Mediastinal fetus in fetu. Case report and embryological discussion

Fetus in fetu is an extremely rare condition defined by the presence of both axial skeleton and organoid formations in the parasitic mass. Besides some authors assuming that fetus in fetu and highly-differentiated teratomas are part of the same pathogenic spectrum, it has been emphasized that these criteria indicate a rather different embryogenetic mechanism. We herein report the first case to our knowledge developed in the ventral mediastinum that was thought to be a tumor before birth. The literature is reviewed and we propose a new explanation for the development of this rare anomaly, based on a defective implantation.