is a rare, aggressive skin cancer that develops in the cells responsible for the sensation of light touch. About 3,000 people are diagnosed with it each year in the United States, making it far less common than melanoma but considerably more dangerous. It grows quickly, spreads early, and is often mistaken for a harmless cyst or pimple before it’s correctly identified.
How MCC Develops
MCC has two distinct pathways. In roughly 80% of cases, a common virus called Merkel cell polyomavirus (MCPyV) plays a central role. Most people are exposed to this virus at some point in their lives without ever getting sick. But in rare cases, the virus’s genetic material integrates into a skin cell’s DNA and produces proteins that override the cell’s normal growth controls, essentially switching off its ability to stop dividing.
The remaining 20% of cases are virus-negative. These tumors instead carry heavy DNA damage from cumulative ultraviolet radiation, similar to what’s seen in other sun-driven skin cancers. Virus-negative tumors have a much higher number of mutations overall, while virus-positive tumors have relatively few mutations beyond the viral disruption itself. This distinction matters because the two subtypes can behave differently and may respond differently to treatment.
Who Is Most at Risk
MCC overwhelmingly affects older, fair-skinned individuals. Most Americans diagnosed are over 70, more than 9 out of 10 are White, and men are affected more often than women. A lifetime of sun exposure is a major driver, which is why tumors frequently appear on the head, neck, and arms.
A weakened immune system is the other major risk factor. Organ transplant recipients who take immunosuppressive medications, people living with HIV, and those with chronic blood cancers all face elevated risk. The immune system normally keeps latent viruses like MCPyV in check. When that surveillance weakens, the virus has a greater opportunity to trigger cancerous changes. MCC that develops in immunosuppressed patients also tends to behave more aggressively.
What It Looks Like
MCC is deceptive because it doesn’t look like what most people imagine skin cancer to be. It typically appears as a firm, dome-shaped bump on the skin, often red, pink, or violet in color. It’s usually painless, which is part of why it gets overlooked. In a study of 195 patients, 88% of tumors were completely painless despite growing rapidly, and 56% were red or pink rather than dark or pigmented.
Dermatologists use a memory aid called the AEIOU criteria to flag suspicious lesions: Asymptomatic (not tender), Expanding rapidly, Immune suppression present, Older than 50, and Ultraviolet-exposed skin on a fair-skinned person. A painless bump that’s visibly growing over weeks, especially in someone who fits the other criteria, warrants a prompt biopsy. In 63% of diagnosed cases, the lesion had noticeably grown in just the prior three months.
Diagnosis and Staging
Diagnosis starts with a skin biopsy, where a sample of the suspicious lesion is examined under a microscope. MCC cells have a distinctive appearance, with specialized staining techniques that can confirm the neuroendocrine nature of the tumor and distinguish it from other cancers that look similar.
Because MCC spreads to nearby lymph nodes early and often, checking the lymph nodes is a critical next step. If the nodes near the tumor feel normal and look clear on imaging, a sentinel lymph node biopsy is typically performed. This procedure identifies the first lymph node(s) where cancer would drain to and removes them for testing. Finding cancer in a sentinel node shifts the staging from localized to regional disease, which has significant implications for treatment planning and outlook.
Treatment Options
Surgery is the first-line treatment for MCC that hasn’t spread beyond the skin and nearby lymph nodes. Surgeons aim for margins of 1 to 2 centimeters around the tumor, and the edges of the removed tissue are examined to confirm they’re cancer-free. For most patients, radiation therapy follows surgery. A standard course delivers targeted radiation to the tumor bed to destroy any microscopic cancer cells left behind. This adjuvant radiation is considered essential when surgical margins are narrow and is recommended for nearly all stages of localized disease. It may only be skipped for the smallest, lowest-risk tumors (under 1 centimeter with no unfavorable features).
For advanced MCC that has spread to distant sites or can’t be surgically removed, immunotherapy has transformed the landscape. Before 2017, no approved drugs existed for advanced MCC. Now two checkpoint inhibitors are approved: avelumab, approved in 2017, and pembrolizumab, approved shortly after. These drugs work by removing a molecular “brake” that cancer cells use to hide from the immune system. In the clinical trial that led to pembrolizumab’s approval, 56% of patients with advanced MCC saw their tumors shrink. That’s a remarkable response rate for a cancer that was previously considered nearly untreatable once it spread. The fact that MCC is so closely tied to viral proteins or has a high number of UV-induced mutations makes it particularly visible to the immune system once these brakes are released.
Survival Rates by Stage
Prognosis depends heavily on how far the cancer has spread at diagnosis. Based on data from patients diagnosed between 2015 and 2021, the five-year relative survival rates are:
- Localized (cancer confined to the skin): 79%
- Regional (spread to nearby lymph nodes): 66%
- Distant (spread to other organs): 31%
The overall five-year survival across all stages is 69%. These numbers reflect a mix of cases diagnosed before the widespread use of immunotherapy, so outcomes for patients diagnosed today with advanced disease may be improving. Still, early detection makes a dramatic difference, which is why any rapidly growing, painless skin bump in a person over 50 deserves medical attention.
Recurrence and Follow-Up
MCC has a high recurrence rate, and the window of greatest risk is narrow. About 80% of all recurrences happen within the first two years after diagnosis. Recurrence can be local (at or near the original site), regional (in nearby lymph nodes), or distant (in organs like the liver, lungs, or brain).
Because of this pattern, follow-up visits are frequent early on. The typical schedule involves appointments roughly every three months during the first year, then every three to six months in the second year, and annually after that. These visits usually include a thorough skin and lymph node exam, and imaging may be ordered if there’s any concern. Staying consistent with this schedule gives the best chance of catching a recurrence while it’s still treatable.