Low-grade fibromyxoid sarcoma: Clinical, morphologic and genetic features.

Low-grade fibromyxoid sarcoma (LGFMS) is a bland spindle cell neoplasm that typically arises in the deep soft tissues of the proximal extremities or trunk of young adults. The majority of LGFMS are characterized by a recurrent (7;16)(q34;p11) translocation, resulting in the FUS-CREB3L2 fusion gene, which generates a chimeric protein with transcriptional regulatory activity. Small numbers harbor a FUS-CREB3L1 fusion resulting from t(11;16)(p11;p11), whilst rare cases harbor the EWSR1-CREB3L1 fusion. LGFMS is of low to moderate cellularity and consists of bland spindle cells with small, angulated nuclei and scant, wispy cytoplasm, arranged in a whorled growth pattern and typically showing abrupt transition from myxoid to fibrous areas. Immunohistochemical expression of MUC4 is a consistent finding. Hyalinized spindle cell tumor with giant rosettes (HSCTGR) is a morphological variant of LGFMS that shares the same balanced translocation, and is also immunoreactive for MUC4. A potential relationship between LGFMS and sclerosing epithelioid fibrosarcoma (SEF), a rare fibroblastic neoplasm that most commonly arises in the deep soft tissues of the lower extremities, limb girdles or trunk, has also been suggested. SEF is classically composed of nests and cords of epithelioid cells with clear or eosinophilic cytoplasm embedded within densely sclerotic stroma. In some cases, areas indistinguishable from LGFMS are present, and these have been shown to contain FUS-CREB3L2 fusion transcripts. The majority of pure SEF tumors harbor EWSR1 rearrangements, with EWSR1-CREB3L1 and more rarely EWSR1-CREB3L2 gene fusions more common than those involving FUS. MUC4 immunoreactivity is also seen in approximately 70% of SEF. Surgical resection of these tumors with clear margins is the treatment of choice. Correct diagnosis is important because of the significant potential for recurrence and late metastatic spread. We review LGFMS and SEF, discussing morphology and immunohistochemistry, genetics and molecular findings, and the differential diagnosis.