International Journal of Health Sciences and Research www.ijhsr.org ISSN: 2249-9571 Review Article Fetus in Fetu: A Review of Rare Malformation with Imaging Findings Manik Mahajan1, Poonam Sharma2, Puneet Gupta3 1Department of Radio-diagnosis and Imaging, GMC Hospital, Jammu, J&K, India. 2Department of Pathology, GMC Hospital, Jammu, J&K, India 3Department of Radiology, Gen-Next Diagnostic Centre, Jammu, J&K, India Corresponding Author: Manik Mahajan ABSTRACT Fetus in fetu is an extremely rare (1/500,000 live births) congenital anomaly and is defined as a malformed fetus which grows within the body of its twin. It is often overlooked in the differential diagnosis of an abdominal mass in infants or children. Unlike teratomas, fetus in fetu is a benign condition and has no malignant potential. Fetus in fetu is a rare condition with less than 200 cases reported in the world to the best of our knowledge. Although rare, fetus in fetu should be considered in differential diagnosis of paediatric and infantile abdominal masses and in presence of typical Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) findings, appropriate diagnosis can be made. The present article is an effort to review the epidemiology, etiopathogenesis and imaging findings in fetus in fetu. Keywords: Computed Tomography; Fetus; Congenital; Malformed; Mass INTRODUCTION Majority of cases consist of Fetus in fetu is a rare congenital pedunculated vertebrate masses within a anomaly and typically presents in infancy capsule containing fluid and with an and early childhood. [1] It is a rare cause of umbilical cord composed of only 2 vessels. abdominal mass in children. The term Fetus [6] Majority of them are seen in the in fetu was first coined by Meckel and retroperitoneal space. [6] Rare locations defined by Willis in 1935 as a fetiform mass include skull, liver, pelvis, scrotum, sacro- in which organ-like tissues are arranged coccygeal region, mouth, adrenal gland, around a vertebral axis. [2,3] Fetus in fetu mesentery and right iliac fossa. [6,7] arises from an inclusion of a monozygotic Normally a fetus-in-fetu is single in number, diamniotic twin within the bearer. Presence but a few cases of more than one have also of vertebral axis and appropriate been reported. [7] Fetus in Fetu represents a arrangement of other organs or limbs with diamniotic, monochorionic, monozygotic respect to the vertebral axis are the twin that becomes incorporated into the diagnostic features of fetus in fetu and help body of the host twin after anastomosis of in differentiating this entity from the highly the vitelline circulation. [8] differentiated teratoma. [4] Majority of cases Pathogenesis: is detected before 18 months of age and Fetus-in-fetu is a rare entity and present as an asymptomatic slow growing results from abnormal embryogenesis in abdominal mass. [5] diamniotic monochorionic twins. It is a malformed or parasitic monozygotic International Journal of Health Sciences & Research (www.ijhsr.org) 388 Vol.8; Issue: 5; May 2018 Manik Mahajan et al. Fetus in Fetu: A Review of Rare Malformation with Imaging Findings diamniotic twin inside the body of a living heterogeneous complex solid cystic mass child or sometimes in an adult. [9] with multiple dense calcifications and The pathogenesis is no exactly presence of fatty areas. Displacement of known but is believed to be due to inclusion adjacent structures by the mass is also seen. of the diamniotic monochorionic twin On both X-rays and USG, findings are non within its host during the ventral folding of specific and differentiation from teratoma is trilaminar embryonic cyst. [10] The inclusion not possible. in the sister embryo is thought to be due to persistent anastomosis of the vitelline circulation during development. [11] It is thought to result from unequal division of the totipotent inner cell mass of the developing blastocyst, causing a small cell mass within a maturing sister embryo. This ultimately results in a vestigial remnant, or fetus in fetu. [12] Superior mesenteric artery develops from vitelline circulation and thereby explaining the retroperitoneal location of the lesion [13] and absence of independent blood supply may account for fetal growth restriction in all cases. Figure 1: Axial CT image in a pathologically proven case of Fetus Clinical Presentation: in Fetu showing presence of large right retroperitoneal mass with Fetus in fetu is mostly detected presence of fatty areas and cystic component. Small associated before 18 months of age and present as an calcific density also seen at the medial aspect asymptomatic slow-growing abdominal mass with abdominal distension. [6] However age of presentation varies from ante natal period upto as old as 47 years. [14] When extremely large, patient may present with dull aching abdominal pain. Vomiting and other symptoms due to pressure effect have also been documented. Imaging Findings: Increasing use of obstetric ultrasound has made prenatal diagnosis of [15] fetus in fetu more common. In the Figure 2: Axial and Coronal CT images in another pathologically