Mesoblastic Nephroma Imaging

Neonatal tumors occur every 12,500 to 27,500 live births and account for 2% of childhood malignancies. Mesoblastic nephroma (also called fetal renal hamartoma) is the most common renal tumor identified in the neonatal period and the most frequent benign renal tumor in childhood. It represents 3-10% of all pediatric renal tumors. This tumor was first described as a separate entity by Bolande et al in 1967. ^[1] Prior to this, it was erroneously confused with congenital Wilms tumor.  ^{[2, 3]}

The diagnosis of mesoblastic nephroma may be made antenatally on ultrasound. Polyhydramnios is reported in 71% of pregnancies associated with mesoblastic nephroma. ^{[4, 5, 6]}   The most common clinical presentation is an asymptomatic abdominal mass. Paraneoplastic syndromes such as hypertension or hypercalcemia may be present. Hypertension is thought to be secondary to increased renin production by the trapped glomeruli in the tumor. The differential diagnosis includes Wilms tumor. ^{[3, 7, 8, 9]}

The radiologic characteristics of mesoblastic nephroma are demonstrated in the images below.

Coronal T2 image demonstrates a solid mass arising from the upper pole of the right kidney. The normal renal tissue is displaced inferiorly. A part of the renal pelvis trapped by the mass appears as a hyperdensity in the mass. The mass is isodense to the renal parenchyma on all sequences.

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Coronal T1 image through the mass arising from the upper pole of the right kidney. The mass is isodense to the kidney. It shows homogeneous signal with no central necrosis or hemorrhage.

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Axial image in a 3-month-old infant with a palpable abdominal mass. The image shows a large, homogeneous, nonenhancing solid mass arising from the upper pole of the right kidney. The normal renal tissue is seen enhancing inferiorly. Excretion of contrast into the pelvis trapped by the mass is seen as central hyperdensity in the center of the mass.

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Pathophysiology

The classic variant usually presents before age 3 months as a solid mass with small foci of necrosis or hemorrhage. It does not invade the perinephric tissues or the vascular pedicle. It is associated with an excellent outcome after complete surgical resection of the tumor. On gross examination, the solid tumor is noncapsulated, with a whorled trabeculated appearance similar to leiomyomas—hence the name leiomyomatous hamartoma of the kidney. Histologically, it consists of uniform spindle cells arranged in bundles with scattered foci of entrapped normal glomeruli and tubules. ^[10]

The cellular type usually presents after age 3 months and demonstrates more aggressive imaging characteristics, with larger areas of necrosis and hemorrhage. It may invade the perinephric fat and connective tissues. It is associated with a higher rate of local recurrence and metastatic disease. On gross examination, the cellular type is a fleshy tumor with multiple foci of necrosis, cystic change, and hemorrhage. Histologically, it consists of spindle cells arranged in haphazard sheets with a limited tendency to form bundles, as seen in the classic form. ^[10]

A frequent genetic alteration is the translocation t(12;15) resulting in a fusion of the ETV6 gene on 12p13 and the NTRK3 gene on 15p15 that occurs almost exclusively in the cellular variant, which is diagnostic. Reverse transcription polymerase chain reaction (RT-PCR) may demonstrate the ETV6-NTRK3 gene fusion, a feature also seen in congenital infantile fibrosarcoma—hence the name infantile fibrosarcoma of the kidney. ^{[11, 12, 13, 14, 15]}  

Preferred examination

The mass may be first diagnosed when the detailed fetal anatomy scan is performed at 18-20 weeks' gestation. Differentiation between a solid and a cystic mass can easily be made on ultrasonography to differentiate between a mass and hydronephrosis. If the mass is very large, it may be difficult to determine the organ of origin in some cases. Fetal magnetic resonance imaging (MRI) may be helpful in determining the organ of origin because of excellent soft tissue detail on MRI and the ability to image in multiple planes. Fetal motion, however, may limit the MRI images. ^{[16, 17, 18, 19, 5, 6]}

Postnatally, ultrasonography is usually the first imaging study performed when the abdominal mass is palpated. Ultrasonography is easily and widely available, is inexpensive, and involves no ionizing radiation. All cross-sectional imaging studies, such as ultrasonography, computed tomography (CT) scanning, ^[20] and MRI, may help define the organ of origin and the relationship to the ipsilateral kidney. However, MRI is the most accurate imaging modality in depicting the local and regional extent of the tumor. This is because of the ability of MRI to display anatomy in different planes in great detail.

In a study of 30 children (15 boys, 15 girls) with congenital mesoblastic nephroma, Chaudry et al found that cystic components were readily identified on ultrasonography (US), central hemorrhage was easily identified on CT scanning, and MRI was highly sensitive for cystic components and central hemorrhage. ^[21]  The authors noted that findings suggestive of the classic variant included a peripheral hypoechoic ring or a large solid component, whereas cystic/necrotic change and hemorrhage were more common in the cellular variant.

