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Mesoblastic Nephroma

Jeanette Callahan, MD candidate
Don DiSalvo, MD

May 11, 1998

Presentation

(September, 1996) A 23-year-old woman (G1P0 at 34.7 weeks) was admitted with premature rupture of membranes. Obstetrical ultrasound obtained at 33.9 weeks had demonstrated severe polyhydramnios.

Imaging Findings

Obsterical ultrasound images
Postnatal magnetic resonance images of the infant

On obstetrical ultrasound, fetal anatomy is normal except for a well-circumscribed 6X6X4cm right-sided solid mass located in the position of the renal fossa (arrows). The mass appears to be contiguous with the mid and lower poles of the right kidney. Transverse sonogram of the fetus clearly shows a homogeneously echogenic mass with well-demarcated borders arising from the right renal fossa (arrows). The left kidney appears normal and no other fetal abnormalities were noted. Doppler applied to transverse and sagittal views of the abdomen showed bidirectional blood flow within the mass.

A postnatal (infant) abdominal MRI demonstrates a large solid mass with residual renal parenchyma at the upper pole of the right kidney (arrows).

Differential Diagnosis

Fetal masses involving the renal fossa are rare. The most common solid intrarenal fetal mass is mesoblastic nephroma, constituting 3% of all renal neoplasms in children. Also referred to as a fetal renal hamartoma, this tumor is benign and is composed primarily of mesenchymal, connective tissue. About 14% are associated with prematurity and polyhydramnios. Other congenital anomalies associated with polyhydramnios include CNS-mediated dysphagia or neural tube defects, GI-esophageal or duodenal atresia or obstruction, and fetal Hydrops, either nonimmune (such as fetal cardiac failure due to structural anomaly or arrhythmia) or immune-mediated (such as Rh incompatibility).

Wilms' tumor (nephroblastoma) is a common renal neoplasm, which might present as a unilateral solid renal mass. However, the tumor rarely presents before 2 years of age and is therefore an unlikely diagnosis. The two most common neonatal abdominal masses are hydronephrosis and multicystic dysplastic kidney disease. These are often discovered incidentally during routine sonographic examination. Also included in the differential is nephroblastomatosis which is closely associated with, and considered a precursor of, Wilms’ tumor. Nephroblastomatosis, however, would differ sonographically from this case by demonstrating multiple solid, subcapsular, hypovascular and hypoechogenic nodules or cysts. Finally, the most common abdominal malignancy in the newborn (although rare with 500 cases per year in USA) is neuroblastoma. Arising in neural crest tissue, an adrenal neuroblastoma may present sonographically similar to this case and would be difficult to rule out without a biopsy.

Diagnosis

Biopsy following complete resection confirmed diagnosis of mesoblastic nephroma.

Discussion

On ultrasound, mesoblastic nephroma is an evenly echogenic mass with concentric echogenic and hypoechoic rings resembling uterine fibroids. It may in time form a heterogeneous mass with hemorrhage and cyst formation secondary to central regions of necrosis. The mechanism behind increased amniotic fluid in cases of mesoblastic nephroma is not known, but the polyhydramnios generally results in preterm labor. The majority of these patients are treated with complete resection postnatally with excellent prognoses.

References

Benacerraf BR. Fetal Ultrasound. The Radiologic Clinics of North America 1990; 28(1).

Krebs CA, Giyananni VL. Ultrasound Atlas of Disease Processes. Norwalk, Conn: Appleton & Lange, 1993:210-481.

Walker JP, McGahan JP. Mesoblastic Nephroma: Prenatal Sonographic Detection. J Clin Ultrasound 1985; 13(9):686-689.

Weissleder R, Wittenberg J. Primer of Diagnostic Imaging. Boston: Mosby, 1994:495-496.

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