Authors : Anju Shukla, Zoya Qamar
DOI : 10.18231/j.achr.2025.005
Volume : 10
Issue : 1
Year : 2025
Page No : 18-22
CD99 is a transmembrane protein, a product of the MIC2 gene with various functions and limited diagnostic utility, routinely used in laboratories for the differential diagnosis of small blue cell tumors. It is used in establishing the diagnosis of Ewing's sarcoma/PNET where it consistently shows strong membranous staining on immunohistochemistry. However, CD99 has high negative and low positive predictive values. Therefore, if CD99 is negative, the likelihood of Ewing's sarcoma/PNET is low. Conversely, if it is positive, it is not specific to the Ewing Sarcoma family of tumors, as this antigen's expression is seen in a wide variety of tumors. Thus for confirmation cytogenetic examination for reciprocal translocation t(11;22)(q24;q12) or t(21;22)(q22;q12) which results in the fusion of EWSR1 gene with FLI1 or ERG genes are essential. Here, I describe two cases where the diagnosis of Ewing's sarcoma was made based on immunohistochemistry. In both cases, the EWSR1-FLI1 fusion was found to be negative. Further molecular testing revealed synovial sarcoma in one case, while total resection of the tumor revealed the morphology of mesenchymal chondrosarcoma in the other case. These cases highlight that strong membranous immunostaining of CD99 with NKX2.2 positivity, which is highly specific for Ewing's sarcoma, is insufficient for diagnosis. Round cell sarcoma cases should be confirmed with molecular testing before starting chemotherapy. Keywords: CD99, Ewings sarcoma, NKX2.2