A Diagnostic Enigma in the Mediastinum: Extraskeletal Ewing Sarcoma

and lactate dehydrogenase (LDH) were normal.Fiberoptic bronchoscopy with endobronchial ultrasound (EBUS) was done.EBUS-transbronchial needle aspiration (TBNA) was taken from the large heterogeneous mass seen in the right paratracheal station.EBUS-TBNA cytology showed tight clusters of small-to medium-sized lymphocytes, and clot core biopsy showed few lymphohistiocytic cells with fibrinoid necrosis; GeneXpert Ultra for tuberculosis was negative.Due to inconclusive reports, a CT-guided biopsy was done from the mediastinal mass, which revealed small round cells with scanty clear cytoplasm suggestive of round cell tumor (Fig. 3).On IHC, tumor cells were positive for synaptophysin and CD99 and focal, weakly positive for NKX2.2, while being negative for CD3, CD20, PanCK, and FLI1, with a Ki67 of 25% (Fig. 4).A suspicion of ES was made.The EWSR1 gene fusion study was done by fluorescence in situ hybridization (FISH) technique, which was positive.Positron IntroductIonEwing sarcoma (ES) is an uncommon and aggressive tumor that typically originates in bone or soft tissue, with the long bones of the extremities are the most commonly involved sites. 1 It may rarely originate outside the bone tissue, with the mediastinal involvement being very rare.The tumor grows rapidly and is frequently associated with locoregional recurrence and distant metastasis.The diagnosis relies on histopathological examination along with immunohistochemistry (IHC) and molecular genetic analysis.