[Pre-natal retroperitoneal mass: teratoma or fetus in fetu?]

Several case reports in the literature brought to discussion the distinction criteria between fetus in fetu and teratoma.

The authors present a case report of a premature newborn boy, evaluated for a capsulated abdominal mass with prenatal diagnosis pointing to possible malignant lesion. A CT scan showed an expansive, retroperitoneal lesion, from kidneys to pelvis, sacrum and coccyx, with solid, cystic and fatty areas; the fine needle aspiration was inconclusive, and he underwent surgery on day 9 with partial resection of the lesion. A new procedure to complete the excision was performed 8 months later. The histopathologic examination of the two fragments showed mature and organized components of nervous system, respiratory and digestive apparatus (liver and pancreas included), thymus, nervous ganglia, skin and adnexa, musculoskeletal and brown adipose tissues; cartilage and several focus of calcification. These components showed a craniocaudal orientation, but no evidence of organoid formation.

This case presents features of both teratoma and fetus in fetu. The assumption of either one of the diagnosis has prognostic implications, with resulting differences in clinical follow up and expectations about the child's health.