Wilms tumor (nephroblastoma) is the most common malignant renal tumor of childhood, accounting for approximately 6% of all pediatric malignancies. It typically occurs between ages 1-5, with 80% of cases diagnosed before age 5. It originates from embryonal metanephric blastema and histologically contains the classic triphasic pattern of blastemal, epithelial, and stromal components. The tumor typically presents as a large, heterogeneous, well-circumscribed abdominal mass. The normal renal parenchyma stretches around the tumor forming a thin capsule — this produces the characteristic 'claw sign' on imaging. Bilateral involvement occurs in 5-10% of cases and is associated with Denys-Drash syndrome, WAGR syndrome, or Beckwith-Wiedemann syndrome. Hematogenous spread occurs most commonly to the lungs (80-85%); renal vein and inferior vena cava (IVC) invasion occurs in 4-10% of cases. Treatment is nephrectomy + chemotherapy ± radiotherapy; prognosis is excellent in early stages (>90% five-year survival). Biopsy is contraindicated because capsule rupture upstages the tumor (Stage I → Stage III) and increases the risk of peritoneal dissemination.
Age Range
0-5
Peak Age
3
Gender
Equal
Prevalence
Rare
Wilms tumor originates from metanephric blastema (embryonal kidney tissue). During normal kidney development, metanephric blastema undergoes nephron differentiation through inductive signals; disruption of this process leads to persistent nephrogenic rests, which are the precursor lesions of Wilms tumor. The WT1 gene (chromosome 11p13) encodes a critical transcription factor for kidney and gonad development; WT1 loss is associated with Wilms tumor + genitourinary anomalies + aniridia (WAGR syndrome). WT2 (11p15.5) is in the IGF-2 (insulin-like growth factor 2) imprinting region; overexpression leads to somatic overgrowth (Beckwith-Wiedemann syndrome) and tumor development. The heterogeneous imaging findings reflect the histological triphasic structure: the blastemal component produces solid and avidly enhancing areas, the stromal component produces myxoid/low-signal areas, and foci of necrosis and hemorrhage create the heterogeneous appearance. The tumor is surrounded by a pseudocapsule composed of compressed renal parenchyma, which is the basis of the 'claw sign.' Renal vein invasion occurs through direct tumor extension into venous structures and may result in IVC thrombus formation.
The claw sign describes a radiological finding where the renal parenchyma stretches around an intrarenal mass. On contrast-enhanced CT, the avidly enhancing normal renal parenchyma is seen as a thin crescent or claw shape at the edges of the tumor. This finding is the most reliable way to confirm that the mass is intrarenal (kidney-origin) — extrarenal masses (such as neuroblastoma) push the kidney by compression but do not wrap around it. In the context of Wilms tumor, the claw sign demonstrates that the tumor originates from the renal parenchyma and compresses normal tissue forming a pseudocapsule as it grows. This finding is critically important in the differential diagnosis of neuroblastoma versus Wilms.
On non-contrast CT, a large (usually >5 cm, often 10-15 cm), heterogeneous-density intrarenal mass is seen in the kidney. The mass may contain solid areas (soft tissue density, ~30-50 HU), cystic/necrotic areas (fluid density, 10-20 HU), and hemorrhagic foci (high density, >50 HU). Calcification is seen in 9-15% of cases — usually amorphous or coarse. The mass is well-circumscribed and surrounded by compressed renal parenchyma (pseudocapsule). The contralateral kidney should be evaluated — bilateral Wilms occurs in 5-10%.
Report Sentence
A well-circumscribed intrarenal mass measuring approximately [X] cm with heterogeneous density containing solid and cystic/necrotic areas is seen in the right/left kidney; in the pediatric age group, Wilms tumor (nephroblastoma) should be considered first.
