2 Prenatally Detected Renal Tumors: Comparison of MRI Findings and Review of the LiteratureLeann E Linam, MD; Maria A Calvo-Garcia, MD; Eva I Rubio, MD; Timothy M Crombleholme, MD; Beth M Kline-Fath, MD. Department of Radiology, Cincinnati Children’s Hospital Medical Center

IntroductionEnlargement of a kidney on prenatal imaging is usually due to hydronephrosis or a cystic renal enlargement, such as multicystic dysplastic kidney or polycystic kidney disease.Solid tumors of the kidneys are rarely seen in the antenatal period, but can be diagnosed by ultrasound.Primary differential considerations for a solid fetal renal mass include: Mesoblastic nephroma Wilms tumor Nephroblastomatosis Renal sarcoma Angiomyolipoma, especially in the presence of tuberous sclerosis

MRI is useful not only in confirming the presence of a renal mass, but also in the evaluation of the contralateral kidney for subtle abnormalities.

Congenital mesoblastic nephroma (CMN)

• CMN is a rare benign hamartomatous renal neoplasm composed of benign spindle cells. This lesion represents 3-6% of all renal tumors. It was first described by Bolande in 1967.

• CMN has been associated with lymphoma, leukemia, neuroblastoma, Beckwidth-Wiedemann syndrome and chromosomal anomalies.

• It is the most common fetal renal neoplasm and accounts for 75% of all tumors identified in the first six months of life.

• Prenatally, CMN may be fatal if lung compression and secondary pulmonary hypoplasia results. Polyhydramnios, often resulting in premature labor, occurs in approximately 70% due increased urine production with renal hyperperfusion. Rarely hydrops may develop secondary to high cardiac output failure.

• There have been 19 cases of CMN by prenatal ultrasound and 5 cases via prenatal MRI .

• Prenatal diagnosis is generally suggested with ultrasound. Typical sonographic findings are noted below: The mass arises from the kidney and is generally homogeneous or heterogeneously hypoechoic in echotecture.

The lesion tends to be well circumscribed and can demonstrate a hypoechoic halo.

Color Doppler may show a ring of venous and arterial waveforms and/or low vascularity.

• Prenatal MRI is useful when body habitus, amniotic fluid volume or fetal position prevent adequate evaluation. In addition, MRI provides better soft tissue contrast, allowing differentiation of the mass from enlarged normal renal tissue. With fetal MRI, it is also easier to separate the mass from adjacent organs. Fetal MRI findings typically noted include:

The mass is well defined and tends to be isointense to normal renal parenchyma on T2 weighted imaging.

There may be distortion of normal renal parenchyma and collecting system.

• Prognosis of the benign variant is excellent with nephrectomy. Infrequently, these tumors can be aggressive and metastasize.

SSFSE T2 weighted axial, coronal, and sagital MRI images through the fetus show a solid mass arising from the right kidney. Preservation of normal right renal tissue and collecting system is noted posteriorly and inferiorly (arrow). This mass compresses the adjacent liver, but is separate. There was polyhydramnios and note is made of severely distended bladder due to renal hyperperfusion. The baby delivered premature at 32 weeks due to preterm labor.

Prenatal MRI

Postnatal abdominal radiograph performed for line placement shows a right abdominal mass displacing bowel inferiorly and into left abdomen.

Postnatal radiograph and ultrasound

Longitudinal and transverse ultrasound images performed after delivery demonstrates a mildly heterogeneous, solid right renal mass with well defined peripheral hypoechoic rim (arrow).

Postnatal MRI

Axial and coronal T2 weighted MRI images performed postnatally show a large predominantly solid mass almost completely replacing the right kidney, most consistent with congenital mesoblastic nephroma.

Congenital Wilms’ tumor

• Wilms tumor is a malignant embryonic neoplasm containing epithelial, blastemal and stromal elements. This lesion is the most common renal childhood malignancy, accounting for 87% cases. It was first described by Wilms in 1899.

• Wilms tumors are primarily sporadic, but 1% will be familial with autosomal dominant inheritance. Extrarenal associated anomalies include genitourinary anomalies and aniridia. Overgrowth syndromes such as congenital hemihypertrophy , Beckwith-Wiedemann, and neurofibromatosis are associated.. Wilms can be seen in the presence of chromosomal syndromes, deletions and translocations. 5% of patients will have bilateral disease.

