Fœtus in fœtu de siège rétropéritonéal

Fetus in Fetu (FIF) is an extremely rare congenital anomaly defined as a mass containing a vertebral axis often associated with other organs or limbs around this axis. We report the case of a female fetus aged 4 months presenting with retroperitoneal mass measuring 7x6x4cm, suggesting teratoma on computerized tomography (CT) scan. The mass was resected. Macroscopic examination showed fetiform mass covered by skin tissue and extending into upper and lower limb buds (A). Sectional views showed that it was centered by several osteocartilaginous fragments arranged in a linear fashion reminding the vertebral axis (B). Histological examination objectified the presence of glial tissue around a cerebral ventricle as well as skin, muscle and bone tissue. The diagnosis of FIF was retained. FIF is mainly reported at the level of the retroperitoneum followed by the sacro-caudal, intra-abdominal, cranial, buccal, mediastinal, pulmonary, renal and scrotal area. Diagnosis is made during the antenatal period in 15% of cases. The etiopathogenesis of FIF includes both the theory of monochorionic diamniotic monozygotic pregnancy in which an aberrant asymmetric twin becomes internalized in the other twin and the theory of a defective embryo implantation in the mesenchyme of its twin instead of the uterine wall. The differential diagnosis includes teratoma, meconium pseudocyst and ectopic pregnancy.