Unveiling the Rarity: A Case of a Mixed Neuroendocrine-Non-Neuroendocrine Tumor of the Gastroesophageal Junction.

Mixed neuroendocrine-non-neuroendocrine neoplasms (MiNENs) are a rare group of heterogeneous tumors, consisting of an endocrine and a nonendocrine component, which can develop throughout the gastrointestinal (GI) tract. This case presents a 70-year-old man with a complex medical history who initially presented with an upper GI bleed. After being stabilized, he underwent an esophagogastroduodenoscopy (EGD) that revealed a suspicious gastroesophageal junction (GEJ) mass. Histopathological studies paired with immunohistochemical investigations of the mass confirmed the rare diagnosis of MiNENs. He then underwent an endoscopic submucosal dissection (ESD) with subsequent chemotherapy and adjunct radiotherapy, with no recurrence noted on post-treatment surveillance. This case highlights the need for an EGD, histopathological examination, and immunohistochemical staining for detecting the underlying etiology of an upper GI bleed.