Abdominopelvic and retroperitoneal low-grade fibromyxoid sarcoma: A clinicopathologic study of 13 cases

Objectives: Low-grade fibromyxoid sarcoma (LGFMS) is a rare tumor often arising in the lower extremities. Only rare examples in the abdominal cavity, pelvis, and retroperitoneum have been reported. Methods: Thirteen cases of abdominopelvic and retroperitoneal LGFMS were retrieved. MUC4, Actin, ALK, β-catenin, desmin, DOG1, KIT, S100 protein, and STAT6 testing was performed, and a subset was tested for FUS rearrangement. Results: Sites included intra-abdominal/abdominal wall (four cases), retroperitoneum (three cases), pelvis (three cases), small bowel (two cases), and kidney (one case) (median size, 15 cm; age range, 5-61 years). Tumors harbored spindled cells with mild to moderate atypia, displaying alternating myxoid nodules and hyalinized areas. All cases were positive for MUC4, and five (of five) cases tested harbored FUS rearrangement. Variable positivity for DOG1 (four of 10) and actin (two of 10) was identified. Six tumors recurred, and one patient developed metastasis. Conclusion: LGFMS arising in these central locations exhibits similar clinicopathologic features to its counterpart in the extremities. LGFMS at these sites may show limited immunoreactivity for DOG1 and actin.