Gastric Mixed Adenoma Well-Differentiated Neuroendocrine Tumor or Mixed Neuroendocrine–Non-neuroendocrine Neoplasm: A Case of Classification Dilemma

ABSTRACT Mixed neuroendocrine–non-neuroendocrine neoplasms (MiNENs) and mixed adenoma with well-differentiated neuroendocrine tumors (MANETs) are rare neoplasms composed of neuroendocrine (NE) and non-NE components. Accurate classification is essential, as MiNENs are typically aggressive, whereas MANETs typically follow an indolent course. We present a case of a gastric mixed tumor in a 69-year-old woman, comprising tubular adenoma, adenocarcinoma in situ, and a low-grade NE tumor. Although the tumor did not meet the current WHO criteria for MiNEN, it demonstrated unusually aggressive behavior including rapid progression and lymph node metastasis, atypical for a MANET. This highlights the need to refine current diagnostic classifications to better treat such mixed tumors, as MANETs are typically managed with polypectomy, whereas MiNENs may require extensive endoscopic or surgical intervention.