Pathology, classification, and grading of  neuroendocrine 

tumors arising in the digestive system

The terminology of gastroenteropancreatic (GEP) neuroendocrine neoplasms has evolved over the past two decades to reflect a separation into two major categories:

●Well-differentiated neuroendocrine tumors (NETs) show a solid, trabecular, gyriform, or glandular pattern, with fairly uniform nuclei, salt-and-pepper chromatin, and finely granular cytoplasm. These tumors were traditionally referred to as carcinoid and pancreatic neuroendocrine (islet cell) tumors. Although carcinoid tumors and pancreatic NETs may have similar characteristics on routine histologic evaluation, they have a different pathogenesis and biology.

●Poorly-differentiated neuroendocrine carcinomas, which are high-grade carcinomas that resemble small cell or large cell neuroendocrine carcinoma of the lung . Poorly differentiated neuroendocrine carcinomas are often associated with a rapid clinical course, while well-differentiated NETs generally have a much better prognosis, with an overall five-year survival of approximately 67 percent 

Histologic appearance of the spectrum of neuroendocrine tumors arising in the gastrointestinal tract

Well-differentiated NET shows an organoid pattern and difficult-to-find mitotic activity on H&E stain (panel A),

( Panel C) the proliferative rate (as measured by the Ki67 labeling index) is very low  (<1 percent="" span="">

poorly-differentiated neuroendocrine carcinoma shows frank features of carcinoma with numerous mitotic figures and tumor necrosis on H&E stain (panel B),the Ki67 labeling index is very high (80% in this case) (panel D).