• Congenital Wilms tumor is exceedingly rare, representing 0.16% of all Wilms tumors. In a study by Ritchey in 1995, 53% of neonates diagnosed with Wilms tumor had evidence of nephrogenic rests, much higher than the childhood population.

• Polyhydramnios has been described but less often than in CMN. Rarely, the tumor may present with fetal hydrops.

• To our knowledge, there have been 5 prenatal ultrasound and 1 prenatal MRI cases reported.

• Prenatal ultrasound imaging defines a mass arising from the right kidney:The mass may present as a solid echogenic mass or a heterogeneous lesion with multiple cystic areas due to hemorrhage or necrosis.The mass may be unencapsulated or well defined.Color Doppler will typically reveal internal vasculature.It is important to exclude thrombus in the inferior vena cava.

Close imaging of the contralateral kidney should be performed

• Fetal MRI is helpful in the evaluation of Wilms tumor for similar reasons as stated in the section for CMN. However, MRI also provides better soft tissue definition of the contralateral kidney to exclude synchronous disease. In addition, the technique provides more information with regard to adjacent organs and may define anomalies that could support an associated syndrome.

The single case report noted above demonstrated a solid T2 hyperintense mass arising from the left kidney displacing but not invading adjacent structures..However, the mass may be complex due to hemorrhage or necrosis.IVC invasion may be present.

Close attention should be made to the contralateral kidney

• Prognosis for the lesion depends on stage, histological type and the presence of metastases.

Prenatal MRI

Sagital, and coronal FIESTA images through the fetus demonstrate a dilated upper pole collecting system with low signal mass extending into the collecting system (arrow). There is a lobulated heterogeneous high signal mass in the lower pole (black arrow).

Axial FIESTA images show a cyst in the contralateral kidney (arrow). Complex right renal mass is noted.

Longitudinal ultrasound performed through the right kidney shows a heterogeneous mass (M) within a dilated collecting system.

Transverse ultrasound image through the left kidney redemonstrates the cyst seen on prenatal MRI. In addition, the collecting system is poorly defined with suggestion of heterogeneous soft tissue within the collecting system.

Postnatal ultrasound

Postnatal CT Postnatal MRI

CT performed after resection of the right kidney. Image at the level of the midpole of the left kidney shows soft tissue density within the left collecting system (arrow).

Coronal T1 weighted MRI performed after the administration of contrast demonstrates a poorly enhancing mass (arrow) in the upper pole causing distortion of normal renal architecture. Corticomedullary differentiation is diminished as well.

MRI 2 months later One year follow up

T1 weighted MRI after the administration of gadolinium at the level of the left kidney performed 2 months later. There has been increase in size in the upper pole mass (arrow). At biopsy, this mass was a nephrogenic rest.

Left upper pole nephrogenic rest remains stable in size.

Conclusion:• Prenatal detection and diagnosis of a renal mass in the fetus is possible with ultrasound.

• Fetal MRI is useful to confirm mass origin within the kidney, evaluate effect on adjacent organs and exclude associated anomalies.

• On prenatal imaging, congenital mesoblastic nephroma is typically a solid lesion with fetal MR T2 signal equal to normal renal parenchyma.

• With prenatal ultrasound and fetal MR, congenital Wilms tumors may demonstrate either a solid or complex cystic mass. In the presence of an abnormality in the contralateral kidney, congenital Wilms should be considered.

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Prenatal Ultrasound

M

Parasagittal antenatal ultrasound images in a female fetus at 34 weeks gestation demonstrates a solid and cystic mass in the area of the right kidney. The opposite kidney appeared normal. Differential included multicystic dysplastic kidney, mesoblastic nephroma, neuroblastoma, teratoma and ovarian tumor.

Prenatal Ultrasound

LK Antenatal ultrasound at 27 week gestation. Sagittal and axial images through the right kidney demonstrates a homogeneous mass. On the axial image, there is a peripheral rim of hypoechogenicity (arrow). Sagittal image of the left kidney (LK) is normal.