Endoscopic Misdiagnosis of a Polypoid Gastric Neuroendocrine Neoplasm: A Case Report and Literature Review

A 75-year-old man presented to our department, complaining of recurrent abdominal pain for the past month. Esophagogastroduodenoscopy (EGD) revealed a single 0.6 cm polypoid lesion, classified as Yamada Type II, at the lesser curvature of the lower body of the stomach. The lesion exhibited a smooth surface, pale pink coloration, and no depression, ulceration, or erosion. Narrow-band imaging (NBI) demonstrated sparse superficial microvascular patterns on the lesion's surface. The observed polypoid lesion was treated via endoscopic mucosal resection (EMR). Pathological examination of the EMR specimen revealed a neuroendocrine neoplasm (NEN) in the gastric body, classified as grade G3. Immunohistochemical staining demonstrated that the tumor cells were positive for pan-cytokeratin (P-CK), CD56, synaptophysin, INSM1, and Ki67 (60% positive). Given the high grade, the patient underwent a subsequent partial gastrectomy. Pathological examination of the surgical specimen demonstrated a microscopic region of approximately 2 mm with dark-staining nuclei. Immunohistochemical staining showed positivity for CD56 and synaptophysin, and negativity for CD20. These findings were consistent with residual NEN, with the residual lesion measuring approximately 2 mm in maximum diameter.