Massive Mixed Adenoneuroendocrine Carcinoma: A Case Report

Mixed adenoneuroendocrine carcinoma (MANEC) of the gastrointestinal (GI) tract is a rare subtype of mixed tumors, and it is scarcely described in the literature. MANEC tumors are composed of adenocarcinoma and neuroendocrine carcinoma components, each of which comprises at least 30% of the lesion. Diagnosing MANEC requires specific histological and immunohistochemistry (IHC) analysis. Typically, MANEC tumors carry a poor prognosis due to their very aggressive nature. We report the case of a 70-year-old female patient with no past medical history who presented with a three-week history of abdominal pain and one episode of hematemesis one week prior to presentation. Initial CT of the abdomen showed a large, 8 x 6 x 6-cm mass arising from the stomach and extending to the lesser sac as well as the central crus of the diaphragm with bilateral retroperitoneal lymphadenopathy. Upper endoscopy revealed an excavated, ulcerated, and partially necrotic mass on the lesser curvature of the proximal gastric body. Tissue biopsy of the lesion showed infiltrating mixed poorly differentiated adenocarcinoma and neuroendocrine carcinoma. On IHC, the adenocarcinoma component stained positively for CDX2 and pancytokeratin, and the neuroendocrine component stained positively for synaptophysin and chromogranin. Further workup included CT of the chest, which demonstrated extensive bilateral pulmonary emboli and new liver lesions with moderate ascites not seen on the initial abdominal CT. The latter was repeated and showed remarkable enlargement of the gastric mass (up to 12 cm) with extensive retroperitoneal adenopathy and mesenteric implants. Given the rapid clinical deterioration and progression of tumor burden, comfort measures were offered and the patient passed away soon after. MANEC tumors are highly aggressive subtypes of "collision" tumors, which are not well described in the medical literature due to their rarity. The etiology is poorly understood with various theories proposing different pathophysiological mechanisms. Standard therapy is not well developed at present; however, a few reports have demonstrated successful outcomes with surgery or combined chemotherapy (cisplatin with irinotecan or etoposide) if diagnosed at an early stage.