Antenatal retroperitoneal mass suspected as teratoma: Postnatal diagnosis of fetus in fetu

INTRODUCTION

Fetus in fetu (FIF) is an exceptionally rare developmental anomaly, with an estimated incidence of approximately 1 in 500,000 live births.^[1] It is believed to arise from abnormal embryogenesis in a monochorionic diamniotic twin pregnancy, where unequal division of the inner cell mass results in incorporation of one twin into the body of the other.^[2] The parasitic twin typically demonstrates varying degrees of organogenesis and is most frequently located in the retroperitoneum, accounting for nearly 80% of reported cases.^[3] Differentiating FIF from teratoma is clinically significant because teratomas are true neoplasms with malignant potential, whereas FIF is generally benign.

CASE REPORT

A 28-year-old gravida 2, abortus 1 woman at 38 weeks and 6 days of gestation underwent routine antenatal ultrasonography. Imaging revealed a well-defined heteroechoic mass measuring approximately 3.9 × 3.7 × 4.3 cm within the fetal abdomen [Figure 1]. The lesion was located in the right infrahepatic retroperitoneal region, adjacent to the upper pole of the right kidney. Internal coarse calcifications were noted, and color Doppler showed no definite internal vascularity. The mass displaced the transverse colon inferiorly and the inferior vena cava laterally without evidence of compression.

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Figure 1: Antenatal ultrasound images demonstrating a fetal retroperitoneal mass. (a) Initial antenatal ultrasound showing a well-defined heteroechoic mass within the fetal abdomen. (b) Follow-up antenatal ultrasound demonstrating persistence of the lesion with minimal interval change. (c) Color Doppler image demonstrating the absence of significant internal vascularity within the mass and depicting its location between the aorta (AO) and inferior vena cava (IVC).

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Serial antenatal ultrasonography demonstrated a marginal increase in size with unchanged internal characteristics of the lesion [Figure 1]. No additional fetal anomalies were identified.

Fetal magnetic resonance imaging (MRI) was performed for further characterization. MRI demonstrated a well-circumscribed complex solid–cystic mass measuring approximately 3 × 3.7 × 3.6 cm in the central retroperitoneum, just inferior to the liver, with mild extension towards the right anterior to the right kidney. The lesion appeared predominantly T2 hyperintense with central solid components and internal signal voids corresponding to calcifications. Sagittal T2-weighted images showed the craniocaudal extent of the lesion without suprarenal, intraspinal, or intrathoracic extension. Coronal MRI images demonstrated the centrally located retroperitoneal mass producing mild displacement of adjacent vascular structures [Figure 2]. An antenatal diagnosis of retroperitoneal teratoma was favored.

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Figure 2: Fetal magnetic resonance imaging (MRI) demonstrating a retroperitoneal mass. (a) Axial MRI image showing a well-defined heterogeneous mass in the central retroperitoneum inferior to the liver the red star in indicates the retroperitoneal mass. (b) Coronal T2-weighted image demonstrating the location of the mass within the retroperitoneum and its relationship with adjacent abdominal structures. (c) Sagittal T2-weighted image depicting the craniocaudal extent of the lesion without suprarenal, intraspinal, or intrathoracic extension.

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The patient delivered a term live female infant weighing 2.67 kg through spontaneous vaginal delivery. The immediate neonatal period was uneventful.

Postnatal imaging findings

Postnatal contrast-enhanced computed tomography (CT) of the abdomen revealed a well-defined, non-enhancing retroperitoneal mass measuring approximately 4.5 × 3.6 × 4.7 cm, located anterior to the aorta and slightly to the right of the midline [Figure 3a-c]. The lesion contained multiple well-formed osseous structures, including tubular bones resembling long bones and bony elements consistent with vertebral bodies, along with peripheral areas of fat attenuation. The mass extended superiorly to the infrahepatic region, mildly indenting the liver without causing biliary obstruction. Adjacent bowel loops and pancreas were displaced anteriorly, while the right kidney was displaced posteriorly without hydronephrosis. No enhancing mural nodules or invasive features were observed.

