A Case of Grade 3 Gastric Neuroendocrine Tumor with Glandular Formation: Diagnostic Process and Differentiation from Gastric Mixed Neuroendocrine-Non-Neuroendocrine Neoplasm.

A 78-year-old man was diagnosed as having a submucosal gastric mass (diameter 4 cm). Preoperative findings from endoscopic ultrasound-guided fine needle aspiration suggested a diagnosis of gastric neuroendocrine neoplasm. Total gastrectomy with excision of a metastatic liver lesion and dissection of gastric lymph nodes was performed. Analysis of frozen sections indicated adenocarcinoma of the peritoneum, which suggested the possibility of a mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN), based on the presence of solid tubules, metastatic spread, and lymphovascular invasion. However, the smooth gastric mucosal surface, organoid architecture with rare atypia or necrosis, immunopositivity for neuroendocrine markers, a Ki-67 index of 21%, and the presence of somatostatin receptor 2 expression confirmed a final pathological diagnosis of grade 3 gastric neuroendocrine tumor (NETG3) with glandular formation. NETG3 with glandular formation can be difficult to distinguish from MiNEN because their histological features overlap. However, gastric NETG3 with glandular formation is distinguishable from MiNEN by the presence of a submucosal tumor with a histological organoid pattern without frequent mitoses and/or necrosis, immunopositivity for neuroendocrine markers, and the absence of an adenocarcinoma or neuroendocrine carcinoma component within the tumor, as determined by immunohistochemistry for somatostatin receptor 2 expression, Ki-67, and Rb1.