Abstract
Fetus in fetu (FIF) is a rare congenital anomaly resulting from abnormal embyogenesis in monochorionic diamniotic twins and appears as a cystic mass containing fetus-like structures mainly in the retroperitoneum of infants. Although there is a theory that FIF is a highly differentiated teratoma, it is commonly distinguished from teratoma as a mass containing a vertebral axis with appropriate arrangement of limbs or other organs around this axis. Here we present a case of FIF with aorta-like structure visualized by contrast-enhanced computed tomography. A 5-day-old girl was pointed out a cystic mass in the abdomen by ultrasound examination. Abdominal contrast-enhanced computed tomography revealed a retroperitoneal cystic mass with spine- and limb-like bone structures and blood vessel-like elongated structures and it was confirmed as FIF by surgery. The presence of major vascular structures along the skeletal axis is clearly different from teratoma and suggests that it occurred as an embryo and underwent some stage of development. Our findings strongly support the monozygotic twin theory.