DOI: https://doi.org/10.36347/sjmcr.2026.v14i05.049

Pages: 1070-1074

Background: Desmoid-type fibromatosis (DTF) is a rare benign fibroblastic neoplasm characterized by local aggressiveness and a high risk of recurrence despite the absence of metastatic potential. Retroperitoneal localization is exceptionally uncommon and may lead to delayed diagnosis because of nonspecific symptoms. Case Presentation: We report the case of a 30-year-old woman presenting two years after vaginal delivery with right-sided lumbar pain and a palpable lumbar mass. Imaging studies revealed a large right retroperitoneal mass measuring 13 × 9 × 13 cm, infiltrating the psoas and quadratus lumborum muscles and causing extrinsic compression of the right ureter with severe ureterohydronephrosis. Magnetic resonance imaging demonstrated a heterogeneous soft-tissue lesion with infiltration of adjacent musculature and displacement of bowel loops without invasion. Following multidisciplinary team discussion, the patient underwent radical surgical resection. Intraoperatively, the tumor was found to encase the right ureter and adhere to surrounding structures, including the hepatic flexure, transverse colon, and diaphragm. Histopathological examination confirmed desmoid-type fibromatosis. Postoperative recovery was complicated by surgical site infection, successfully managed with antibiotics and local wound care. Conclusion: Retroperitoneal desmoid-type fibromatosis should be considered in the differential diagnosis of retroperitoneal masses causing ureteral obstruction, particularly in young women with a recent history of pregnancy or childbirth. Complete surgical excision remains the cornerstone of treatment in resectable symptomatic lesions, although emerging targeted therapies are expanding therapeutic options.