Case Report: Robotic-assisted resection of intra-abdominal aggressive fibromatosis and Boari flap ureteroneocystostomy for hydronephrosis.

Background: Aggressive fibromatosis (AF), also known as desmoid-type fibromatosis, is a rare, locally infiltrative soft-tissue tumor with no metastatic potential but a significant risk of local recurrence. Intra-abdominal AF may encase adjacent organs and vascular structures, occasionally leading to organ dysfunction. Ureteral involvement resulting in hydronephrosis is uncommon and presents complex diagnostic and therapeutic challenges, particularly when reconstruction of the urinary tract is required. Case summary: We report a 28-year-old male patient in whom right-sided hydronephrosis was incidentally detected during routine examination. Imaging revealed a 6 cm pelvic mass encasing the distal ureter and displacing the iliac vessels. Biopsy suggested a low-grade spindle-cell tumor. Due to progressive ureteral obstruction, surgical intervention was indicated. The patient underwent robotic-assisted laparoscopic tumor resection with segmental ileal resection and intracorporeal anastomosis. Distal ureteral involvement necessitated resection and reconstruction using a 7-cm Boari bladder flap ureteroneocystostomy over a double-J stent. Histopathological examination confirmed aggressive fibromatosis. Postoperative recovery was uneventful. At three-month follow-up, imaging demonstrated resolution of hydronephrosis and no evidence of recurrence. Conclusions: Intra-abdominal aggressive fibromatosis with ureteral encasement is rare and may require simultaneous oncologic resection and complex urinary tract reconstruction. A single-stage robotic-assisted approach can facilitate precise tumor excision and tension-free ureteral reconstruction while preserving renal function. Individualized surgical planning is essential in managing organ-compromising desmoid tumors.