The vertebral axis in retroperitoneal fetus in fetu: a case report on the definitive diagnosis.
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BACKGROUND: Fetus in fetu (FIF) is an exceptionally rare congenital anomaly (1 in 500 000 live births) involving a fetiform mass within a host twin. It is characterized by tissue organization around a vertebral axis rather than the mere presence of three germ layers. Distinguishing retroperitoneal FIF from a mature teratoma is critical, given the latter's malignant potential. CASE PRESENTATION: A 4-year-old boy presented with progressive abdominal distension. Imaging revealed a 150 × 100 × 136 mm retroperitoneal mass containing calcifications and axial structures, suggesting FIF. The patient underwent a successful en bloc complete excision via a transverse infraumbilical laparotomy without capsular breach. Histopathology confirmed the diagnosis by identifying vertebral bodies, limb-bud-like structures, and tissues from all three germ layers, with a complete absence of immature or malignant elements. The boy's recovery was uneventful, and follow-up confirmed normal alpha-fetoprotein (AFP) levels with no recurrence. CONCLUSIONS: Distinguishing FIF from mature teratomas relies on identifying a vertebral axis and organized internal structures. Complete surgical excision is the definitive treatment and is associated with an excellent prognosis. While standard clinical follow-up is sufficient for completely excised, mature FIFs, long-term AFP surveillance may be considered, particularly in cases exhibiting immature elements with malignant potential.