Merkel Cell Carcinoma

• Rare, highly aggressive malignant neuroendocrine carcinoma; 5-year mortality approximately 30%
• Presents as red to pink dome-shaped rapidly growing nodule typically involving head, neck, leg, or buttock; 40% metastasis rate at time of diagnosis
• Histology: poorly defined dermal mass of small blue monomorphic round cells with scanty cytoplasm and nuclear molding, abundant mitotic figures, often see necrosis and crush artifact; “ball in mitt” cellular pattern with crescentic neoplastic cells wrapping around round neoplastic cell; three growth patterns:
  • Trabecular: islands of neoplastic cells connecting to one another via anastomosing cords
  • Intermediate - cell type: large solid collection of cells with peripheral trabecular pattern
  • Small -cell type: diffuse sheet-like infiltration mixed with intermediate cells
• Immunohistochemically: CK20, CK8/18/19 (CAM 5.2), chromogranin, somatostatin, calcitonin, synaptophysin, vasoactive intestinal peptide, neuron-specific enolase
• Treatment: wide local excision (3cm margins) often with adjuvant chemotherapy and/or radiation, sentinel lymph node biopsy