Maho Takuwa

Introduction: Mixed neuroendocrine–non-neuroendocrine neoplasms (MiNENs) are rare tumors composed of neuroendocrine and non-neuroendocrine components. Their occurrence in the duodenum is extremely uncommon. Preoperative diagnosis is difficult because the neuroendocrine component may not be detected in biopsy specimens. We report a rare case of non-ampullary duodenal MiNEN initially diagnosed as adenocarcinoma. Case presentation: A 56-year-old man was found to have a tumor in the duodenal bulb during routine endoscopic examination. Biopsy revealed moderately to poorly differentiated adenocarcinoma, and computed tomography demonstrated bulky regional lymph node metastasis without distant disease. Neoadjuvant chemotherapy with S-1 plus oxaliplatin was administered based on treatment strategies used for advanced gastric cancer in Japan. Post-treatment evaluation showed stable disease, and pancreaticoduodenectomy with regional lymph node dissection achieved R0 resection. Histopathological examination revealed both adenocarcinoma and neuroendocrine carcinoma (NEC) components, leading to a diagnosis of MiNEN. Lymph node metastases demonstrated component-specific heterogeneity, with NEC involvement in station No. 8a and adenocarcinoma in the remaining metastatic nodes. Postoperative platinum-based chemotherapy was administered, and the patient remains recurrence-free at 16 months after surgery. Discussion: Duodenal MiNEN presents diagnostic and therapeutic challenges because biopsy specimens may fail to detect the neuroendocrine component preoperatively. In the present case, component-specific lymph node metastases suggested biological heterogeneity and may have influenced treatment selection. Conclusion: Non-ampullary duodenal MiNEN is a rare and diagnostically challenging disease. This case highlights the importance of accurate pathological evaluation and multidisciplinary management.