Monophasic Synovial Sarcoma in the Elbow Misclassified but Successfully Treated as Ewing's Sarcoma with Chemotherapy.

To the best of the authors' knowledge, this is the first published case of monophasic synovial sarcoma (SS) initially diagnosed as Ewing's sarcoma (ES), yet successfully treated with chemotherapy in a 24-year-old patient. The initial diagnosis showed a monotonous round cell tumor and positivity for CD99, characteristic of ES; however, the cytology was negative for the classic EWSR1 rearrangement of ES. The patient was treated with the standard chemotherapy protocol of ES - COG AEWS1031 Regimen A with vincristine, doxorubicin, cyclophosphamide, and mesna - as well as with wide resection. Post-resection tissue submission showed additional morphologic features which led to a re-evaluation of the classification of the tumor as well as additional molecular studies; these revealed positivity for translocations of SS18 (18q11.1) in 100% of the nuclei, which is most characteristic of SS, thus, reclassifying the neoplasm as a SS tumor. This case underscores the importance of considering several pathologic entities in the differential diagnosis of small, round blue cell tumors, including ES, SS, and lymphoma. It also demonstrates the importance of using chromosomal identification for a more definitive diagnosis, rather than relying on histological features and markers which are found in more than one tumor classification. There is conflicting evidence of the impact of chemotherapy on survival in SS, as it is primarily treated with radiation therapy. Since SS is rare, prospective studies on the effect of chemotherapy on survival are limited in number. However, our case study demonstrates that chemotherapy is another modality that can be used in the treatment of SS neoplasms.