---

Pediatric Renal Mass: Complete Radiology Reference

---

1. AGE-WISE CLASSIFICATION OF PEDIATRIC RENAL MASSES

---

2. IMAGING MODALITIES - OVERVIEW

Ultrasound (US) - First-Line Always

• Always the first imaging modality for any pediatric abdominal mass
• Assesses: renal origin, vascularity (Color Doppler), IVC/renal vein thrombus, contralateral kidney, liver
• Limitations: small nephroblastomatosis foci (<1 cm) may be missed; operator dependent
• Contrast-enhanced US (ce-US) improves detection of nephrogenic rests

CT - Gold Standard for Staging

• Preferred for: initial staging, surgical planning, pulmonary metastases
• Wilms on CT: heterogeneous, areas of low attenuation, calcification rare (<10%), enhances less than normal parenchyma; "claw sign" of normal renal tissue around the mass
• Chest CT for pulmonary metastases (though staging systems traditionally use CXR)
• Avoids CT when bilateral disease suspected - use MRI instead

MRI - Gold Standard for Bilateral Disease

• Preferred for: bilateral tumors (Stage V), nephroblastomatosis surveillance, follow-up, suspected vascular extension to atrium
• Wilms on MRI: hypointense T1, variably hyperintense T2, heterogeneous enhancement
• Nephrogenic rests: homogeneous, low signal, minimal enhancement (poorer perfusion than cortex)
• Gadolinium required to assess contralateral kidney for nephroblastomatosis
• No ionizing radiation - preferred for repeated surveillance

Nuclear Medicine

• 99mTc-DMSA: Mesoblastic nephroma shows uptake (unlike Wilms); used when diagnosis uncertain
• 99mTc-MDP bone scan: Clear cell sarcoma staging (bone metastases first)
• FDG-PET/CT: Staging of rhabdomyosarcoma (replaces bone scan for RMS)

---

3. PERCUTANEOUS BIOPSY - INDICATIONS

1. Bilateral renal masses (SIOP protocol: biopsy then preoperative chemo)
2. Suspected non-Wilms diagnosis (e.g., tumor in teen, atypical imaging)
3. Solitary kidney - preoperative chemotherapy planned
4. Unresectable tumor
5. Metastatic disease at presentation with bilateral disease
6. SIOP protocol: some centers biopsy to confirm before giving preoperative chemotherapy (12% in UK had non-Wilms diagnosis on biopsy)

---

4. WILMS TUMOR (NEPHROBLASTOMA) - COMPREHENSIVE

Epidemiology

• Most common renal tumor in children; 12% of all childhood cancers
• Peak age: 3 years; equal gender distribution; highest incidence in Black populations
• ~10% bilateral (2/3 synchronous, 1/3 metachronous)
• 75% in otherwise normal children; 15% with associated anomalies

Associated Syndromes / Predisposing Conditions

• Genitourinary anomalies (horseshoe kidney, cryptorchidism, hypospadias) in 4.5%
• Acquired von Willebrand disease in 8% - check coagulation before surgery

Classic Imaging Features

• Large, heterogeneous solid-cystic mass; fibrous pseudocapsule
• Variable areas of hemorrhage, necrosis, cysts (cystic components may predominate)
• Normal renal tissue stretched at periphery - "claw sign"
• Assess renal vein and IVC carefully for thrombus (can extend to right atrium)

• Heterogeneous, low-attenuation areas
• Calcification uncommon (<10%)
• Enhances less than normal renal parenchyma
• "Claw" of remaining normal renal tissue at periphery
• Check: contralateral kidney, para-aortic nodes, liver, lungs

• T1: hypointense
• T2: variably hyperintense
• Post-gadolinium: heterogeneous, often poor enhancement
• Best modality for bilateral disease (Stage V) and nephroblastomatosis

Pathology

• Triphasic: blastemal, stromal, epithelial components
• Favorable histology (FH) - majority
• Unfavorable histology (UH): diffuse anaplasia (nuclear enlargement + hyperchromatic multipolar mitoses) - worst prognosis

