Late Presentation of Fetus in Fetu in a 10‐Year‐Old Boy

Fetus in fetu (FIF) is a rare congenital anomaly in which a dysmorphic, parasitic twin is retained within the body of its host twin. It typically presents in infancy, shows a slight male predominance, and is most often located in the retroperitoneum. Differentiating FIF from an abdominal teratoma on imaging is important, as the two entities can closely resemble one another. We report the case of a 10‐year‐old boy who presented with abdominal pain, low‐grade fever, and unintentional weight loss. Antenatal history was unremarkable. Abdominal examination revealed mild distension and left‐sided tenderness. Ultrasonography demonstrated an 8.5 × 10.4 × 11.1 cm mass‐like lesion in the left hypochondrium, and a provisional diagnosis of teratoma versus FIF was considered. Contrast‐enhanced computed tomography (CT) of the abdomen showed a large, well‐defined, heterogeneous mesenteric mass measuring 10.0 × 11.0 × 14.0 cm containing macroscopic fat, dense tooth–like calcifications, and osseous elements with attenuation of up to 1450 Hounsfield units, together with a well‐formed fetal skeleton. Serum tumor markers were within normal limits. A presumptive diagnosis of FIF, supported by imaging and by intraoperative findings that satisfied the Willis criteria, was made at exploratory laparotomy; histopathological confirmation could not be obtained because the specimen was inadequately fixed prior to processing, so a highly differentiated fetiform teratoma cannot be entirely excluded. The child remained well at last follow‐up, about 10 weeks after discharge; longer surveillance is required. This case illustrates that FIF, although classically a diagnosis of infancy, should remain in the differential diagnosis of a complex abdominal mass in an older child, and highlights the role of the Willis criteria, radiological correlation, and intraoperative findings when histopathology is unavailable.