Objective: The purpose of this study was to describe the imaging and pathologic findings of 2 cases of complex neonatal abdominal masses and to discuss the probable common embryologic etiology of fetus in fetu (FIF) and fetaform teratomas.
Methods: Two male neonates had complex cystic abdominal masses, 1 of which was diagnosed prenatally, and both had abdominal sonography. One patient also had abdominal computed tomography. Both patients subsequently underwent surgical resection, with pathologic and genetic analysis of these masses.
Results: One patient had typical imaging, pathologic, and genetic findings of FIF. The second patient had a well-formed mass that was diagnosed pathologically as a teratoma yet had most of the criteria for FIF that have been set forth in recent embryologic literature.
Conclusions: Recent concepts regarding the origin of FIF suggest that it is part of a spectrum of monozygotic twinning gone awry, ranging from conjoined twins at one end to fetaform teratomas at the other. The imaging and pathologic features of these 2 cases serve to reinforce this concept.