A Case of Cecal Mixed Neuroendocrine-Non-Neuroendocrine Neoplasm Requiring Adjuvant Chemotherapy Based on...

Colorectal mixed neuroendocrine-non-neuroendocrine neoplasms (MiNENs) are rare and aggressive tumors characterized by marked intratumoral heterogeneity, often making preoperative diagnosis difficult. Consequently, definitive diagnosis is frequently established only after surgical resection, and postoperative pathological findings may influence subsequent treatment strategies.

A 66-year-old man presented with bloody stool. Colonoscopy revealed a cecal tumor, and biopsy demonstrated moderately differentiated adenocarcinoma. Laparoscopic ileocecal resection with D3 lymph node dissection was performed. Histopathological examination of the resected specimen revealed a collision-type MiNEN composed of neuroendocrine carcinoma (NEC) (60%) and adenocarcinoma (40%). The NEC component showed lymphovascular invasion, deep mesenteric invasion, and a Ki-67 index of approximately 50%-60%. Although regional lymph node metastasis involved only the adenocarcinoma component, comprehensive pathological assessment suggested biologically aggressive behavior of the NEC component, raising concern for potential systemic dissemination. Accordingly, NEC-oriented adjuvant chemotherapy with carboplatin plus etoposide was administered after curative resection. The patient remained recurrence-free until death from other causes 2 years after surgery.

Colorectal MiNEN poses substantial diagnostic challenges because biopsy specimens may not adequately represent both tumor components. Comprehensive pathological assessment of the resected specimen is important for identifying the aggressive component most likely to influence prognosis and for optimizing postoperative therapeutic strategy.