Mixed Neuroendocrine-NonNeuroendocrine Neoplasm of the Sigmoid colon: the first case reported in Perú

Mixed Neuroendocrine-NonNeuroendocrine Neoplasms (MiNENs) are rare and aggressive entities, with scarce reports worldwide, especially from Latin America. We describe the first documented case of sigmoid MiNEN in Perú in a 77-year-old woman with a history of hypertension, diabetes, and osteoporosis who presented with left lower quadrant pain and abdominal distension. Colonoscopy revealed polyps, one of which was histologically consistent with a large-cell neuroendocrine carcinoma (NEC) and a high-grade tubular adenoma, each accounting for ≥30% of the lesion, fulfilling MiNEN criteria. Immunohistochemistry confirmed the diagnosis with an Antigen Kiel 67 [Ki-67] index of 70%. Positron Emission Computed Tomography (PET/CT) demonstrated focal uptake without nodal involvement. The patient underwent laparoscopic radical sigmoidectomy with colorectal anastomosis; histopathology confirmed submucosal invasion and metastasis in 5/14 lymph nodes. She was subsequently started on adjuvant carboplatin/etoposide chemotherapy and remains under oncological follow-up. This case underscores the diagnostic and therapeutic challenges of colorectal MiNENs, highlights the role of PET/CT as well as multidisciplinary management, and emphasizes the importance of considering radical resection even after endoscopic removal, given the high risk of recurrence and aggressive biological behavior.