Congenital mesoblastic nephroma (CMN) is the most common renal tumor of the neonatal period, typically diagnosed within the first 3 months of life. Histologically, it comprises three subtypes: classic (infantile), cellular (atypical), and mixed. The classic type consists of mature fibroblastic cells and follows a benign course; the cellular type may behave more aggressively and harbors the ETV6-NTRK3 fusion gene. The tumor typically presents as a large, solid, unilateral renal mass that infiltrates surrounding tissues. Ultrasonography is the primary imaging modality; typically appearing as a large, solid, heterogeneously echogenic mass. CT demonstrates a solid, heterogeneously enhancing mass. Surgical resection (nephrectomy) is curative, and prognosis is excellent, particularly for the classic type. Unlike Wilms tumor, CMN typically occurs in infants under 6 months, and this age distinction is an important differential diagnostic criterion. The ring sign (a halo of normal renal parenchyma surrounding the tumor) is a characteristic ultrasound finding highly specific for CMN.
Age Range
0-1
Peak Age
-
Gender
Male predominant
Prevalence
Rare
Congenital mesoblastic nephroma originates from metanephric mesenchyme (the embryonal mesodermal tissue that forms the kidney). The classic type consists of mature spindle cells (fibroblasts/myofibroblasts) and is the renal equivalent of infantile fibrosarcoma. The cellular arrangement is in concentric fibers and tumor borders are not well-defined but infiltrative — hence the expected inability to clearly delineate margins on imaging. The cellular type harbors the ETV6-NTRK3 translocation t(12;15)(p13;q25); this fusion gene increases cell proliferation through tyrosine kinase activation, producing a more aggressive growth pattern. The cellular type has high cellularity, which creates a more homogeneous hypoechoic appearance on ultrasound and more homogeneous enhancement on CT. In the classic type, fibrous stroma predominates — therefore heterogeneous enhancement on CT is seen (fibrous areas enhance late, cellular areas enhance early). The ring sign results from the tumor not completely destroying normal renal parenchyma as it grows, leaving a peripheral shell — this shell is compressed normal renal cortex and appears as an isoechoic-hyperechoic ring around the hypoechoic tumor on ultrasound. The large tumor size reflects growth beginning in the intrauterine period; polyhydramnios and fetal hydrops may result from renal vein compression or high-output cardiac failure caused by this mass.
A thin isoechoic-hyperechoic halo of normal renal parenchyma seen surrounding the tumor on ultrasound. This ring results from the tumor compressing but not completely destroying normal renal cortex as it grows, leaving a thin shell peripherally. The ring sign is considered pathognomonic for CMN among neonatal renal masses and is the most important imaging finding for differential diagnosis from Wilms tumor. In Wilms tumor, the renal parenchyma is typically completely displaced and the ring sign is not seen.
Ultrasound demonstrates a large (typically >5 cm), solid, heterogeneously echogenic renal mass. The mass usually occupies the majority of the kidney with minimal remaining normal parenchyma. Ring sign: a thin isoechoic-hyperechoic halo of normal renal parenchyma is seen surrounding the tumor — this finding is pathognomonic for CMN. Cystic areas are rare and when present suggest the cellular subtype. The mass generally appears well-defined, though margins may not be entirely sharp due to infiltrative growth pattern. Calcification within the tumor is rare and when seen is punctate.
Report Sentence
A large, solid, heterogeneously echogenic mass is seen in the left/right kidney with a thin halo of normal parenchyma surrounding the tumor (ring sign); findings are consistent with congenital mesoblastic nephroma.
Color Doppler ultrasound demonstrates prominent internal vascularity within the tumor. Feeding vessels are typically traced from the periphery toward the center. In the classic type, vascularity is less prominent; in the cellular type, markedly increased vascularity is observed — this difference may provide a clue for subtype differentiation. The renal artery and vein may be displaced or encased by the tumor but invasion is not expected.
Report Sentence
Color Doppler examination demonstrates prominent internal vascularity within the mass; peripheral feeding vascular structure is observed.
Non-contrast CT demonstrates a large, solid, homogeneous to mildly heterogeneous density renal mass. The mass typically occupies the majority of the kidney or completely replaces it. Density values are generally between 30-50 HU (soft tissue density). Calcification is rare and when seen is fine and punctate. Areas of necrosis and hemorrhage may be seen in large tumors creating heterogeneity. The mass is generally well-defined, though the tumor-kidney interface may not be sharp.
Report Sentence
A large, solid, soft tissue density mass occupying the majority of the left/right kidney is seen; no calcification is observed.
Contrast-enhanced CT shows heterogeneous enhancement. Cellular areas enhance prominently in the early arterial phase while fibrous stromal areas show delayed enhancement. Necrotic or cystic areas do not enhance and remain hypodense. The tumor generally enhances less than normal renal parenchyma — this finding creates a contrast difference between surrounding normal kidney and tumor, providing the CT equivalent of the ring sign. Renal vein thrombosis may rarely be seen, particularly in the cellular type.
