Pediatric low-grade fibromyxoid sarcoma mimicking ossifying fibromyxoid tumor: adding to the diagnostic spectrum of soft tissue tumors with a bony shell.

We describe a case of superficial low-grade fibromyxoid sarcoma (LGFMS) in a 12-year-old boy, confirmed by the detection of FUS-CREB3L2 fusion transcripts by reverse-transcription polymerase chain reaction and FUS rearrangement with fluorescence in situ hybridization, which had morphological features similar to ossifying fibromyxoid tumor (OFMT), including an almost complete rim of mature, metaplastic bone. LGFMS and OFMT can appear morphologically similar, with bland ovoid cells within a fibrous to myxoid matrix. Both can occur superficially; and whereas MUC4 immunoreactivity is characteristic of LGFMS, this can also be seen in some OFMTs. As the morphological spectrum of LGFMS is wide, we highlight the potential for diagnostic confusion with OFMT, which is clinically pertinent as most OFMTs behave in a benign manner whereas LGFMS is a malignant neoplasm with a propensity for local recurrence and a significant metastatic rate.