Fetus in fetu occurs most commonly in the upper retroperitoneum, while teratoma usually occurs in the lower abdomen, ovaries, or sacrococcygeal region . In this case our patient's mass was in the retroperitoneum.
The fetus was always anencephalic. The vertebral column and the limbs were present in the fetus in fetu in almost all cases [91% and 82.5%, respectively]. There is controversy as to whether fetus in fetu is a distinct entity or represents a highly organized teratoma. A teratoma is defined as a neoplasm with a slight potential for malignancy that is composed of multiple tissues foreign to the part in which they are located . It often is difficult to make a distinction between teratomas and vestigial remnants that result from abortive attempts at twinning. As a result, some authors think that fetus in fetu is within the spectrum of abnormalities that can result from the anomalous embryogenesis in a diamniotic monochorionic pregnancy. The spectrum includes conjoined symmetric twins; parasitic fetuses; embryonic vestigial fetal inclusions; and organized teratoma, Thus, some authors claim that fetus in fetu is only a well-differentiated highly organized teratoma .
However, many other investigators suggest that fetus in fetu is a pathologic entity that is distinct from teratoma for several reasons ; Malignant degeneration associated with fetus in fetu is extremely rare, with only one reported case to our knowledge .
A final important feature that has been used to distinguish between fetus in fetu and teratoma is the presence of a vertebral column. Willis  emphasized that the identification of a vertebral column secures the diagnosis of fetus in fetu and differentiates this entity from teratoma. Identification of the vertebral column indicates that fetal development of the included twin must have advanced at least to the primitive streak stage to develop a notochord, which is the precursor of the vertebral column . Occasional cases have been reported in which the spinal column could not be identified at imaging . These cases probably were due to an underdeveloped and markedly dysplastic spinal column that prevented the identification of vertebral bodies at imaging . The intraabdominal fetus in fetu is usually contained in a complete sac, without any major vascular connections to the host, which was the situation in our reported case.
In case reports in the medical literature up to 1980 preoperative diagnosis of fetus in fetu was made in only 16.7% of cases because CT scan was not performed. Since that time CT scan has proven very helpful in suggesting the preoperative diagnosis . The differential radiological diagnoses are teratoma and meconium pseudocyst. Indeed, these masses often have calcified components, so they were sometimes difficult to differentiate with fetus in fetu .
In our case imaging did not play an important role in our ability to make a preoperative diagnosis.