In the arterial (corticomedullary) phase, the mass demonstrates heterogeneous enhancement. Solid blastemal and stromal areas show variable degrees of enhancement, while necrotic and cystic areas show no enhancement. The compressed normal renal parenchyma around the mass enhances avidly, creating the 'claw sign' — this is the most important indicator confirming the intrarenal origin of the mass. The enhancing renal parenchyma partially or completely wraps around the tumor, showing the kidney stretched in a crescent or claw-like configuration. Functional renal parenchymal thickness and enhancement pattern should be assessed.
Report Sentence
On contrast-enhanced CT arterial phase, the mass demonstrates heterogeneous enhancement with the claw sign formed by stretching of normal renal parenchyma around the mass, confirming the intrarenal origin of the lesion.
In the portal venous (nephrographic) phase, the renal vein and inferior vena cava (IVC) must be evaluated. Tumor thrombus appears as a filling defect within the contrast-filled venous structure. The renal vein may be distended and may contain non-enhancing or minimally enhancing soft tissue within the lumen. IVC thrombus can show cranial extension — it may extend up to the right atrium. The cranial extent of the thrombus directly affects surgery (cardiopulmonary bypass may be required for suprahepatic extension). Enhancing tumor thrombus (unlike bland thrombus) may show contrast enhancement — this supports the tumoral nature of the thrombus. Renal vein invasion occurs in 4-10% of cases.
Report Sentence
The renal vein is distended with a tumor thrombus seen as a filling defect within the lumen; the IVC shows extension to the [infrahepatic/suprahepatic/intra-atrial] level.
On T2-weighted images, Wilms tumor shows a heterogeneous signal pattern. Solid areas show moderate-high signal intensity (hyperintense or isointense relative to renal parenchyma), cystic/necrotic areas show markedly hyperintense signal (free fluid signal), and hemorrhagic foci show variable signal (subacute hemorrhage hyperintense on T2, hemosiderin-containing areas hypointense). T2 sequences are particularly superior in differentiating solid and cystic components of the tumor, assessing the extent of necrosis, and visualizing the pseudocapsule. In bilateral Wilms tumor, a mass with heterogeneous signal is also seen in the contralateral kidney. Nephrogenic rests (precursor lesions) may be seen as small, homogeneous, mildly hyperintense foci on T2.
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On T2-weighted images, a large intrarenal mass with heterogeneous signal pattern containing solid and cystic/necrotic components is seen in the kidney.
On diffusion-weighted imaging (DWI), the solid blastemal components of the tumor show high signal, and low ADC values are seen on the ADC map (typically <1.0 × 10⁻³ mm²/s). This diffusion restriction reflects the high cellularity of solid blastemal areas. Necrotic and cystic areas show low signal on DWI and high values on the ADC map (T2 shine-through effect should be differentiated). DWI is particularly useful for detection of nephrogenic rests and for post-treatment response assessment (ADC increase with treatment = necrosis/apoptosis). In suspected bilateral Wilms, DWI adds value in detecting small lesions in the contralateral kidney.
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Diffusion restriction is seen in the solid components of the mass on DWI (low signal on ADC map), consistent with high cellularity.
On B-mode ultrasonography, a large, usually well-circumscribed, heterogeneous echogenic mass is seen in the kidney. Solid areas appear as moderate-to-high echogenicity, cystic/necrotic areas as anechoic or hypoechoic, and hemorrhagic foci as hyperechoic. The tumor is often surrounded by normal renal parenchyma — the claw sign on US may be seen as a hypoechoic rim of normal kidney tissue around the mass. Calcifications are detected as hyperechoic foci with posterior acoustic shadowing. US is the first-line imaging modality: it provides initial assessment in a child presenting with a painless abdominal mass. The contralateral kidney must be evaluated (bilateral involvement 5-10%). The renal vein and IVC should be checked with Doppler for tumor thrombus.
Report Sentence
On ultrasonography, a heterogeneous echogenic mass measuring approximately [X] cm containing solid and cystic components is seen in the kidney; contrast-enhanced CT/MRI is recommended for further characterization.