---

5. WILMS TUMOR STAGING - COG (NWTSG) vs SIOP

COG/NWTSG Staging (Post-surgical - surgery first, then stage)

SIOP Staging (Post-chemotherapy surgery - preoperative chemo given first)

• Essentially same staging criteria as COG/NWTSG
• Key difference from COG: Masses that have been biopsied are regarded as Stage I disease when later excised (not upstaged to Stage II)
• Advantage: tumor shrinkage before surgery, less intraoperative spill (0% vs 14.6% spill rate)
• Disadvantage: ~5.4% of patients given preoperative chemo found to have non-Wilms diagnosis at nephrectomy

COG vs SIOP Treatment Philosophy

Prognosis

• Stage I-III (4-year OS): 86-96%
• Stage IV: 83%
• Stage V (bilateral): 70%
• Diffuse anaplasia Stage III: 45%; Stage IV: only 7%

---

6. WILMS TUMOR vs NEUROBLASTOMA - IMAGING DIFFERENTIATION

---

7. NEPHROGENIC RESTS (NR) - TYPES AND ASSOCIATIONS

Definition

Two Fundamental Types

Key Facts

• Found in ~1% of kidneys at autopsy (vast majority never become Wilms)
• Nephroblastomatosis: 41% of unilateral Wilms, 94% of metachronous bilateral, 99% of synchronous bilateral Wilms
• Multiple rests in one kidney implies rests in the other kidney
• Children <12 months with Wilms + PLNRs: high risk for contralateral disease - needs surveillance

NR Natural History

Shape Distinction from Wilms

• Wilms tumor: spherical shape, pseudocapsule at interface with normal parenchyma
• Hyperplastic NR: elliptical/lenticular shape (retains original rest shape)
• MRI: NRs are homogeneous, low T2 signal, minimal enhancement; Wilms is heterogeneous with stronger enhancement

---

8. NEPHROBLASTOMATOSIS - COMPLETE

Definition

Forms

1. Unifocal - single rest
2. Multifocal - multiple discrete rests
3. Diffuse - massive involvement; may produce thick rind enlarging kidney while preserving its shape

"Diffuse Hyperplastic Perilobar Nephrogenic Rest" (DHPLNR)

• Thick rind of PLNR tissue enveloping the kidney, compressing normal parenchyma centrally
• Wilms tumor developed in 44% (23/52 cases); median 30 months
• Increased anaplasia in Wilms developing after chemotherapy for nephroblastomatosis

Imaging Features

• Individual foci: homogeneous, low echogenicity
• Diffuse form: thick rind of reduced echogenicity
• Multifocal: may be subtle - slightly nodular or plaque-like lesions
• Lesions <1 cm difficult to detect on US alone; ce-US helps
• CDS improves detection

• Homogeneous lesions of low signal intensity (T1 and T2)
• Do NOT enhance with gadolinium (poorer perfusion vs. renal cortex)
• This lack of enhancement differentiates NR from Wilms tumor

• Renal lymphoma
• Leukemic infiltration

• Regular US or MRI surveillance preferred over biopsy of individual lesions
• Despite known malignant risk, biopsy of individual lesions is not standard

---

9. DIFFUSE CONFLUENT PERIPHERAL TUMOR (Diffuse Hyperplastic PLNR)

• Kidney is enlarged but maintains its reniform shape (unlike Wilms which distorts it)
• On CT/MRI: homogeneous low-density rind surrounding compressed but normal-appearing medullary tissue
• On US: thick hypoechoic peripheral rind compressing brighter medullary tissue centrally
• Key distinction from Wilms: uniform shape preservation, no discrete mass, no central necrosis
• May be treated with chemotherapy (as for Wilms protocol) or close surveillance

---

10. CONGENITAL MESOBLASTIC NEPHROMA (CMN)

Key Facts

• Most common renal tumor in first 3 months of life (mean age 3.5 months)
• Most common renal neoplasm on antenatal ultrasound
• Accounts for 3-10% of all pediatric renal tumors

