Mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN) of the rectum.

A 75-year-old man presented to our hospital complaining of abdominal distention. He underwent lower gastrointestinal endoscopic examination, which indicated a raised tumor of 20 mm in diameter in the sigmoid colon transition region of the rectum. Endoscopic mucosal resection was performed with the aims of treatment and diagnosis. Histopathological examination revealed a moderately differentiated adenocarcinoma with neuroendocrine carcinoma, which was classified as a mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN). The tumor had highly invaded the submucosa and positive vertical margins. Computed tomography showed no obvious lymph node metastasis or distant metastasis, so the patient underwent high-level anterior resection of the remaining lesion. The final diagnosis was MiNEN, pT3(SS), INFb, Ly1c, V0, Pn1a, pPM0, pDM0, pRM0, pN1a, and pStage IIIb (TNM Classification of Malignant Tumors, 8th Edition). The patient is recurrence free at 3 years without postoperative adjuvant therapy. We report this case with a review of the literature.