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Figure 3: Postnatal imaging and surgical specimen confirming fetus in fetu. (a and b) Axial contrast-enhanced computed tomography (CT) images demonstrating a well-defined retroperitoneal mass containing multiple internal calcified osseous structures. (c) Coronal CT image depicting the extent of the retroperitoneal mass and internal skeletal elements. (d) Postnatal abdominal radiograph showing coarse calcifications in the right paravertebral region. (e) Postnatal ultrasound image demonstrating a complex echogenic retroperitoneal mass with internal calcific foci. (f) Radiograph of the excised specimen demonstrating organized skeletal elements. (g) Gross photograph of the excised fetiform mass.

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The presence of an organized vertebral axis and appendicular skeletal elements was diagnostic of FIF.

A postnatal abdominal radiograph demonstrated coarse calcifications in the right paravertebral region [Figure 3d]. Postnatal abdominal ultrasound showed a complex echogenic retroperitoneal mass with internal calcific foci [Figure 3e].

DISCUSSION

The primary diagnostic challenge in this case was differentiating FIF from retroperitoneal teratoma. Both entities may present as complex masses containing cystic components, fat, and calcifications on antenatal ultrasonography and fetal MRI.

Some authors have suggested that FIF and fetiform teratoma may represent entities along the same developmental spectrum. However, the presence of a well-organized axial skeleton and limb buds remains the most reliable distinguishing feature of FIF.^[4] In contrast, fetiform teratomas lack a recognizable vertebral axis and demonstrate less organized tissue differentiation.^[2,3]

Spencer proposed diagnostic criteria for FIF, suggesting that the parasitic twin should demonstrate one or more of the following characteristics: (1) Enclosure within a distinct sac; (2) partial or complete coverage by normal skin; (3) recognizable anatomic parts; (4) attachment to the host by relatively few large blood vessels; and (5) location near typical sites of conjoined twin attachment or association with the neural tube or gastrointestinal system.^[1] In contrast, teratomas are true neoplasms composed of disorganized tissues derived from multiple germ layers and typically show a broader attachment site with numerous small blood vessels.

Prenatal ultrasonography and fetal MRI are invaluable for early detection and anatomical assessment of fetal masses but may lack specificity in identifying organized skeletal structures, particularly when ossification is incomplete. Postnatal CT remains the most sensitive modality for demonstrating skeletal organization and for delineating the relationship of the mass to adjacent organs and vascular structures.^[4,5]

Complete surgical excision is considered curative in nearly all cases of FIF. Malignant transformation is exceedingly rare, although isolated cases of recurrence and malignant transformation have been reported.^[6] Therefore, periodic follow-up is recommended.

DIFFERENTIAL DIAGNOSIS

The primary differential diagnosis for a fetal/neonatal retroperitoneal mass with calcifications includes: Retroperitoneal teratoma – The most important differential, as it may show fat, cystic components, and calcifications but lacks an organized vertebral axis or limb-like structures.

Neuroblastoma–Common neonatal abdominal mass, typically arising from the adrenal gland, often showing calcifications but without organized skeletal elements. Meconium pseudocyst – May present as an intra-abdominal cystic mass with calcifications secondary to meconium peritonitis.

Enteric duplication cyst–Usually cystic and lacks calcifications or complex internal structures.

Adrenal hemorrhage–Can mimic a suprarenal mass but typically evolves over time and lacks calcified organized structures.

The presence of a well-organized vertebral axis and limb-like skeletal elements in our case favors the diagnosis of fetus in fetu over these differentials.

CONCLUSION

Fetus in fetu is a rare congenital anomaly that can mimic teratoma on prenatal imaging. Identification of an organized vertebral axis and limb-like structures is key to diagnosis. Postnatal CT plays a crucial role in confirming the diagnosis and guiding surgical management. Complete excision is curative with excellent prognosis.

TEACHING POINTS

1. Identification of an organized vertebral axis with limb-like structures on imaging is the most reliable criterion for diagnosing FIF

2. Postnatal CT plays a decisive role in differentiating FIF from teratoma when prenatal imaging is inconclusive.

MCQS

• Q1. The most reliable imaging feature distinguishing FIF from teratoma is:

  1. Presence of fat

  2. Cystic components

  3. Organized vertebral column

  4. Lack of vascularity

  Correct Answer: C

• Q2. The most common anatomical location of FIF is:

  1. Thorax

  2. Cranial cavity

  3. Retroperitoneum

  4. Pelvis

  Correct Answer: C

• Q3. Which imaging modality is most sensitive for confirming FIF postnatally?

  1. Ultrasonography

  2. MRI

  3. Plain radiography

  4. Computed tomography

  Correct Answer: D