Histologic Subtypes

Pathognomonic Sign

• ETV6-NTRK3 fusion (t(12;15)(p13;q25)) - present in cellular variant and congenital fibrosarcoma - this chromosomal translocation is the molecular pathognomonic feature
• On imaging: solid, homogeneous, firm mass with hypoechoic vascular periphery (ring sign on US)
• Heterogeneity suggests cystic change/necrosis (more common in cellular variant)

Imaging Features

• Solid, homogeneous mass
• Relatively hypoechoic vascular periphery ("ring sign")
• May be indistinguishable from Wilms on US/CT

• Cannot reliably distinguish from Wilms
• Does NOT typically invade vascular pedicle (unlike Wilms)
• Usually does not metastasize

• Shows 99mTc-DMSA uptake (functional tubular elements) - unlike Wilms

• Excellent prognosis with radical surgery alone (nephrectomy)
• Local recurrence possible if capsule penetrated (cellular variant)
• Chemo not routinely recommended; consider for incompletely resected cellular variant

---

11. METANEPHRIC ADENOMA

• Rare benign tumor, any age but mean ~41 years; F:M = 2:1
• May occur in children
• Polycythemia in ~12% (due to EPO secretion) - characteristic clinical association
• Can coexist with Wilms tumor

Imaging Features

• Well-defined, homogeneous
• Calcification in ~20%
• Minimal enhancement (hypovascular) - key feature
• Hypodense relative to renal parenchyma

• T1: hypointense to isointense
• T2: variable (often hypointense - hypocellular stroma)
• Minimal gadolinium enhancement

---

12. CYSTIC RENAL MASSES IN CHILDREN

Bosniak Classification Applied to Children

Specific Entities

• Bimodal: boys <4 years and women in 5th-6th decade
• Well-defined capsule/margins; multicystic architecture; enhancing septa
• May herniate into collecting system (pathognomonic feature)
• Cannot distinguish from cystic partially differentiated nephroblastoma (CPDN) on imaging
• Treatment: nephrectomy (partial if possible) due to malignant potential

• Majority in first 2 years of life; male predominant
• Imaging identical to MCN
• Contains blastemal elements in septa (unlike MCN)
• Treated as Wilms (more aggressive)

• Uncommon in children; if present, consider syndromic cause
• Von Hippel-Lindau, TSC, ADPKD, ARPKD

• Large cystic mass; thick irregular walls; may have solid components
• Treated same as standard Wilms

---

13. OSSIFYING RENAL TUMOR OF INFANCY (ORTI)

• Extremely rare; exclusively in infants (usually <6 months)
• Benign (only 15 reported cases)
• Arises from medullary collecting ducts
• Presents with gross hematuria (mass in collecting system)

Pathognomonic Imaging Feature:

• Calcified intrapelvicalyceal/collecting system mass - calcification is internal/central (unlike peripheral in other tumors)
• US: echogenic mass within collecting system with acoustic shadowing
• CT: dense calcification within the renal pelvis or calyx; may have soft tissue component
• IVU: filling defect in collecting system with central calcification
• Must be distinguished from staghorn calculus, TCC

---

14. ANGIOMYOLIPOMA (AML)

Key Features

• In children: almost exclusively associated with Tuberous Sclerosis Complex (TSC)
• Isolated AML extremely rare in children
• Multiple bilateral AMLs in TSC (>80% of TSC patients)
• Risk: spontaneous hemorrhage when >4 cm (Wunderlich syndrome)

Imaging - Pathognomonic Features

• Hyperechoic renal mass (fat content)
• Hyperechoic = echogenicity equal to or greater than renal sinus fat
• May show acoustic shadowing (fat)
• Absence of posterior acoustic shadowing distinguishes from calcification

• Fat attenuation (<-10 HU) within the mass - pathognomonic for AML
• Heterogeneous enhancement of non-fat components
• Beware "fat-poor AML" - may lack measurable fat; resembles RCC

• T1: hyperintense (fat)
• Fat-suppressed T1: signal drops (confirms fat) - "India ink" artifact
• T2: variable

• Hypervascular; "onion peel" or coiled/tortuous aneurysms
• Renal angiography pre-embolization for hemorrhage control

