Merkel Cell Carcinoma

Introduction & Epidemiology - Rare But Nasty

• Rare, aggressive neuroendocrine skin carcinoma.
  • From Merkel cells (tactile cells) in stratum basale.
• Incidence: Rising, ~0.7/100,000 (US).
• Median age: ~75 years.
• Key Risk Factors:
  • UV radiation (fair skin)
  • Immunosuppression (transplant, CLL, HIV)
  • Merkel Cell Polyomavirus (MCPyV) in ~80% tumors
  • Age >50 years
• High local recurrence & distant metastasis.

⭐ High incidence in elderly, fair-skinned individuals with significant UV exposure, and immunosuppressed patients (e.g., organ transplant recipients, CLL, HIV).

Pathogenesis & Etiology - Viral Villain & UV

• Dual oncogenesis: MCC arises from two main pathways.
• Viral (MCPyV-positive):
  • Merkel Cell Polyomavirus (MCPyV) integration.
  • Viral oncoproteins (LT, sT) disrupt cell cycle (pRb, p53).
  • Common in immunosuppression.
• UV Radiation (MCPyV-negative):
  • Chronic sun exposure causes DNA damage (UV signature mutations).
  • Leads to uncontrolled cell proliferation.

⭐ Approximately 80% of MCCs are associated with MCPyV; viral-positive tumors often have better prognosis than UV-induced tumors. oka

Clinical Features & Diagnosis - Spotting Suspects

• Solitary, rapidly growing, painless, firm, dome-shaped nodule or plaque.
• Often erythematous or violaceous; typically on sun-exposed areas (head, neck, extremities).
• Surface may be shiny; ulceration can occur.

⭐ 📌 AEIOU clinical aid:

• Asymptomatic/lack of tenderness
• Expanding rapidly (≤ 3 months)
• Immunosuppression
• Older than 50 years
• UV-exposed site

• Biopsy: Full-thickness (incisional/excisional) for diagnosis.
• Immunohistochemistry (IHC) is key:
  • CK20 (characteristic paranuclear dot-like pattern).
  • Synaptophysin, Chromogranin A (neuroendocrine markers).
  • TTF-1 negative (differentiates from small cell lung cancer).

Staging & Prognosis - Sizing the Threat

• AJCC 8th Edition (TNM) is used for staging.

-   Tumor size (T stage)
-   Nodal status (N stage) - most important
-   Distant metastasis (M stage)
-   Immunosuppression (worse prognosis)
-   Lymphovascular invasion

⭐ Sentinel Lymph Node Biopsy (SLNB) is critical for staging in clinically node-negative patients and significantly impacts prognosis and adjuvant therapy decisions.

• Overall 5-year survival: ~60%; Stage IV: <20%

Management & Treatment - Fighting Back

• Localized Disease:
  • Wide local excision (WLE) with 1-2 cm margins.
  • Sentinel lymph node biopsy (SLNB) is crucial for staging.
  • Adjuvant radiotherapy (RT) to primary site & regional nodes improves locoregional control, especially for high-risk features (e.g., large size, lymphovascular invasion, positive margins, immunosuppression).
• Metastatic/Advanced Disease:
  • Systemic therapy is the mainstay.
  • Chemotherapy (e.g., platinum-etoposide) offers transient responses.
  • Immunotherapy is preferred first-line.

⭐ Immune checkpoint inhibitors (e.g., Avelumab, Pembrolizumab) have shown significant efficacy and are approved for metastatic or locally advanced MCC, revolutionizing treatment for advanced disease.

High‑Yield Points - ⚡ Biggest Takeaways

• Aggressive neuroendocrine skin cancer, strongly linked to Merkel Cell Polyomavirus (MCPyV).
• Typically affects elderly, immunosuppressed individuals, predominantly on sun-exposed skin (head, neck).
• Presents as a rapidly growing, painless, firm, shiny, red-violaceous nodule.
• AEIOU mnemonic is key: Asymptomatic, Expanding rapidly, Immunosuppression, Older than 50 years, UV-exposed site.
• Histopathology shows small round blue cells; characteristic paranuclear dot-like CK20 positivity.
• High rates of local recurrence and lymph node metastasis; management includes wide surgical excision, often with radiotherapy.