Intracranial Fetus In Fetu—a Pediatric Rarity

Background: Intracranial fetus in fetu is an extremely rare entity in which a discrete vertebrate fetiform mass is found inside a diamniotic, monochorionic twin. It is a benign mass and can manifest with symptoms owing to mass effect. To establish the diagnosis, a vertebra must be present within the mass. Case Description: A 5-year-old child presented at a multispecialty hospital with gradual weakness of both lower limbs. Magnetic resonance imaging of the brain revealed a midline intraventricular mass with lobulated margins having both cystic and fatty components with areas of blooming within. A provisional diagnosis of teratoma/primitive neuroectodermal tumor was made. The patient subsequently presented to our hospital with drowsiness and vomiting for 1 day. Noncontrast computed tomography revealed a mass of heterogeneous density occupying the third ventricle. The mass contained a few well-formed long bones representative of the appendicular skeleton and a vertebra-like bone representative of the axial skeleton, fulfilling the Willis criteria. A biopsy sample was taken from the mass; no malignant cells were seen on histopathologic examination. Based on noncontrast computed tomography findings of well-formed long bones and a vertebra and no significant increase in the size of the mass over 2 years, an intracranial fetus in fetu was diagnosed. Conclusions: Whenever bony structures are identified in an intracranial mass in a pediatric patient, we should always look for bones of the axial skeleton, as this finding will point toward a diagnosis of intracranial fetus in fetu and will help in differentiating it from teratoma, which can have malignant transformation.