A Case of Gastric Neuroendocrine Neoplasm with Mixed Grade: a Distinct Type of "High"-grade Well-Differentiated Neuroendocrine Neoplasm.

Grade 3 neuroendocrine tumor (NET G3) is a rare new entity that has recently been introduced in the classification of neuroendocrine neoplasms of the pancreas. It is a well-differentiated form, with a high proliferative activity (Ki67 > 20%), and it represents a category whose prognosis is intermediate between NET G2 and poorly differentiated neuroendocrine carcinoma (NEC G3). In sites other than the pancreas, this new category is by far less coded. Herein is reported a case of NET G3 of the stomach, the 13th described in literature. The patient was a 71-year-old man with a mass in the gastric fundus. Histological examination revealed a neuroendocrine tumor that was mostly composed by areas with conventional well-differentiated features and intermediate proliferative rate (< 20 mitoses/10 HPFs and 7% Ki67); part of the tumor was characterized by increased nuclear atypia, a confluent growth pattern, intermediate mitotic activity, and high Ki67 value (25%). The tumor was studied immunohistochemically and molecularly, and the only alteration that came out was MEN1 gene mutation (Lau584SerfsTer5) in both the high- and low-grade components. According to the actual classification of gastric neuroendocrine neoplasms, this case is labeled as NEC G3 but it should be named NET G3, with a much better prognosis than the former. Overall, both morphological pattern and the Ki67 value should be mentioned in the histological report in cases of high-grade neuroendocrine neoplasms because of its clinical significance in defining the prognosis and for new potential therapeutic approaches.