antenatal period, the diagnosis may be proven case of Fetu in Fetu showing presence of large left retroperitoneal mass with presence of large cystic and fatty mass suggested when a defined encapsulated causing displacement of adjacent bowel loops. Multiple irregular cystic mass with solid and calcified vertebral bodies, rib like structures and long bones also seen in the components is seen. The mass often mass. increases in volume throughout the CT findings are highly diagnostic for gestation period and the calcified fetus in fetu. CT reveals a large well defined components take on the appearance of fetal mass, commonly in the retroperitoneal skeletal bones. [16] region (Figure 1) with presence of cystic Post natal imaging modalities abd fatty components. Stomach and adjacent include X-rays, Ultrasonography (USG), CT bowel loops are commonly displaced due to and MRI. X-rays reveal an amorphous soft significantly large size of the mass. Few or tissue density mass with numerous multiple bony structures can be seen in the calcifications resembling bony structures. mass, predominantly irregular vertebral On USG, fetus in fetu appears as a large bodies, rib like bones and a well formed International Journal of Health Sciences & Research (www.ijhsr.org) 389 Vol.8; Issue: 5; May 2018 Manik Mahajan et al. Fetus in Fetu: A Review of Rare Malformation with Imaging Findings femur like long bones (Figure 2). Facial and and well formed long bones suggesting calvarial bones are uncommonly seen. Fat remnants of a second fetus. [20] Limb buds plane between the mass and surrounding can be seen with primitive phalanges and an structures were well maintained. The orbital socket. [21] MRI has emerged as an presence of bones is a significant finding excellent alternate safe technique in which differentiates the lesion from evaluation of the pathology and can be used teratoma. However, non visualization of in antenatal period also as there is no known vertebral column on CT does not exclude adverse biological effect on the fetus in the diagnosis as stated by Hoeffel et al [17] utero. (Figure 3). Spinal axis in fetus in fetu may be markedly dysplastic and underdeveloped Pathology: and so may not be visualised on CT scan. So Grossly, the fetus in fetu is pathological examination of the specimen is surrounded by thin fibrous sac containing always recommended to establish the final straw colored fluid along with an diagnosis. anencephalic fetus (100%) with limb buds (83%) and vertebral column. [20] Pathological diagnosis is confirmed by the presence of vertebral column. Another diagnostic criterion includes presence of a membrane covering fetal vasculature connections. [6] Rounded or tubular collection of very low density fat surrounding amorphous soft tissue is characteristic. [20] Other structures include skin, skin extensions, bone, bone marrow, venous structures, skeletal muscles and peripheral nerves. Uncommonly teeth, tongue, salivary glands, lymphatics, trachea, thyroid, parathyroid, lower respiratory tract, Figure 3: Axial and Coronal CT images in a proven case of Fetus in Fetu showing presence of a large right retroperitoneal mass with pancreas, spleen, kidney, gonads, testis, fatty and cystic areas. Small irregular long bone like structure and ovaries, urinary bladder, digits and nails are few calcific foci are also seen. Despite lack of spinal axis, [14,21,22] pathological diagnosis of Fetus in Fetu was made. present in FIF. By virtue of Karyotyping, serological markers and Few case reports have advocated the restriction, fetus in fetu represents a use of MRI in diagnosis of Fetus in Fetu. [17- diamniotic monochorionic twin of the host, 19] MRI has many advantages like high thus confirming a separate etiology as [6] contrast resolution, multiplanar capability compared to teratoma. No placenta or and choice of various pulse sequences, thus chorionic villi are found. making the diagnosis accurately. MRI has distinct advantage over CT in evaluation of Management: Fetus in Fetu: (1) lack of ionizing radiation, Treatment of Fetus in Fetu involves (2) it does not depend on detection of tissue complete resection of the lesion along with calcification, this helps in identification of the surrounding sac as it may cause insufficiently calcified vertebral column, [17] symptoms because of mass effect over intra [20] (3) absence of bony artifacts, (4) no need of abdominal organs. Associated risks iodine based contrast agents. MRI may include chances of infection, bleeding and reveal a large fetiform retroperitoneal mass pleura-peritoneal inflammation due to leak [20] with mixed high, intermediate and low of sac contents. Though benign, single intensities and bony structures in the form case report of fetus in fetu with malignant of crumpled vertebral bodies, rib like bones degeneration has been described in literature International Journal of Health Sciences & Research (www.ijhsr.org) 390 Vol.8; Issue: 5; May 2018 Manik Mahajan et al. Fetus in Fetu: A Review of Rare Malformation with Imaging Findings in which recurrence occurred after 4 month 5.