Metastatic merkel cell carcinoma of the pancreas mimicking primary pancreatic endocrine tumor diagnosed by endoscopic ultrasound-guided fine needle aspiration cytology: a case report.

BACKGROUND: Merkel cell carcinoma (MCC) is a relatively infrequent, rapidly progressive and often fatal cutaneous malignancy exhibiting neuroendocrine differentiation. It has a penchant for local recurrence and distant metastasis to various sites, including regional lymph nodes, distant skin, lung, liver, testis and other rare organs, such as the pancreas. There are only 4 cases of MCC metastatic to the pancreas reported in the English-language literature, and they were all diagnosed by histology from pancreatic resection. CASE: A 79-year-old woman with a large pancreatic tail mass underwent endoscopic ultrasound guided fine needle aspiration (EUS-FNA). She had a history of MCC of the upper extremity with wide local excision 15 months earlier. Metastatic MCC was diagnosed based on the cytomorphology, characteristic immunohistochemical staining pattern, clinical history and comparison of the morphology with that of the primary tumor. CONCLUSION: The cytomorphology and immunohistochemical profile of this neoplasm mimicked a pancreatic endocrine tumor. We discuss the diagnostic pitfalls and differential diagnoses of the metastatic pancreatic MCC, highlighting the importance of thorough clinical history, attention to cytologic detail and corroborating immunohirtochemistry in arriving at the correct diagns. This is the first case ofa metastatic pancreatic MCC diagnosed by EUS-FNA cytology.