Picture of the Month—Diagnosis FREE

The initial radiographs indicated a calcified mass in the abdomen. Both ultrasonography (Figure, B) and abdominal CT scan (Figure, C) were performed, which confirmed the presence of the mass. The differential diagnosis of an abdominal mass in an infant is broad and includes both benign and malignant etiologies. Benign lesions can include teratomas, omental or mesenteric cysts, fecalomas, and gastrointestinal duplications. Abdominal malignancies most commonly found in this age group are Wilms tumor, neuroblastoma, hepatoblastoma, and rhabdomyosarcoma.¹ Because of calcifications in the mass noted on CT scan, teratoma was felt to be the most likely diagnosis. On closer review, distinct body parts, including a spinal column, skull, ribs, and tibia/fibula, were identified (Figure, D), leading to the diagnosis of fetus in fetu. The patient was discharged home and returned for resection 1 week later. Pathologic evaluation confirmed the diagnosis of mature teratoma consistent with fetus in fetu. The patient will have serial α-fetoprotein values measured every 3 months. Although the initial α-fetoprotein value was normal in this patient, malignant transformation of a recurrent teratoma remains a possibility and α-fetoprotein value is a useful screening test for this entity.

Fetus in fetu is a rare condition estimated to occur in 1:500 000 deliveries.² There have been approximately 100 prior case reports on this condition. The condition is associated with abnormal embryogenesis in which a monozygotic twin fetus is incorporated into the body of another normally developing fetus.³ This uncommon malformation is distinguished from a teratoma by the presence of a vertebral column and limbs.⁴ However, the vertebral column can be radiolucent so is not always detected by diagnostic imaging.

A review of 87 published cases of fetus in fetu found that 87% of cases were in children younger than 18 months, 80% of cases were retroperitoneal, and 70% of cases presented with complaints of an abdominal mass.³ Diagnosis is typically made by a combination of imaging modalities including abdominal radiography, ultrasonography, and CT scan, all of which were used in this case.

Although fetus in fetu is a rare diagnosis, it represents an important lesson in maintaining a broad differential diagnosis in an infant with feeding intolerance, which includes obstruction as seen in this case. The differential diagnosis for such an obstruction could include mass, pyloric stenosis, obstructing foreign body or bezoar, malrotation with or without volvulus, intestinal atresias, or gastrointestinal motility defect. Careful physical examination and selected imaging studies should aid the clinician in distinguishing between these diagnoses.

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