Diagnosis:
Small Blue Cell Tumors and Associated Neoplasms
• Merkel cell carcinoma, a primary cutaneous or subcutaneous neuroendocrine carcinoma thought to arise from the Merkel cell found in the basal layer of the epidermis and hair-shaft, must be distinguished from the more aggressive metastatic small cell carcinoma.
• Merkel cell carcinomas, unlike pulmonary small cell carcinomas, are cytokeratin 7 (CK7) negative and show punctate, perinuclear positivity with CK20.
• Merkel cell carcinomas are negative for thyroid transcription factor 1 TTF-1, which stains the vast majority of small cell lung carcinomas, as well as a significant proportion of nonpulmonary small cell carcinomas.
• Merkel cell carcinomas are CD99 and Fli-1 positive, and their expression in an adult tumor is not indicative of Ewing sarcoma/primitive neuroectodermal tumor (ES/PNET), which affects younger individuals and is CK20 negative.