Aggressiveness in Well-Differentiated Small Intestinal Neuroendocrine Tumors: A Rare Case and Narrative Literature Review.

Small intestinal neuroendocrine tumors (SI-NETs) are the most common malignancies of the small bowel. Although typically well differentiated and slow-growing, they may exhibit aggressive behavior, especially when diagnosed at an advanced stage. To illustrate the diagnostic and therapeutic challenges of advanced SI-NETs through a rare case presentation and a narrative review of recent studies in the literature. A narrative literature review was conducted using the PubMed database to examine the incidence, risk factors, diagnostic modalities, and treatment strategies for advanced-stage SI-NETs. The search included studies published between January 2010 and June 2025 and focused on human subjects, using keywords such as "small intestinal neuroendocrine tumor", "metastasis", "tumor grade", and "treatment". We report the case of a 68-year-old man who presented with bowel obstruction. Imaging and surgical exploration revealed a jejunoileal SI-NET with extensive liver and peritoneal metastases, mesenteric fibrosis, and ascites. Histopathology confirmed a well-differentiated grade 2 tumor (Ki-67: 3%) positive for chromogranin A and CD56. Despite a low proliferative index, the tumor demonstrated aggressive clinical behavior. The patient underwent emergency enterectomy with ileostomy and was referred for further evaluation, including somatostatin receptor imaging and consideration for peptide receptor radionuclide therapy (PRRT). This case highlights the potential for aggressive progression in well-differentiated SI-NETs with low Ki-67 indices. Histological grade alone may not predict clinical behavior. Early diagnosis, comprehensive staging, and individualized multidisciplinary management-guided by functional imaging and receptor profiling-are critical to improving outcomes in advanced SI-NETs.