Introduction: Mixed Adenoneuroendocrine cancer (MANEC) is an extremely rare, relatively novel gastrointestinal cancer that is characterized by the presence of both adenocarcinomatous and neuroendocrine differentiation; each component representing at least 30% of the tumor. We present a case of a patient presenting with gastric outlet obstruction and was found to have a primary duodenal MANEC.

Case Presentation: A 57-year-old female with history of diverticulitis presented with intractable nausea, vomiting, epigastric pain, and weight loss. She was afebrile with mild epigastric tenderness. Labs were unremarkable except for hypokalemia. CT abdomen showed thickening of the third portion of the duodenum with surrounding abnormal lymph nodes. EGD revealed a large infiltrating mass in D3 portion of the duodenum. Pathology was positive for adenocarcinoma.

After ruling out metastatic disease, patient underwent pancreatoduodenectomy. Surgical pathology revealed poorly differentiated mixed adenoneuroendocrine tumor (Ki67-70%) forming a 3.2 cm mass in the duodenum and invading through the wall of the duodenum into the adjacent pancreas and peripancreatic soft tissue. Patient was treated in ICU for post-op pancreatic leak and infection. She was eventually discharged with oncology follow-up for adjuvant chemotherapy.

Discussion: MANECs appear to be highly malignant tumors with a high risk for distant metastases. The aggressiveness depends mainly on the endocrine component, independent of its proportion. Identification of the neuroendocrine component has significant therapeutic relevance; therefore, a very attentive microscopic evaluation should be performed in adenocarcinomas with dedifferentiated areas. Surgery is the first line of treatment for cases with a resectable tumor. Because of its aggressive nature and high recurrence rate, adjuvant chemotherapy constitutes a critical part of treatment and significantly improves survival.