---

15. TRANSLOCATION RENAL CELL CARCINOMA (Xp11.2 RCC / TFE3 RCC)

• Most common RCC subtype in children and young adults (up to 40% of pediatric RCC)
• Mean age: ~15 years; female predominance
• Associated with prior chemotherapy (especially cyclophosphamide)

Molecular: TFE3 gene fusions (Xp11.2)

• PRCC-TFE3 (t(X;1)(p11.2;q21))
• ASPL-TFE3 (t(X;17)(p11.2;q25))
• Also: TFEB amplification (6p21)

Pathognomonic Imaging Features:

• Abundant psammomatous calcifications within the tumor - key CT feature
• Bulky lymphadenopathy at presentation (despite small primary)
• Hypervascular on CT; papillary architecture with clear cells
• Lymph node metastases early; relatively indolent course despite lymph node involvement

• Heterogeneous with extensive calcification
• Enhancement present
• Often large lymph nodes disproportionate to tumor size

---

16. PAPILLARY RCC

• Second most common RCC subtype overall; rare in children
• Two subtypes:
  • Type 1: Low grade, foamy macrophages, small cells; associated with hereditary papillary RCC (MET gene)
  • Type 2: High grade, eosinophilic cytoplasm; associated with hereditary leiomyomatosis RCC (FH gene)

Imaging - Pathognomonic Features:

• Hypovascular on CT/MRI (hallmark - less enhancement than clear cell)
• Homogeneous on CT; central necrosis uncommon in small lesions
• T2 hypointense on MRI (hemosiderin deposition, compact cellularity)
• On US: homogeneous, solid, iso- or slightly hypoechoic
• May have intratumoral hemorrhage with T1 hyperintensity
• Bilateral and multifocal papillary RCC: hereditary papillary RCC syndrome

---

17. MEDULLARY RCC (Renal Medullary Carcinoma)

Pathognomonic Association:

• Sickle cell trait (or other sickle hemoglobinopathies - HbSC, HbSS) - nearly pathognomonic association
• Young Black males; median age ~22 years
• Highly aggressive; most present with metastases

Imaging Features:

• Centrally located within the kidney (medullary origin)
• Infiltrative, ill-defined borders
• Heterogeneous on CT; often with central necrosis
• Hydronephrosis due to collecting system involvement
• Lymphadenopathy and distant metastases common at presentation
• Molecular: INI1/SMARCB1 loss (same as rhabdoid tumor)
• Poor prognosis regardless of treatment

---

18. MALIGNANT RHABDOID TUMOR OF THE KIDNEY (RTK)

Key Imaging & Clinical Features:

• Most aggressive renal tumor in childhood
• Usually diagnosed in first year of life
• Survival rate only 20-25%
• Accounts for 2% of pediatric renal neoplasms
• Resistance to chemotherapy; early widespread metastases

Imaging Features:

• Indistinguishable from Wilms tumor on imaging
• Metastases: lungs, liver, brain
• Subcapsular fluid collections (crescent sign) - somewhat characteristic
• Lobulated contour
• May have areas of necrosis/hemorrhage

Pathognomonic Associations:

1. Synchronous posterior fossa primitive neuroectodermal tumors (PNETs) - seen in ~15% - must check brain with MRI
2. Hypercalcemia (not specific - also in mesoblastic nephroma)
3. INI1/SMARCB1 gene inactivation (chromosome 22q11.2) - molecular hallmark
4. Also associated with primary CNS atypical teratoid/rhabdoid tumors (AT/RT)

---

19. CLEAR CELL SARCOMA OF THE KIDNEY (CCSK)

• Rare; marked male preponderance
• Peak age similar to Wilms (3-5 years)
• No known genetic associations; no bilateral tumors; no association with nephrogenic rests
• Alternative name: "Bone Metastasizing Renal Tumor" (BMRT)

Imaging Features:

• No specific radiological features to distinguish from Wilms (CT/MRI appearance similar to Wilms)
• Heterogeneous large mass on CT/MRI (see Fig 72.52 in Grainger & Allison: coronal CT and STIR MRI show large heterogeneous right renal mass)
• Key distinguishing clue: bone metastases (not lung) - only 5% present with metastases but bone first
• Staging requires 99mTc-MDP bone scintigraphy

