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- Presentation
Overview of Merkel Cell Carcinoma: Diagnosis, Staging, Prognosis, and Management
Description
The talk provided an overview of Merkel cell carcinoma, a rare but highly aggressive cutaneous neuroendocrine cancer that often presents on sun-exposed skin and can rapidly metastasize. Diagnosis relies on histology plus immunohistochemistry, classically CK20 positive and TTF1 negative, but up to 25% of tumors are CK20 negative, so additional markers such as neurofilament, INSM1, SATB2, and Merkel cell polyomavirus testing may help, along with imaging to exclude metastatic small cell lung cancer. Pathogenesis is linked either to Merkel cell polyomavirus or to UV-driven mutations in virus-negative tumors. Prognosis is influenced by viral status, stage at diagnosis, immunosuppression, age, sex, and tumor immune infiltration; virus-positive tumors and tumors with brisk CD8 infiltrates tend to do better, while transplant recipients do worse. Clinical clues include asymptomatic, rapidly enlarging lesions in older or immunosuppressed patients, often mistaken for cysts or other skin cancers. Staging ranges from localized disease to nodal disease, in-transit disease, and distant metastasis, and management is best done in a multidisciplinary setting. For primary tumors, wide local excision is standard, with adjuvant radiation used for positive margins or high-risk features; some low-risk cases may be observed. Sentinel lymph node biopsy is recommended for clinically localized disease, and node-positive disease is generally treated with surgery and/or radiation. For unresectable or metastatic disease, immunotherapy is central, with agents such as avelumab, pembrolizumab, nivolumab, and retifanlimab showing meaningful responses. The speaker emphasized that newer immunotherapy-era data suggest improved survival compared with historical outcomes and highlighted ongoing trials testing adjuvant and neoadjuvant immunotherapy.
View moreConclusions
- Merkel cell carcinoma is rare, but it is clinically important because it is highly aggressive and frequently presents with nodal or distant spread.
- Accurate diagnosis depends on pathology plus immunohistochemistry, with CK20 and TTF1 being classic markers, but CK20-negative tumors require additional stains and imaging to exclude metastatic small cell lung cancer.
- Newer markers such as neurofilament, INSM1, SATB2, and Merkel polyomavirus T antigen can improve diagnostic confidence, especially in CK20-negative cases.
- Merkel cell carcinoma arises through two major biological pathways: ultraviolet-driven, virus-negative tumors and Merkel polyomavirus–associated tumors.
- Polyomavirus-positive Merkel cell carcinoma is associated with better prognosis, while virus-negative disease often shows a UV mutational signature.
- Prognosis is also influenced by stage, immunosuppression, advancing age, male sex, and tumor immune infiltration, with brisk CD8+ infiltration indicating better outcomes.
- Clinical recognition is difficult because MCC lesions are often small, painless, rapidly growing, and nonspecific, so clinicians should maintain a low threshold for biopsy and think in terms of upstaging.
- Even very small primary tumors can metastasize to lymph nodes, so sentinel lymph node biopsy is important for clinically node-negative disease.
- Management should be multidisciplinary and stage-adapted, combining surgery, radiation, systemic therapy, and imaging-based surveillance as needed.
- For low-risk localized disease with clear margins, observation after excision may be reasonable, but adverse features such as positive margins, large size, immunosuppression, or head-and-neck location favor adjuvant radiation.
- For sentinel-node–positive disease, current practice generally favors nodal radiation or dissection, often guided by tumor board review and individualized risk factors.
- Unknown-primary nodal MCC can behave better than nodal disease with a known primary and is staged differently because of its more favorable prognosis.
- Stage III and unresectable stage IIIB disease can respond dramatically to neoadjuvant immunotherapy, particularly nivolumab, and pathologic complete responses are achievable.
- For metastatic stage IV disease, immune checkpoint inhibitors are the preferred systemic treatment, with meaningful response rates for avelumab, pembrolizumab, retifanlimab, and nivolumab.
- Overall survival for advanced MCC appears to be improving in the immunotherapy era compared with historical outcomes.
- Adjuvant immunotherapy is promising and may reduce recurrence risk, but final results from ongoing phase III trials are still pending.
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