Merkel Cell Carcinoma.Care & specialists in Canada

Merkel cell carcinoma is a rare, aggressive neuroendocrine skin cancer, with US incidence of about 2,500 cases per year. Approximately 80% of cases are caused by Merkel cell polyomavirus and 20% by cumulative ultraviolet damage. It typically appears as a rapidly growing, painless, red or violet skin nodule on the head, neck, or arms in older or immunosuppressed adults.

Merkel cell carcinoma is a type of skin cancer, but it differs from the more common basal and squamous cell carcinomas. It arises from neuroendocrine precursor cells, grows much faster, and has a much higher risk of spreading to lymph nodes and distant sites. Treatment and prognosis are also distinct from common skin cancers.

AEIOU is a clinical mnemonic for features that raise suspicion for Merkel cell carcinoma: Asymptomatic (painless), Expanding rapidly (over weeks), Immunosuppressed, Older than 50, and on UV-exposed skin. Approximately 90% of MCC patients have three or more AEIOU features at presentation. The mnemonic helps clinicians biopsy promptly when MCC is suspected.

Localized Merkel cell carcinoma (stages I-II) can be cured by wide local excision and adjuvant radiation, with 5-year survival around 51%. Stage III disease is treated with surgery, radiation, and increasingly adjuvant immunotherapy. Stage IV metastatic disease is rarely curable but is now controllable for many years in roughly one-third of patients treated with PD-1/PD-L1 inhibitors.

Merkel cell polyomavirus (MCPyV) is a common skin virus carried asymptomatically by most adults. In about 80% of Merkel cell carcinoma cases, the virus integrates into the host cell DNA with truncating mutations that drive uncontrolled cell growth. MCPyV-positive tumors are highly responsive to immune checkpoint inhibitors because they express foreign viral proteins.

Diagnosis requires a skin biopsy with histology and immunohistochemistry. The pattern of small round blue cells with positive CK20 in a characteristic perinuclear dot pattern confirms MCC and distinguishes it from melanoma, basal and squamous cell carcinoma, and metastatic small cell lung cancer. Sentinel lymph node biopsy and imaging complete the staging workup.

Five-year overall survival depends on AJCC stage at diagnosis: approximately 62% for stage I, 44-52% for stage II, 39-42% for stage III, and 14% historically for stage IV. With first-line pembrolizumab, 3-year survival in stage IV disease is now around 59%, a major improvement over the chemotherapy era.

Most Merkel cell carcinomas are painless, even when they are growing rapidly. This is captured by the A in the AEIOU mnemonic — Asymptomatic. The lack of pain often leads to delayed diagnosis because patients mistake the lesion for a harmless cyst or bump. Any new, firm, rapidly growing skin nodule should be biopsied regardless of whether it hurts.

Merkel cell carcinoma is notable for rapid growth — most tumors are noted by the patient to double in size over weeks rather than months. By the time of diagnosis, the median size is 1.5-2 cm. Rapid growth combined with location on sun-exposed skin in an older adult should prompt urgent dermatology referral.

Merkel cell tumors are among the most immunogenic of solid tumors. Virus-positive tumors express foreign viral oncoproteins that the immune system can recognize, while virus-negative tumors carry one of the highest mutational burdens of any cancer from UV damage. Both pathways produce abundant neoantigens that PD-1 and PD-L1 inhibitors unmask for immune attack.

Avelumab is an anti-PD-L1 monoclonal antibody given by intravenous infusion every 2 weeks. It was the first immunotherapy approved by the FDA for metastatic Merkel cell carcinoma in 2017. In chemotherapy-pretreated patients, avelumab produces an objective response in 33% of cases, with many responses lasting more than 2 years.

Merkel cell carcinoma is not considered an inherited cancer. Genetic predisposition does not appear to play a major role. The dominant risk factors are environmental and immune: cumulative ultraviolet exposure, fair skin, advanced age, and chronic immunosuppression from transplant, leukemia, or HIV.

You can lower your risk by limiting cumulative UV exposure with daily sunscreen, sun-protective clothing, and avoidance of tanning beds. People on long-term immunosuppression should have regular skin checks. There is no vaccine against Merkel cell polyomavirus, and most adults carry the virus harmlessly throughout life without developing MCC.

Yes. Lymphatic spread is common — sentinel lymph node biopsy is positive in about 30% of patients with no clinically detectable nodes. Distant metastasis develops in 30-40% of patients overall, most often to skin, distant lymph nodes, liver, lung, or bone. This is why staging at diagnosis with PET-CT and sentinel biopsy is essential.

Approximately 50% of cases appear on the head and neck, 35% on the upper limbs, and 15% on the lower limbs — all chronically sun-exposed sites. Rare cases occur on the trunk or covered areas, particularly in immunosuppressed patients. Lesions on the head and neck tend to have worse outcomes than those on the trunk or limbs.

Treatment of recurrent MCC depends on the site and extent. Isolated local or nodal recurrence is treated with surgery and radiation. Distant recurrence is managed with immune checkpoint inhibitors as first-line, with chemotherapy reserved for patients who cannot tolerate or have failed immunotherapy. Clinical trials should be considered for any recurrent disease.

Most patients tolerate PD-1 or PD-L1 inhibitors well, but 10-20% develop significant immune-related adverse events. These include colitis, pneumonitis, hepatitis, dermatitis, and thyroid, pituitary, or adrenal dysfunction. Early steroid therapy controls most events. Patients should report any new diarrhea, cough, rash, or severe fatigue to their oncology team the same day.

Merkel cell carcinoma is rare under age 50, with median age at diagnosis around 75. When it does occur in younger adults, immunosuppression — solid organ transplant, HIV infection, or chronic lymphocytic leukemia — is almost always involved. Genetic predisposition is not an established cause of early-onset MCC.

Adjuvant radiation to the primary site after wide local excision is recommended in most cases and reduces local recurrence from about 30% to under 10%. Radiation to the nodal basin is added when sentinel nodes are positive or in selected high-risk situations. The exact plan is individualized by your radiation oncologist.