On color Doppler ultrasonography, variable vascularity is seen within the tumor — increased blood flow in solid areas, avascular zones in necrotic areas. Normal arcuate and interlobar arterial flow may be seen along the compressed renal parenchyma forming the claw sign. The most critical Doppler assessment is evaluation of the renal vein and IVC: distension of the renal vein and tumor thrombus showing solid echoes within the lumen may be detected. Doppler evaluates the flow pattern in the IVC — loss of flow or turbulent flow at the thrombus level may be observed. Screening of the contralateral renal vein and IVC for tumor thrombus is mandatory.
Report Sentence
Color Doppler ultrasonography shows a heterogeneous vascularity pattern within the mass; the renal vein and IVC should be evaluated for tumor thrombus.
On chest CT, lung metastases appear as bilateral, multiple, round, well-circumscribed, variable-sized (from a few mm to >1 cm) solid nodules. The nodules are usually subpleural and basal in location, showing a peripheral distribution. Lung metastases are present at diagnosis in 10-15% of cases and represent the most common route of hematogenous spread (80-85%). Cavitation is rare. Pleural effusion may accompany. Mediastinal lymphadenopathy is rare, and when detected, should raise concern for advanced stage. Post-treatment size and number changes of metastases are critical for response assessment. Chest CT at diagnosis is a mandatory staging investigation in all Wilms tumor patients.
Report Sentence
Bilateral multiple well-circumscribed pulmonary nodules are seen in both lungs on chest CT; in the context of known Wilms tumor, these are consistent with lung metastases.
Criteria
Classic triphasic pattern: coexistence of blastemal, epithelial, and stromal components without anaplasia. Accounts for approximately 90% of cases.
Distinct Features
Typical heterogeneous, well-circumscribed mass appearance on imaging. Prognosis is excellent: >90% five-year survival in Stage I-II. Treated with standard chemotherapy (actinomycin-D + vincristine) and nephrectomy. May contain necrosis and hemorrhage areas, but solid areas contain homogeneous blastemal tissue.
Criteria
Widespread nuclear anaplasia: markedly enlarged, pleomorphic, hyperchromatic nuclei and abnormal mitotic figures present in multiple areas of the tumor. Accounts for approximately 5-10% of cases. Associated with p53 mutation.
Distinct Features
Cannot be distinguished from favorable histology on imaging — definitive diagnosis is only made by pathological examination. Prognosis is significantly worse: five-year survival drops below 50% in advanced stages. Requires aggressive chemotherapy regimen (doxorubicin + cyclophosphamide added). Relapse risk is high. Large, irregular masses with extensive necrosis may raise suspicion for anaplasia, but this is not a reliable imaging criterion.
Criteria
Simultaneous Wilms tumor in both kidneys. Accounts for 5-10% of cases. Frequently associated with nephrogenic rests — perilobar or intralobar nephrogenic rests are precursors of bilateral involvement. More common in syndromic patients: Beckwith-Wiedemann, Denys-Drash, WAGR.
Distinct Features
On imaging, heterogeneous masses and/or multiple small nodules are seen in both kidneys. Nephron-sparing surgery (partial nephrectomy) is preferred — bilateral nephrectomy requires dialysis. Neoadjuvant chemotherapy to shrink tumors followed by partial nephrectomy is planned (SIOP protocol). MRI is superior to CT for detailed assessment of renal parenchymal and tumor margins. Nephrogenic rests may appear as small foci that are homogeneously hyperintense on T2 MRI and hypodense on CT.
Criteria
Completely cystic variant of Wilms tumor. Thin-septated, multilocular cystic mass — minimal or no solid component. Septae contain blastemal elements but do not form expansile solid tumor.
