Intra-articular Extraskeletal EWSR1-Negative NR4A3-Positive Myxoid Chondrosarcoma: A Case Report.

Case: Extraskeletal myxoid chondrosarcomas (EMCs) are rare soft-tissue malignancies. Intra-articular occurrence is even more rare. To our knowledge, this case is one of only 2 reported intra-articular EMC cases of the knee free of local recurrence and/or amputation at follow-up. This case is also distinctive for being fluorescence in-situ hybridization-negative for the typical EMC-balanced translocation t(9;22) which fuses EWSR1 with NR4A3, harboring instead a variant translocation resulting in fusion of NR4A3 with a less common gene fusion partner.

Conclusion: This is a unique case of intra-articular EMC of the knee with a rare molecular fingerprint and an unusually positive outcome.