Bone Sarcoma With EWSR1-NFATC2 Fusion: Sarcoma With Varied Morphology and Amplification of Fusion Gene Distinct From Ewing Sarcoma.

Ewing sarcomas are typified by EWSR1 fusion to ETS gene family members. Tumors with fusion partners other than ETS family members and atypical histologic features pose significant diagnostic challenges and controversies as to their classification. In this article, we report a tumor with EWSR1-NFATC2 fusion in the left femur of a 43-year-old man and with unusual morphologic features that resemble undifferentiated high-grade sarcoma. Analysis together with reported cases in the literature shows that tumors with EWSR1-NFATC2 exhibit distinctive clinicopathologic features, including predilection for young male adults, highly variable histology that varies from round cell tumors frequently associated with nuclear irregularity, short spindle cells with nuclear pleomorphism, to myoepithelial tumor-like with or without myxohyaline matrix. They show variable positivity to CD99, frequent expression of cytokeratins, and consistent high-level amplification of EWSR1-NFATC2 fusion gene with distinctive gene expression profile. These tumors thus deserve classification separate from Ewing sarcoma.