Neonatal tumors occur every 12,500 to 27,500 live births and account for 2% of childhood malignancies. Mesoblastic nephroma 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.  Prior to this, it was erroneously confused with congenital Wilms tumor. The radiologic characteristics of mesoblastic nephroma are demonstrated in the images below.
The classic and the cellular variants of mesoblastic nephroma differ in the age at presentation, histologic characteristics, imaging characteristics and the biologic behavior of the tumors.
The classic variant presents earlier, usually before age 3 months. It usually presents 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. 
On the other hand, the cellular type presents later, usually after age 3 months. It 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 a haphazard sheets with a limited tendency to form bundles as seen in the classic form. 
The cellular variant may also demonstrate the t(12;15) translocation, which is diagnostic. On reverse-transcription polymerase chain reaction (RT-PCR), it may demonstrate the ETV6-NTRK3 gene fusion, a feature also seen in congenital infantile fibrosarcoma, hence the name infantile fibrosarcoma of the kidney. 
Mesoblastic nephroma is the most common renal tumor in neonates and infants and represents 3% of the pediatric renal tumors.  It was initially described as a benign tumor, but recent advances suggest that a spectrum of disease exists, with the classic type representing the benign disease and the cellular variant representing the aggressive disease, with the mixed tumors falling somewhere in the middle. Recent studies have indicated that the incidence of the cellular variant may be from 42-63%. 
Sex- and age- related incidence
Mesoblastic nephroma has a slight male predominance, with a male-to-female ratio of 1.5:1. 
Mesoblastic nephroma is the most common renal tumor presenting in the neonatal period and accounts for approximately 54% of tumors in this age group.  The classic variant presents early, usually in the first 3 months, whereas the cellular type presents later.
The diagnosis may be made antenatally on ultrasound. Polyhydramnios is reported in 71% of pregnancies associated with mesoblastic nephroma. 
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.
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. [7, 8, 9, 10]
Postnatally, ultrasonography is usually the first imaging study performed when the abdominal mass is palpated. Ultrasonography is easily and widely available, it is inexpensive, and it involves no ionizing radiation.
All cross-sectional imaging studies, such as ultrasonography, computed tomography (CT) scanning, and MRI, may help to define the organ of origin and the relationship to the ipsilateral kidney. However, MRI is the most accurate imaging modality, as it most accurately depicts 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.
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, but it is still considered experimental. 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.
Chaudry et al found that in a study of 30 children (15 boys, 15 girls) with congenital mesoblastic nephroma, 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. 
The authors conducted the study to determine whether various imaging findings can identify the classic variant versus the cellular variant of mesoblastic nephroma. Ultrasonography was performed in 17 children, CT in 19, and MRI in 7. The investigators 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.
The radiographic findings in mesoblastic nephroma may be nonspecific, and images may show a large abdominal mass. Mass effect on the surrounding organs and bowel may be present, but plain radiographs are not indicated.
Degree of confidence
The findings do not help in identifying the organ of origin or in making a diagnosis.
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.
Magnetic Resonance Imaging
MRI demonstrates a solid mass that replaces all or part of the ipsilateral kidney (as shown in 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.
Degree of confidence
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.
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,  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.
Degree of confidence
The ultrasonographic findings may cause underestimation of the local and regional extent of the mass.
Nuclear medicine studies do not play a big role in the diagnosis or follow-up of patients with a mesoblastic nephroma. The studies may demonstrate a mass arising from the kidney.
Entrapment of the renal tissue in the mass may lead to some excretion of radionuclide in the mass, but the mass itself does not have any functional renal tissue.
Degree of confidence
No specific characteristics confirm the diagnosis of mesoblastic nephroma.