S6446 Linitis Plastica With Multiple Expressions Masquerading as Mixed Adenoneuroendocrine Carcinoma (MANEC) of the Gastric Antrum: An Unusual Case Report

Introduction: Mixed adenoneuroendocrine carcinoma (MANEC) is a very rare tumor of the gastrointestinal tract that consists of 2 components each with an adenocarcinomatous and neuroendocrine differentiation, with at least 30% of the tumor from either component. Gastric linitis plastica (LP) is an aggressive tumor with poor prognosis and abysmal outcome. Timely diagnosis is important for effective management. Our case describes a 41-year-old man with LP gastric adenoma with polypoid tumor of antrum exhibiting glandular component characteristics. Case Description/Methods: Forty-one-year-old man referred to our hospital and evaluated for epigastric pain and weight loss. Computed tomography (CT) of the abdomen and pelvis was obtained and demonstrated thickening of the fundus and body, punctate air foci adjacent to the gastric pylorus, without extra luminal contrast. Moderate volume low density ascites. Diffuse intra-abdominal fatty stranding and simple ascites possible contained perforation at the gastric fundus with some surrounding fluid, without obvious metastatic disease. He underwent esophagogastroduodenoscopy (EGD) and diagnostic laparoscopy with washings and biopsies, a large, infiltrative and polypoid, non-circumferential mass with no bleeding and no stigmata of recent bleeding was found on the lesser curvature of the stomach possibly a type 2 tumor in the antrum of the stomach. The polypoid tumor of the antral had metastatic tissue which represented glandular component mimicking neuroendocrine carcinoma. The Immunohistopathological findings showed a moderately differentiated HER2-negative adenocarcinoma and Positive BerEP4, MOC31, CDX2 without mucinous component. Pathology demonstrated Linitis Plastica involving the proximal and mid stomach. The antrum was spared. The disease went to the Z-line but did not appear to invade the esophagus. On diagnostic laparoscoscopy, 5-6 lymphatic nodules were noted on the left diaphragm and 1 nodule on the right diaphragm, and ascites, all highly concerning for peritoneal disease. carcinoembryonic antigen was 1.1 and CA 19-9 was 3.1. Patient was referred to higher tertiary institution for a higher level of care and endoscopic ultrasound fine needle aspiration (EUS-FNA) confirmed LP. Discussion: MANECs usually parade to be highly malignant tumors, with extremely high risk for metastasis. This aggressive character relies on the endocrine component, independent of its adenomatous portion. The adenomatous component may be a dedifferentiated tubular adenocarcinoma like LP but with a neuroendocrine phenotype.