Limitations of techniques

Imaging findings can suggest the diagnosis prenatally or after birth, and they can be used to identify the organ of origin. They may suggest a probable or most likely diagnosis based on the imaging characteristics. However, imaging results cannot be used to definitively differentiate a mesoblastic nephroma from a congenital Wilms tumor. Histologic examination is the only definitive test.

Ultrasonography is widely available and routinely performed antenatally. However, it has the disadvantage of being the most operator-dependant modality. Margins of the tumor may not be accurately visualized, especially with large tumors. In cases of large tumors, the organ of origin may also not be definitively determined.

CT scanning is not useful as an antenatal imaging modality because of exposure to ionizing radiation and the use of intravenous contrast. Postnatally, the same disadvantages exist. In addition, accurate delay after contrast injection may be difficult to determine because of the small volume of contrast used in neonates. The soft tissue contrast in this age group is also limited, because of the lack of peritoneal fat. In older patients, CT would also require sedation.

MRI is being increasingly used as a prenatal diagnostic tool. Antenatal MRI examination may be limited by fetal motion and maternal discomfort. In addition, MRI may require sedation, as the typical exam times are longer. Intravenous contrast may be required to fully characterize the tumor.

CT scans demonstrate a solid mass arising from the kidney, as shown in the images below. The tumor may replace part or all of the ipsilateral kidney. Areas of necrosis may be seen with the aggressive variant. Entrapment of urine or collecting system may lead to the excretion of contrast material within the mass. The mass itself shows no enhancement or calcification. The tumor may invade the perinephric connective tissue, but it does not extend into the renal pelvis or invade the vascular pedicle.

The extent of local infiltration may be underestimated with CT, but the modality has an important role in the evaluation of recurrent disease or metastases after initial surgery.

Axial image through the kidneys demonstrates a mass arising from the right kidney, displacing the normal renal parenchyma inferiorly. The mass shows no enhancement.

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Axial image in a 3-month-old infant with a palpable abdominal mass. The image shows a large, homogeneous, nonenhancing solid mass arising from the upper pole of the right kidney. The normal renal tissue is seen enhancing inferiorly. Excretion of contrast into the pelvis trapped by the mass is seen as central hyperdensity in the center of the mass.

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MRI demonstrates a solid mass that replaces all or part of the ipsilateral kidney (see the images below). The signal intensity characteristics are similar to those of the normal renal parenchyma. The mass shows intermediate signal intensity, similar to the renal cortex and the skeletal muscle on T1-weighted sequences. The signal is lower than that of the surrounding fat and higher than that of the renal medulla. The mass shows increased signal on T2-weighted images. Contrast-enhanced MRIs show no or minimal contrast enhancement in the mass. The benign or the typical mesoblastic nephroma may demonstrate a peripheral, markedly enhancing ring on the postcontrast T1 turbo-spin-echo sequence, and this may correspond to the vascular ring seen on US.

MRI is most accurate in determining the organ of origin and the size of the tumor, along with the local and regional extent of the mass. MRI offers exquisite delineation of the soft tissue planes and anatomy.

The distinction of a mesoblastic nephroma from the main differential diagnosis of a prenatal Wilms tumor is not possible, even with MRI.

Coronal T2 image demonstrates a solid mass arising from the upper pole of the right kidney. The normal renal tissue is displaced inferiorly. A part of the renal pelvis trapped by the mass appears as a hyperdensity in the mass. The mass is isodense to the renal parenchyma on all sequences.

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Coronal T1 image through the mass arising from the upper pole of the right kidney. The mass is isodense to the kidney. It shows homogeneous signal with no central necrosis or hemorrhage.

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Ultrasonography is usually the first test performed. The modality demonstrates a large, echogenic mass with a homogeneous echotexture arising from the kidney. The vascular ring sign, as described by Chan et al, ^[22] is an anechoic or a hypoechoic vascular ring surrounding the tumor on ultrasonography and is considered a feature of the typical or nonaggressive mesoblastic nephroma. This peripheral hypoechoic ring is also reported in other benign tumors, such as benign thyroid nodules.

On color Doppler examination, this vascular ring demonstrates significant vascularity, and on spectral Doppler examination, it demonstrates arterial and venous waveforms. On histopathologic correlation, the vascular ring corresponds to dilated blood vessels at the periphery of the mass.

Heterogeneity may suggest areas of necrosis or hemorrhage. The atypical mesoblastic nephroma has a more heterogeneous appearance, and the peripheral vascular ring has never been described in association with atypical tumors.

The mass does not invade the vascular pedicle or the renal pelvis.

The ultrasonographic findings may cause underestimation of the local and regional extent of the mass.