Contributed by
Michigan State University College of Human Medicine
Connor Latterman, and Joseph Junewick MD, FACR.
History
A 32-year-old pregnant female was referred at 26 weeks gestation for evaluation of a right upper quadrant fetal mass detected on routine prenatal ultrasound.
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Question
What is the most likely diagnosis?
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Correct answer
Fetus in fetu
Discussion
Fetus in fetu is a rare congenital anomaly characterized by an abnormal embryologic development of monozygotic, diamniotic twins, with an estimated incidence of 1 in 500,000 births.1 It most commonly presents as a retroperitoneal mass detected in infancy or early childhood, but may be detected prenatally on ultrasound or MRI.
A fetus in fetu will typically show as a complex cystic mass with soft tissue, resulting in pressure on its adjacent tissues. Two of the most important distinguishing factors from a teratoma are an identifiable fetal structure or an established vertebral axis which indicates a certain level of embryonic organization.2 Additionally, the karyotype of the tumor will be identical to the host.
In this case, a complex cystic mass with soft tissue and no identifiable organ of origin was identified prenatally on ultrasound and MRI studies. Interval growth as well as effect to surrounding tissues was seen on postnatal imaging. Surgical resection resulted in the finding of an encapsulated mass containing well organized fetal structures consistent with a fetus in fetu.
The awareness of a fetus in fetu diagnosis is of high importance due to the potential to confuse it with other types of suprarenal masses such as a neuroblastoma and/or teratoma leading to the proper surgical intervention and avoidance of unnecessary oncologic treatment.
Differential diagnosis
Fetal suprarenal masses can have many different possible causes. Some of these possible causes include neuroblastoma, teratoma, and organizing hematoma. Neuroblastoma is the most common kind of mass found in the fetal suprarenal area and has a solid or heterogeneous appearance that frequently has areas of calcification and internal vascularity.3 Neuroblastomas nearly always originate in the adrenal gland.
Teratomas may appear as mixed cystic and solid masses containing fat, calcification, and/or fluid; however, unlike fetus in fetu, teratomas do not have a well-defined vertebral axis or recognizable fetal structures.4
Organizing hematomas may appear echogenic on ultrasound but change to a more cystic appearance over time, and do not usually develop evidence of either internal vascularity or organized soft tissue structures.
In this case, the presence of a complex cystic mass with evidence of soft tissue components and organized evidence of fetal structures supported the diagnosis of fetus in fetu.
Additional images
References
- 1. Stevanny B, Mirani P, Thene ERF, Kesty C. Prenatal diagnosis of well-developed fetus in fetu with spontaneous movement in a resource-limited setting: A case report. Case Rep Womens Health. 2024;41:e00581. doi:10.1016/j.crwh.2024.e00581
- 2. Mazhoud I, Skhiri W, Hafsa C, et al. Fetus-in-fetu: mimicking teratoma on antenatal ultrasound. Case Rep Perinat Med. 2023;12(1):20220024. doi:10.1515/crpm-2022-0024
- 3. Chu CM, Rasalkar DD, Hu YJ, Cheng FWT, Li CK, Chu WCW. Clinical presentations and imaging findings of neuroblastoma beyond abdominal mass and a review of imaging algorithm. Br J Radiol. 2011;84(997):81-91. doi:10.1259/bjr/31861984
- 4. Hanquinet S, Damry N, Heimann P, Delaet MH, Perlmutter N. Association of a fetus in fetu and two teratomas: US and MRI. Pediatr Radiol. 1997;27(4):336-338. doi:10.1007/s002470050144