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 in situ hybridization. Fusion testing was unavailable, so the diagnosis remains morphologically favored but molecularly unconfirmed. At 6 months, abdominal and chest computed tomography showed no local recurrence or metastatic disease. At 12 months, non-contrast abdominal computed tomography showed no significant abnormality or abdominal recurrence; chest imaging was not performed. The narrow peripheral mesenteric pedicle provides a directly documented anatomic explanation for the marked positional variability in this case.