Retroperitoneal Teratoma in Newborn Mimicking Neuroblastoma With Rapid Growth.

Adrenal-region masses during the perinatal period are uncommon and may include neuroblastoma and teratoma. These two entities often show overlapping features on initial imaging. Differentiation is challenging because teratomas may initially lack visible fat or coarse calcification, whereas congenital neuroblastoma may present with normal catecholamine levels and no abnormal 123I-MIBG uptake. In the present patient, a rapidly enlarging right suprarenal mass was identified during late gestation. The mass was initially interpreted as neuroblastoma based on its cystic-solid appearance and nonspecific biochemical findings. After birth, the mass continued to grow, and the initial postnatal evaluation was inconclusive, showing no abnormal MIBG uptake and normal catecholamine metabolites. However, serial imaging revealed time-dependent changes, including new calcification, emerging fatty components, and progressive enlargement, prompting reconsideration of the initial diagnosis. Complete resection confirmed an immature retroperitoneal teratoma without adrenal involvement. These observations illustrate that a retroperitoneal teratoma can closely mimic an adrenal neuroblastoma during the perinatal period, particularly during the fetal and early neonatal periods, when fat or calcification is not yet apparent. Time-dependent radiologic evolution is therefore a key diagnostic clue, and continuous fetal-to-neonatal imaging follow-up plays a crucial role in accurately distinguishing these tumors.