Mixed Poorly-Differentiated Neuroendocrine Carcinoma and Well-Differentiated Neuroendocrine Tumor in the Extrahepatic Common Bile Duct: A Unique Rare Case

The World Health Organization classified neuroendocrine neoplasms of the digestive system into well-differentiated neuroendocrine tumor (NET) and poorly-differentiated neuroendocrine carcinoma (NEC) based on their unique morphological, clinical, epidemiological, histological, and prognostic differences. We pres-ent a case of an 80-year old female found to have a 31x22x21 mm mass in the perihilar common bile duct on CT scan. A tan-yellow mass within the common bile duct wall, extending into the surrounding fibroconnective tissue was noted on gross examination. Histologic examination revealed a well-circumscribed tumor with a biphasic appearance consisting of predominantly well-differentiated NET (approximately 80%) arranged in a trabecular architecture with round nuclei, finely granular chromatin, moderate cytoplasm, rare mitosis (6/2mm 2 ), and minor poorly differentiated NEC (approximately 20%) with markedly pleomorphic cells, necrosis, and abundant mitosis (40/2mm 2 ). Tumor cells in both morphologies showed immunoreactivity for AE1/AE3, CD56, synaptophysin and chromogranin. The Ki-67 proliferation index in the well-differentiated component was low (approximately 3-20%) and unequivocally high in the poorly-differentiated component (focally >50%). In the well-differentiated component, p53 staining was patchy and weak (wild-type), whereas it was negative (null-type) in the poorly-differentiated component. RB1 immunostaining showed weak staining in the well-differentiated component and diffusely strong staining in the poorly-differentiated component. The final diagnosis of mixed well-differentiated NET and poorly-differentiated NEC is made, which does not fit neatly into a specific category in the current classification of neuroendocrine neoplasms of the digestive system. Reporting more cases like this will be helpful for the revision of the current classification system.