A case with sacrococcygeal primitive myxoid mesenchymal tumor of infancy: a case report and review of the literature.

A girl, aged 19 months, presented with a sacrococcygeal tumor that developed at 5 months after birth and gradually enlarged. Serum tumor marker levels were negative. Ultrasound imaging showed abundant blood flow in the tumor. However, neither computed tomography (CT) nor magnetic resonance imaging (MRI) showed contrast agent incorporation. The surgically resected tumor consisted of immature cells with myxoid stroma and proliferating small blood vessels. Immunostaining showed extensive vimentin expression. However, smooth muscle actin, muscle-specific actin, and S-100 protein expression was negative. Neither the ETV6-NTRK3 fusion gene nor the FUS gene rearrangement was detected. Thus, the patient was diagnosed with a primitive myxoid mesenchymal tumor of infancy. This tumor primarily consisted of a mucosal stroma with a low absorption on CT, a low signal on T1-weighted MRI, and a high signal on T2-weighted MRI. A diagnosis of primitive myxoid mesenchymal tumor of infancy should be considered in cases of soft tissue tumors in infants that show prominent vascularity but little contrast enhancement on MRI or CT.