An update on the diagnosis of gastroenteropancreatic neuroendocrine neoplasms

Figure 1 Morphologic features of low-grade well-differentiated neuroendocrine tumors and poorly differentiated neuroendocrine carcinoma. A-D: Gastroenteropancreatic neuroendocrine neoplasms have a variety of architectural patterns (hematoxylin and eosin staining); A and B: Low-grade well-differentiated neuroendocrine tumors (NETs) typically have monotonous cells with round nuclei and “salt and pepper” chromatin; C: High-grade well-differentiated NETs tend to have more nuclear pleomorphism with readily identifiable mitoses; D: Small cell carcinoma, a variant of neuroendocrine carcinoma, has significant atypia with nuclear molding and scant cytoplasm. Mitoses are also readily identified; E and F: In addition to mitotic count, the Ki-67 proliferation index is necessary for grading. Low-grade well-differentiated NETs have a low proliferation index, < 20% on Ki-67 immunohistochemical (IHC) stain (E), while high-grade well-differentiated NETs have a high proliferation index, > 20% on Ki-67 IHC stain (F). Scale bars: 100 µm (A), 200 µm (B, E and F), 50 µm (C and D).