Report Sentence
On contrast-enhanced CT, the mass shows heterogeneous enhancement with less enhancement than normal renal parenchyma; hypodense areas are consistent with necrosis.
On T2-weighted MRI, CMN shows heterogeneous intermediate-to-high signal. Cellular components show intermediate signal on T2 due to high cellularity, while fibrous stroma areas show low T2 signal. Necrotic or cystic areas show high T2 signal. In the classic type, fibrous stroma predominates so a more heterogeneous appearance is seen, while in the cellular type, more homogeneous intermediate signal is observed. The ring sign may also be seen on MRI — peritumoral normal renal parenchyma shows different signal characteristics from the tumor on T2.
Report Sentence
On T2-weighted MRI, the mass shows heterogeneous intermediate-to-high signal; solid and cystic/necrotic components are seen together.
On diffusion-weighted imaging (DWI), cellular components show marked diffusion restriction (hyperintense on DWI, hypointense on ADC map). This finding is more prominent in the cellular type. In the classic type, fibrous stroma areas do not show diffusion restriction. Necrotic areas show low signal on DWI and high signal on ADC (facilitated diffusion). DWI may be helpful for differentiating between cellular and classic types.
Report Sentence
On DWI, marked diffusion restriction is observed in the solid components of the mass (hypointense on ADC map); this finding is consistent with high cellularity.
Prenatal ultrasound reveals a large, solid, echogenic mass in the fetal kidney. Polyhydramnios may accompany — this finding results from the tumor disrupting amniotic fluid dynamics through renal vein compression or high-output cardiac failure. Fetal hydrops may be seen in the most severe cases. The mass size creates a kidney appearance larger than expected for fetal age. Prenatal diagnosis directly impacts delivery planning (to confirm the contralateral kidney is healthy).
Report Sentence
Prenatal ultrasound reveals a large, solid, echogenic mass in the fetal left/right kidney with accompanying polyhydramnios; congenital renal tumor (CMN) should be primarily considered.
Criteria
Mature fibroblastic/myofibroblastic spindle cells, low mitotic activity, growth in concentric fibers
Distinct Features
More heterogeneously echogenic on US (fibrous stroma areas hyperechoic), delayed enhancement on CT, no significant diffusion restriction on DWI, more benign course, no recurrence expected after surgery
Criteria
High cellularity, increased mitotic activity, ETV6-NTRK3 fusion gene t(12;15)(p13;q25), histology similar to infantile fibrosarcoma
Distinct Features
More homogeneously hypoechoic on US, more homogeneous enhancement on CT, marked diffusion restriction on DWI (low ADC), tendency for larger size, higher local recurrence risk, rarely metastasizes
Criteria
Coexistence of both classic and cellular components, variable proportions of fibrous and cellular areas
Distinct Features
Imaging features vary according to the dominant component, heterogeneous appearance on CT/MRI, prognosis depends on cellular component proportion
Distinguishing Feature
Wilms tumor typically presents between 6 months-5 years (CMN <3 months); Wilms shows claw sign (normal parenchyma drapes around tumor) while CMN shows ring sign; Wilms is generally more heterogeneous with cystic components
Distinguishing Feature
Neuroblastoma originates from adrenal/extrarenal and compresses kidney externally (CMN is intrarenal); coarse calcification is common in neuroblastoma (80-90%) but rare in CMN; urinary catecholamines are elevated in neuroblastoma
Distinguishing Feature
Clear cell sarcoma of kidney rarely occurs in the same age group as CMN; bone scintigraphy is recommended for clear cell sarcoma due to bone metastases; histological differentiation is required
Distinguishing Feature
Renal rhabdoid tumor can be confused with CMN in the neonatal period; subcapsular fluid collection and aggressive appearance (invasion, metastasis) in rhabdoid tumor distinguishes from CMN; hypercalcemia may accompany
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
6-monthPrimary treatment for CMN is surgical nephrectomy. In the classic type, complete resection is curative and recurrence is virtually absent. In the cellular type, there is local recurrence risk (5-10%) and positive surgical margins increase recurrence risk. Chemotherapy is generally not required but may be considered for incomplete resection or recurrence in the cellular type. Prognosis is excellent, particularly for the classic type (>95% survival). Follow-up with ultrasound at 6-month intervals is recommended for the cellular type. Prenatal diagnosis impacts delivery planning — coordination with pediatric urology is needed to confirm contralateral kidney health and plan surgery.
Mesoblastic nephroma is treated with nephrectomy and prognosis is excellent. The classic type almost always has a benign course. The cellular type carries a risk of local recurrence and requires close follow-up. Chemotherapy is usually not needed. It is the first diagnosis to consider when a renal mass is detected in a neonate.