In addition, the combination of strong CD99 membranous positivity and nuclear NKX2.2 positivity seems to be very reliable for ESFT diagnosis in an appropriate clinicoradiological setting.
Membranous CD99 and nuclear Fli-1 staining was seen in 10 and 16 tumors, respectively, and concurrent expression of both markers was seen in both central and Ewing sarcoma/peripheral PNETs.
CK20 and neurofilament staining was undertaken to investigate whether some of these neoplasms might exhibit a Merkel cell immunophenotype and CD99 staining to assess whether there is immunohistochemical overlap with neoplasms in the Ewing family of tumors (EFT).
Establishing histological diagnosis can be difficult; CD99 and FLI1 immunohistochemical staining has improved diagnosis, but these markers are not specific for EFTs.
Our results suggest that in ESFT, high CD99 expression levels contribute to the malignant properties of ESFT by promoting growth and migration of tumor cells and identify KCMF1 as a potential metastasis suppressor gene downregulated by high constitutive CD99 expression in ESFT.
Pathologic studies revealed an Ewing sarcoma/peripheral primitive neuroectodermal tumor (EWS/pPNET) with typical features including periodic acid Schiff positive diastase sensitive cytoplasmic substance; strong membranous pattern of immunoreactivity for CD99, and a reciprocal translocation of t(11;22)(q24;q12) that was demonstrated by fluorescent in situ hybridization (FISH).
CD99 expression is identified in nearly all ESFT and constitutes a useful positive marker when used as part of a panel of immunostains that can help rule out other differential diagnostic considerations.
By analogy to heterologous cellular systems, it is possible to postulate an important functional role for CD99(MIC2) as it contributes to the malignant phenotype of EFT.