Primary Pediatric Cardiac Small Round Cell Sarcoma Consistent with Ewing Sarcoma: A Rare Case Report

Primary malignant cardiac neoplasms are very uncommon, with sarcomas being the most common. Primary tumors of the Ewing sarcoma family of the heart are exceptionally uncommon in children and carry a poor prognosis. Here, we report a case of a 10-year-old girl who presented with breathlessness, chest pain, and generalized swelling. Imaging revealed a large right atrial mass prolapsing into the right ventricle with pleuro-pericardial effusions. Positron emission tomography-computed tomography showed heterogeneous uptake confined to the cardiac lesion and did not reveal an extracardiac uptake. The excised mass showed sheets of small, round, blue cells with rosettes, brisk mitoses, necrosis, and myocardial invasion. Tumor cells demonstrated positivity for a diffuse membranous cluster of differentiation (CD99), nuclear Friend Leukemia Integration 1 fusion, and strong diffuse nuclear NKX2.2, whereas CD45, desmin, and myogenin showed negativity. In the absence of molecular confirmation, the tumor is best classified as a primary right atrial small round cell sarcoma, favoring Ewing sarcoma based on the morphology and immunohistochemistry. This pediatric presentation is exceedingly rare. Management relies on surgical resection with adjuvant chemotherapy due to the lack of standardized guidelines. Given its aggressive behavior and high recurrence risk, long-term follow-up is essential, and greater awareness is needed to guide future management strategies.