With reported incidence of about 1 in 500,000 live births, fetus in fetu (FIF) stands as one of the extremely rare recorded anomalies. There is increased incidence in families with previous history of twin pregnancies. They are usually single and genetically identical to their host . It is controversial whether to consider FIF as a highly differentiated teratoma or a distinctive pathology.
FIF is differentiated from a highly organized teratoma (with potential malignancy) prenatally by the presence of the spatially organized organs (anterior-posterior, cranio-caudal, and lateral/symmetrical development) depending on the vertebral bodies, which presence indicate that notochord stage of fetal development has been reached. Willis considered the presence of vertebral column as a mandatory differentiating point between FIF and fetiform teratoma . Nevertheless, spatially organized and well-recognized organs such as limbs may favor the same fetal developmental stage.
Moreover, each condition has a different genetic pattern supporting the hypothesis of distinct etiopathogenesis: FIF is genetically identical to its host; on the contrary, fetiform teratomas are homozygous . Based on that, having a vertebral column will differentiate FIF from teratomas, yet it fails to do so with other spectrum of monozygotic twin pregnancies, such as conjoined parasitic twin, detached external twin, and embryonic vestigial inclusion.
FIF is derived from totipotent inner cell mass (which can give rise to both embryonic or extraembryonic cells), while teratomas arise from pluripotent cells without organogenesis or vertebral segmentation, both containing different tissues from one or more germinal cell layers . Spencer sets criteria for a mass to earn the definition of FIF; one or more of the following characteristics must be achieved: having a separate sac, normal skin coverage either partially or completely, the presence of grossly recognizable anatomical parts, one or more vascular connections with its host .
The role of tumor markers is confined to the differentiation between FIF and other causes of intra-abdominal calcification, including teratoma, neuroblastoma, adrenal hemorrhage, meconium pseudocyst, and viral infection. The most used markers are β-human choriogonadotropin (hCG), AFP, and urine homovanillic acid .
Despite being a benign condition, and complete surgical excision is curative, the potential of malignancy should be kept in mind. Till now, there is only one case reported in literature with malignant transformation . Follow-up with imaging and tumor markers is advised in cases showing immature components; teratomas require a longer and a closer follow-up .
Owing to mass effect, FIF may exhibit its symptoms by compressing the surrounding structures leading to abdominal distension, feeding difficulties, vomiting, jaundice, renal function abnormalities, and respiratory distress . It is worth mentioning that few numbers of cases were diagnosed antenatally by ultrasound; moreover, the sonographic features of FIF differ considerably .
In our first case, the main presentation was a palpable abdominal mass and anemia which could be nutritional because of feeding difficulties from the mass effect. In the histopathological examination, all the findings were in keeping with the diagnosis of fetus in fetu rather than retroperitoneal teratoma such as the presence of a sac, partial skin coverage, presence of hand, and there was no evidence of immature cells. Moreover, tumor markers were within the normal range for age.
In the second case, the diagnosis was made based on the antenatal ultrasound findings and the CT scan after birth, the mass was enclosed in a sac, and there was a single vascular connection with the host. Fetal parts were present, and histopathological examination showed no immature elements. Both cases in our report are likely to be FIF rather than a mature teratoma.