A puzzling case report of well-differentiated neuroendocrine tumor mixed with gastric adenocarcinoma of the fundic gland type associated with autoimmune gastritis.

Gastric adenocarcinoma of the fundic gland type (GA-FG) is a rare gastric neoplasm. We present a unique case of multiple GA-FG that coexisted with the well-differentiated neuroendocrine tumors in a patient with autoimmune gastritis. To our knowledge, this is the first documented instance of such a co-occurrence and the molecular mechanism of their origin has been reviewed systematically. A 47-year-old male presented to our hospital with abdominal distension for over 10 years. Gastroscopy revealed multiple gastric eminence lesions (0.2-1.5 cm). After endoscopic mucosal resection, the pathological morphology showed mixed tumor components infiltrating into the submucosa with puzzling similarity. One with uniform-sized tumor cells arranged in nests or tubes and the other a well-differentiated tubular adenocarcinoma with irregular branching and visible gland fusion. Immunohistochemistry findings revealed the first component expressed typical markers of neuroendocrine tumor, whereas the second component expressed pepsinogen and mucin-6, indicating the presence of oxyntic gland adenocarcinoma. Due to the tumors' proximity to the surgical margins, the patient underwent laparoscopic subtotal gastrectomy three months after the diagnosis without any tumor residue and showed no recurrence or metastasis occurred in the following regular checkups.