A case report of rare primary extraosseous Ewing sarcoma of the bladder is presented, with confirmation by pathological, immunohistochemical, and molecular analyses

Abstract

Extraosseous Ewing sarcoma (EES) of the bladder is an extremely rare, highly aggressive small round cell malignancy. Because its clinical manifestations and imaging features are nonspecific and its histological appearance closely mimics that of other small round cell tumors, misdiagnosis is common both before surgery and during pathological evaluation. This study reports a case of EES of the bladder confirmed by histomorphology, immunohistochemistry (IHC), and molecular testing. The patient, a 43-year-old woman, presented with frequent urination, urinary urgency, and dysuria. An enhanced abdominal computed tomography (CT) scan revealed a nodular soft-tissue density lesion on the right posterior bladder wall. Histologically, tumor cells were arranged in dense sheets or lobules. IHC showed positive expression of CD99 and NKX2.2, while fluorescence in situ hybridization (FISH) revealed break-and-rearrangement of the EWSR1 gene. Based on clinical data, imaging, histomorphology, IHC, and FISH, the final pathological diagnosis was EES of the bladder. By integrating clinical data, imaging findings, histological features, IHC results, and FISH test results, this study aims to improve clinicians’ and pathologists’ understanding of this rare tumor, reduce misdiagnosis and inappropriate treatment, and offer a reference for precision diagnosis and therapy in clinical practice.