We report a case of a 49-year-old female with desmoplastic small round cell tumor of the uterus (DSRCT). Histologically, in some areas the tumor showed typical features with ample desmoplastic stroma, while in other areas the tumor cells diffusely infiltrated myometrium with only focal desmoplastic reaction. Immunohistochemically, the tumor cells showed diffuse positivity for desmin, CD56, CD57, EMA and cyclin D1. Focal positivity was present for antibodies against cytokeratin AE1/3, BerEP4, NSE, IFITM1 and CD10. The WT-1 antibody (against the N-terminus) showed cytoplasmic positivity in some tumor cells, while the nuclei were negative. P53 expression was wild-type. The Ki-67 index (MIB1 antibody) was about 55%. Other markers examined including transgelin, myogenin, synaptophysin, chromogranin, h-caldesmon, PAX8, and CD117 were all negative. NGS analysis revealed a fusion transcript of the EWSR1 and WT1 genes. DSRCT of the uterus is a rare neoplasm, as only two cases have been reported so far. However, only one of these cases was examined molecularly with a confirmation of the characteristic EWSR1-WT1 fusion. We report a second case of molecularly confirmed DSRCT of the uterus and discuss its clinical features, differential diagnosis and the significance of molecular testing.

Department of Obstetrics and Gynecology 3rd Faculty of Medicine University Hospital Kralovske Vinohrady Charles University 10034 Prague Czech Republic

Department of Pathology 1st Faculty of Medicine General University Hospital Prague Charles University 12808 Prague Czech Republic

Department of Pathology 3rd Faculty of Medicine University Hospital Kralovske Vinohrady Charles University 10034 Prague Czech Republic

Department of Pathology and Molecular Medicine 3rd Faculty of Medicine Thomayer Hospital Charles University 14059 Prague Czech Republic

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