Occluded sigmoid tumor with a complete common mesentery discovering: clinical dilemma

• Complete common mesentery is exceptionally diagnosed in adults. • Its association with obstructive sigmoid cancer is extremely rare. • Preoperative CT is essential to identify anatomical and vascular anomalies. • Surgical strategy must be adapted to vascular constraints. • Tailored oncological resection can achieve safe and effective outcomes. Intestinal malrotation with complete common mesentery is a rare congenital anomaly that is exceptionally diagnosed in adulthood. When associated with obstructive colorectal cancer, it represents a diagnostic, anatomical, vascular, and oncological challenge that can significantly influence the surgical strategy. A 69-year-old woman was admitted for acute large bowel obstruction. Computed tomography revealed a stenosing sigmoid tumor with upstream colonic dilatation, associated with complete common mesentery. Emergency surgery confirmed the abnormal intestinal and vascular anatomy, with a short and atypically oriented mesenteric pedicle. After ligation of the inferior mesenteric vessels, oncological resection of the lower sigmoid segment was performed, deliberately adapted to the vascular constraints rather than an extended left colectomy, followed by a Hartmann procedure. Postoperative recovery was uneventful. Histopathological examination revealed pT3N+ adenocarcinoma. The patient received adjuvant chemotherapy, and intestinal continuity was restored six months later by mechanical colorectal anastomosis. In adults, the association between colorectal cancer and complete common mesentery is extremely rare. Preoperative imaging is essential for identifying anatomical and vascular variations, anticipating technical difficulties, and guiding oncological decision-making, particularly in emergency situations. This case highlights the importance of recognizing congenital anatomical and vascular anomalies in adults with obstructive colorectal cancer, as they may require adapted surgical oncology strategies to ensure both radicality and safety.