Fatal Retroperitoneal Desmoid-Type Fibromatosis Masquerading as Uterine Fibroid with Colonic Invasion and Fecal Peritonitis: A Case Report

Desmoid-type fibromatosis is a rare, locally aggressive mesenchymal neoplasm that accounts for fewer than 0.03% of all tumors. Its retroperitoneal variant in young women closely mimics uterine leiomyoma on pelvic ultrasonography. When the diagnosis of desmoid-type fibromatosis is delayed, the tumor can silently invade adjacent viscera with fatal consequences. A 21-year-old woman presented with lower abdominal pain and a large pelvic mass on ultrasonography. The mass was labelled a uterine fibroid, and she was discharged. Two days later, she returned with an acute abdomen; CT revealed a 29 cm retroperitoneal mass with pneumoperitoneum. Emergency laparotomy confirmed colonic invasion and perforation with gross fecal peritonitis. Debulking colectomy with colostomy was performed; complete resection was not achievable. Histopathology was morphologically consistent with desmoid-type fibromatosis; confirmatory immunohistochemistry was not performed. Postoperatively, the patient developed refractory septic shock driven by an MDR Escherichia coli wound infection. Serum lactate never normalized across thirteen serial measurements over nine days from admission to death. She died nine days after presentation with a terminal lactate of 4.8 mmol/L and combined metabolic and respiratory failure. This case shows how retroperitoneal desmoid-type fibromatosis (DTF) can precisely replicate a uterine fibroid on ultrasound when it compresses but does not invade the uterus. Sporadic desmoid disease can reach catastrophic size silently and cause hollow-viscus perforation. Any large pelvic mass with bilateral ureteric obstruction in a young woman should prompt cross-sectional imaing before discharge.