---

20. RENAL CELL CARCINOMA IN CHILDREN - OVERVIEW

• Less than 1% of all RCC cases in children; mean pediatric presentation age 9 years
• More common presentation: abdominal mass/flank pain > hematuria (unlike adults)
• Cannot be distinguished from Wilms on imaging; age is the best discriminating factor
• 20% have metastases (lung, liver, skeleton, brain) at diagnosis
• Histologically: high-grade, high-stage, papillary tumors with ring-like calcifications

RCC in Children - Association with Other Diseases:

• Tuberous Sclerosis (TSC)
• Prior neuroblastoma treatment
• Saethre-Chotzen syndrome
• Chronic renal failure
• Hereditary disorders (VHL, hereditary papillary RCC, HLRCC)

---

21. PNET / EWING SARCOMA OF KIDNEY

• Primary renal PNET is rare
• Part of the Ewing sarcoma family of tumors (ESFT)
• t(11;22)(q24;q12) - EWS-FLI1 fusion - pathognomonic molecular marker
• Usually in older children/young adults; male predominance

Imaging:

• Large, heterogeneous mass with necrosis and hemorrhage
• Aggressive behavior; early metastases (lung, liver, bone)
• Cannot be distinguished from other large renal masses on imaging alone
• FDG-PET avid

---

22. SYNOVIAL SARCOMA OF THE KIDNEY

• Rare primary renal tumor; any age
• Pathognomonic molecular marker: SYT-SSX fusion (t(X;18)(p11;q11))
• Predominantly monophasic spindle cell type in the kidney

Imaging:

• Large multiloculated cystic mass (can mimic MCN/CPDN)
• Thick, irregular walls and septa
• Enhancing solid components
• May be predominantly cystic - mimics benign cystic nephroma

---

23. NEUROBLASTOMA - IMAGING IN DETAIL

Imaging Features:

• Extrinsic to kidney; crosses midline in ~50%
• Calcification in ~90% - stippled/amorphous/dystrophic
• Encases (not invades) major vessels - "vascular draping" or "vascular encasement"
• "Dumbbell" or intraspinal extension through neural foramina
• Inferior/lateral displacement of kidney ("drooping lily" sign on IVU)
• Heterogeneous on CT; irregular borders
• MIBG scan: positive in ~90% of neuroblastomas (staging, response)

Staging (INRG/INSS):

• Stage 4S: infant (<12 months) with localized primary + liver/skin/bone marrow (but not cortical bone) metastases - may spontaneously regress

---

24. LEUKEMIA AND LYMPHOMA - RENAL INVOLVEMENT

Leukemia:

• Diffuse bilateral renal enlargement with altered echogenicity on US
• Loss of corticomedullary differentiation
• May have discrete hypoechoic nodules
• Subtle on CT; more conspicuous on contrast-enhanced MRI
• Obstructive uropathy from uric acid nephropathy (during chemotherapy - tumor lysis)
• US recommended in all children with leukemia before starting chemo

Lymphoma (Non-Hodgkin, Burkitt most common):

• Renal involvement in 12% of children with NHL
• Typically multiple bilateral nodules (most common pattern)
• Or diffuse renal infiltration (may mimic nephroblastomatosis)
• Generally widespread disease elsewhere at presentation
• Burkitt lymphoma: B-cell; most common pediatric renal lymphoma
• On US: multiple hypoechoic nodules; enlarged kidneys
• Differential of bilateral nodular disease: lymphoma, Wilms (Stage V), metastases, AML (TSC)

---

25. BILATERAL RENAL MASS - DIFFERENTIAL DIAGNOSIS AND SYNDROME ASSOCIATIONS

Differential Diagnosis of Bilateral Renal Masses in Children

Syndrome-Bilateral Renal Mass Associations

---

26. HYPERECHOIC vs HYPOECHOIC RENAL MASS - DIFFERENTIAL DIAGNOSIS

Hyperechoic Renal Mass

Hypoechoic Renal Mass

---

SUMMARY TABLE - Quick Reference

---