Primary undifferentiated small round cell sarcoma of the right ventricle with rapid recurrence and aggressive clinical presentation.

A middle childhood boy presented with fatigue, pallor and vomiting that had been persistent for half a month and significantly worsened over the past day. On clinical examination, he exhibited tachycardia with a heart rate of 126 bpm. Transthoracic echocardiography, chest CT and cardiac magnetic resonance were performed. A round, dense mass measuring 5.60 cm×5.17 cm was identified in the right ventricle and right ventricular outflow tract, without involving the right atrium, causing obstruction of both the right ventricular inflow and outflow tracts. Subsequently, the patient underwent tricuspid valve (TV) replacement and surgical resection of the mass. The patient was ultimately diagnosed with undifferentiated small round cell sarcoma. One month after the surgery, the tumour recurred, and severe stenosis of the artificial TV suggested a poor prognosis for the sarcoma.