Fetus in fetu (FIF) is a very rare condition, with a reported incidence of one in 500,000 live births. It most likely represents a monozygotic diamniotic twin that implants itself and grows within the body of its normal karyotypically identical sibling, which typically manifests as a fetiform abdominal mass in a newborn or infant.
The mass is located in the retroperitoneum in most cases, including our example, and is commonly surrounded by encapsulated fluid. However, FIF has been reported to occur in other locations, such as within the cranium, the scrotum, and the oral cavity. Usually only one fetus is present but very rarely multiple fetuses may also be present. Five fetuses in the cranium of a 1-day-old female infant with hydrocephalus (although three of these fetuses were composed of extremities only) had been reported. The fetus itself is incomplete, containing a variable number of identifiable organs. The lung, the liver, the adrenal gland, the pancreas, and the genital organs may be seen in the more complex specimens. The presence of a head with eyes, hair, and teeth has been reported, but most of these fetuses are anencephalic. We are presenting a FIF mimicking a solid and cystic renal mass in a 6-month-old boy.