Distinct Features
Presents as a multilocular cystic mass on imaging — may be confused with Bosniak IV cystic renal mass or multilocular cystic nephroma. May show thin enhancement in septae but no solid nodular component. Prognosis is excellent — nephrectomy alone is usually curative, chemotherapy may not be needed. Usually occurs in boys under 2 years. Cystic nephroma and cystic RCC should be considered in the differential diagnosis.
Distinguishing Feature
Clear cell RCC typically occurs in adults (50-70 years) and is extremely rare in children under 5 years. RCC shows prominent hypervascular arterial enhancement with rapid washout; Wilms tumor shows more heterogeneous enhancement with the claw sign typical in pediatric intrarenal masses. Opposed-phase signal drop on MRI (intracellular lipid) in RCC is not seen in Wilms. Age criterion is the most reliable distinguishing factor: <5 years = Wilms, >40 years = RCC.
Distinguishing Feature
Renal medullary carcinoma occurs in sickle cell trait carriers (usually young adults, 10-30 years) and often originates from the medullary region of the kidney. Medullary carcinoma shows an infiltrative growth pattern — it is not well-circumscribed, and unlike Wilms tumor, the claw sign is typically not seen. Sickle cell history is the critical distinguishing clinical finding. Medullary carcinoma appears as a hypodense solid mass on CT, and infiltration into surrounding tissues (perinephric fat, collecting system invasion) is more prominent at earlier stages compared to Wilms.
Distinguishing Feature
Renal lymphoma in children typically appears as bilateral, multifocal, and homogeneously hypodense lesions — unlike Wilms tumor, it does not show a single large heterogeneous mass pattern. Lymphoma is hypovascular and shows minimal enhancement on contrast CT; Wilms tumor shows heterogeneous but definite enhancement. Systemic findings (diffuse lymphadenopathy, splenomegaly, bilateral renal involvement) are prominent in lymphoma. On DWI, lymphoma shows very marked diffusion restriction (ADC values are generally lower than Wilms). The claw sign is not seen in lymphoma — lymphoma infiltrates the renal parenchyma or originates from perinephric fat.
Distinguishing Feature
Neuroblastoma originates from the adrenal gland (or sympathetic chain) and is the most important differential diagnosis of Wilms in the pediatric age group. The critical distinction: in neuroblastoma, the kidney is displaced inferiorly and laterally but the claw sign is not seen (renal parenchyma does not wrap around the tumor because the tumor is extrarenal in origin). Neuroblastoma frequently contains calcification (80-90% vs 9-15% in Wilms), grows by encasing vascular structures (encasement of aorta/IVC is characteristic — Wilms does not do this, it invades the IVC). Neuroblastoma crosses the midline, Wilms typically does not. Urine catecholamines (VMA, HVA) are elevated in neuroblastoma.
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralWilms tumor requires urgent pediatric oncology and pediatric surgery consultation. BIOPSY MUST ABSOLUTELY NOT BE PERFORMED: capsule rupture upstages the tumor (Stage I → Stage III) and increases the risk of peritoneal dissemination — this dramatically changes the treatment protocol and prognosis. Treatment approach is based on two main protocols: (1) COG (Children's Oncology Group) protocol — nephrectomy first, then chemotherapy (North American approach); (2) SIOP (International Society of Paediatric Oncology) protocol — neoadjuvant chemotherapy first (4-6 weeks), then nephrectomy (European approach). Staging requires chest CT (lung metastases), abdominal CT/MRI (local extent, IVC thrombus, contralateral kidney), and complete blood count/biochemistry. In bilateral Wilms (Stage V), nephron-sparing surgery is the goal. Prognosis for favorable histology Stage I-II is >90%, dropping significantly in anaplastic histology and advanced stages. Post-treatment close follow-up (US + chest CT every 3 months) is maintained for at least 5 years.
Wilms tumor is treated with neoadjuvant chemotherapy and nephrectomy. Prognosis is generally good (90% survival). Bilateral involvement occurs in 5-10% of cases requiring nephron-sparing surgery. Anaplastic histology is a poor prognostic factor.