A mobile intra-abdominal low-grade fibromyxoid sarcoma arising from the jejunal mesentery in an adolescent: a case report and literature review.

Low-grade fibromyxoid sarcoma (LGFMS) is a rare fibroblastic neoplasm that only exceptionally involves the intra-abdominal cavity. We report a 17-year-old girl with a one-year history of a progressively enlarging, painless, freely mobile abdominal mass. Computed tomography and magnetic resonance imaging showed a localized, well-circumscribed, pedunculated lesion with positional variability, initially suggesting a completely resectable gastrointestinal stromal tumor. Preoperative biopsy was not performed because systemic treatment was not anticipated and intact resection was planned while minimizing the risks of tumor rupture and intraperitoneal dissemination. A non-contrast chest computed tomography examination on admission, three days before surgery, showed no significant abnormality. Laparoscopic exploration identified an approximately 10 × 10 cm encapsulated tumor attached by a narrow peripheral stalk to the mesenteric border of the proximal jejunum, approximately 40 cm distal to the ligament of Treitz. Complete en-bloc tumor excision with segmental jejunal resection required conversion to open surgery. Histology showed bland spindle cells in alternating fibrous and myxoid stroma with low mitotic activity. The tumor was MUC4-negative, showed absent RB expression and CDK4/p16 positivity, and was essentially negative for MDM2; weak nuclear MDM2 staining was observed predominantly in vascular endothelial cells, concordant with the absence of MDM2 amplification by fluorescence