An unusual cystic presentation of pelvic skeletal Ewing sarcoma: a case series.

Ewing sarcoma (ES) is the second most common primary malignant bone tumour in children and adolescents. About 14.5% of primary malignancies develop in pelvic bones, where they typically have worse prognoses than extremity or acral sarcomas. It usually presents with aggressive features on radiology scans, but may also present with different radiological characteristics. In this series, we describe rare appearances of pelvic skeletal Ewing sarcoma, with large extraosseous cystic component on imaging, defined by the presence of fluid-filled spaces in the extraosseous tumour lesion, which distinguishes it from the solid nature of conventional ES. We report 3 cases of cystic presentation of ES, with imaging features supporting diagnosis of a primary malignant bone tumour arising from the superior pubic ramus with associated massive intrapelvic solid and cystic mass. CT-guided biopsy provided diagnosis of ES, with large intrapelvic soft tissue and cystic component. These patients underwent neo-adjuvant chemotherapy and proton beam therapy with significant reduction in size of the solid components, while the cystic components remained relatively unchanged. Two patients underwent surgical resection of the tumour (navigated P3 internal hemipelvectomy and hemipelvis P2/P3 resection, respectively), and one patient died while on treatment. In both who underwent surgery, histology showed ES with margins clear and more than 99% of treatment-induced necrosis. To the authors' knowledge, this unusual presentation of pelvic ES is described for the first time in the literature as a case series, with particular reference to atypical extraosseous cystic changes, along with the clinical and radiological characteristics, and their treatment.