Distinct Adenocarcinoma and Neuroendocrine Carcinoma Components (Suspected Mixed Neuroendocrine‐Non‐Neuroendocrine Neoplasm) Identified by Targeted Biopsy Based on Differences in Endoscopic Findings in an Esophagogastric Junction Carcinoma: A Case Report

ABSTRACT Mixed neuroendocrine‐non‐neuroendocrine neoplasm (MiNEN) is a rare, highly malignant tumor comprising both neuroendocrine neoplasm (NEN) and non‐neuroendocrine neoplasm (non‐NEN) components. MiNEN arising at the esophagogastric junction (EGJ) is uncommon, and pretreatment identification of both components remains challenging. A man in his seventies presented with back pain and liver and bone metastases. Esophagogastroduodenoscopy revealed an erythematous, elevated lesion with central depression at the EGJ. Magnifying endoscopy with narrow‐band imaging (ME‐NBI) showed a granular microsurface (MS) pattern and a dense, irregular microvascular network in the elevated area, while the depressed area exhibited the absence of the MS pattern and fragmented, irregular microvessels. Biopsy of the elevated area revealed well‐differentiated tubular adenocarcinoma, whereas the depressed area was positive for synaptophysin and CD56, consistent with neuroendocrine carcinoma (NEC). Although both components were clearly identified endoscopically and histopathologically, evaluation of the proportion of each component within the entire tumor was difficult based solely on biopsy specimens; the lesion was therefore diagnosed as EGJ carcinoma (suspected MiNEN). The patient achieved marked tumor shrinkage after the first course of irinotecan plus cisplatin chemotherapy, but treatment was discontinued due to adverse effects, and he ultimately died of disease progression. This rare case demonstrated identification of both adenocarcinoma and NEC components before treatment through detailed ME‐NBI‐guided biopsy, leading to suspicion of MiNEN. Precise endoscopic evaluation may play an important role in determining the therapeutic strategy. Clinical Registration : N/A