Fetus in fetu: A rare differential diagnosis for an antenatally identified ultrasonographic intra-abdominal mass

Abstract

Background: Fetus in fetu (FIF) is a rare congenital anomaly in which a parasitic fetus is trapped inside the body of its twin. The incidence is 1 in 500,000 births. Initially, considered as a mature teratoma, it was later identified as a separate entity due to the presence of an axial skeleton and organized limbs. Clinical Description: A 48-day-old girl presented with an antenatally detected calcified intra-abdominal mass. She had a palpable retroperitoneal mass measuring 6 cm × 4 cm at the left lumbar area. Sonology showed a heteroechoic cystic mass in the left lumbar region. Serum markers were normal except for a slightly elevated serum lactate dehydrogenase. Contrast-enhanced computerized tomography of the abdomen showed a well-defined cystic lesion measuring 7.6 cm × 6.5 cm × 5.9 cm with enhancing septae, multiple calcific foci, teeth, and bones in the left suprarenal region displacing the left kidney. Management: Laparotomy showed a cystic appearing mass with solid components within. Cystic structure was the amniotic covering and solid component turned out to be the FIF with a face, limbs, and umbilical cord. No adjacent infiltration was seen. Pathology confirmed the finding of FIF with an identifiable vertebral column, umbilical cord, pelvis and lower limb bones, skin, and retinal and brain tissue. The infant has been under follow-up for a year and is thriving. Conclusion: FIF is a rare condition of infancy which can be diagnosed preoperatively by radiological examination, treated by complete excision, and confirmed by gross and histopathology. Differentiation of FIF from teratoma is mandatory as the latter can be malignant.