GI NET in which tumor cells have both phenotypic features of endocrine and neural cells and express markers such as chromogranin-A and synaptophysin can be categorized into neuroendocrine tumor grade 1 (well-differentiated endocrine tumor, carcinoid tumor), neuroendocrine tumor grade 2 (well-differentiated endocrine carcinoma), and neuroendocrine carcinoma grade 3, large-cell or small-cell type (poorly differentiated endocrine carcinoma/small-cell carcinoma) according to WHO 2010 classification based on tumor histopathology, proliferative activity, and other tumor characteristics including site of origin, size, infiltration/invasion into other organ, and metastasis (Rindi et al. 2010). Mainly due to increased application of endoscopy, the frequency of gastric NET has been rising to 41 % of all GI NETs. In most of patients with gastric NET, there are no specific symptoms related to gastric NETs because these tumors are usually detected incidentally. In contrast to small bowel NETs, typical carcinoid syndrome is very rare in patients with gastric NETs. Because these tumors can give early hepatic metastases and regional lymph node metastases even in small size, the adequate staging evaluation should be mandatory. Based on the presence of hypergastrinemia, well-differentiated gastric NETs can be categorized into type I, associated with hypergastrinemia, chronic atrophic gastritis, and enterochromaffin-like cell hyperplasia; type II, seen in the setting of hypergastrinemia due to multiple endocrine neoplasia type I (MEN-I) and Zollinger-Ellison syndrome; and type III, not associated with hypergastrinemia (sporadic type). Type I gastric NETs are the most common type (75–80 % of gastric NETs) and more common in women patients (two or three times more than men). They usually appear as multiple and small (usually less than 1 cm) polypoid lesions in gastric body and fundus area. Type I gastric NETs generally represent benign disease course and rarely metastasize to lymph nodes (2 % of patients with type I tumor). Type II gastric NETs are associated with MEN1 syndrome and tend to be larger (often more than 1 cm) and show more aggressive clinical behavior (local metastasis in up to 30 %). In patients with type II NETs, there are multiple polyps or masses in which ulceration is sometimes present in the thickened rugal folds and the gastric wall. Type III gastric NETs, that are sporadic tumor, generally appear as usually large (often more than 2 cm), solitary tumors that may show ulceration. These tumors have an aggressive clinical behavior with local invasion and early metastases, especially when the tumor size is greater than 3 cm (Christopoulos and Papavassiliou 2005; Johnson et al. 2010).

On UGIS, gastric carcinoid tumors appear as small (almost less than 2 cm) sessile polypoid lesion. These tumors are usually seen as the small enhancing polyp on CT. In patients who have the associated Zollinger-Ellison syndrome, diffuse rugal folds thickening and multiple polyps or masses can be seen. The differential diagnosis of gastric carcinoid tumor that presents as small enhancing polypoid lesion can include small gastrointestinal stromal tumor, glomus tumor, and heterotopic pancreas. For the gastric neuroendocrine carcinoma, stomach is the most frequent site of GI neuroendocrine carcinomas (up to 59 % of all GI neuroendocrine carcinoma). On CT, gastric neuroendocrine carcinomas typically show obvious and homogeneous enhancement on arterial phase and further enhancement on venous phase since these tumors have an abundant blood supply. When these tumors spread to the liver, hepatic metastatic lesions usually show a homogeneous enhancement (Johnson et al. 2010).