Low-grade fibromyxoid sarcoma mimicking solitary fibrous tumor: a report of two cases.

Low-grade fibromyxoid sarcoma (LGFMS) is an uncommon fibroblastic neoplasm with many morphologic mimics. Solitary fibrous tumor is a more common fibroblastic neoplasm, but the two rarely enter the same differential diagnosis. However, here, we report two unusual cases of LGFMS, containing dilated, hemangiopericytoma-like blood vessels, which prompted diagnostic considerations of solitary fibrous tumor. Both cases presented were confirmed to harbor FUS gene rearrangement, thereby confirming a diagnosis of LGFMS. One case is that of an 18-year-old male with a left forearm mass, and the other a 50-year-old man with a left popliteal mass. While both cases show some histologic features of LGFMS, the non-classical, dilated blood vessel pattern seen here may serve as a diagnostic pitfall, as LGFMS normally exhibits fine, curvilinear blood vessels. To our knowledge, there is only one other report of LGFMS displaying such hemangiopericytoma-like blood vessels. In summary, when encountering a bland spindle cell neoplasm with classic hemangiopericytoma-like blood vessels, it is prudent to consider a diagnosis of LGFMS besides solitary fibrous tumor--particularly in the absence of CD34 immunoreactivity as it may be a rare, mimicking variant of LGFMS.