Wilms Tumor

Introduction And Epidemiology

• Most common primary malignant renal tumor of childhood.
• Represents sixth most common childhood malignancy; accounts for 6% of pediatric malignancies.
• Accounts for >95% of childhood kidney tumors.
• Peak incidence occurs between 2-3 years of age.
• 75% of cases diagnosed in children <5 years old.
• Median age at presentation: 44 months for unilateral disease, 32 months for bilateral disease.
• Bilateral involvement occurs in 7% of cases; multicentricity seen in 11%.
• Gender distribution: Male-to-female ratio 0.92:1 (unilateral) and 0.6:1 (bilateral).
• Ethnic variation: Asian children exhibit approximately half the incidence rates of White and Black children.

Genetics And Pathophysiology

• Derived from incompletely differentiated renal mesenchyme.
• Nephrogenic rests (foci of benign, undifferentiated mesenchyme) persist abnormally and serve as precursor lesions.
• Nephrogenic rests found in 1% of general population but up to 90% of children with bilateral Wilms or associated syndromes.
• WT1 gene (11p13): Encodes zinc finger transcription factor; mutated or deleted in 10-15% of tumors.
• WT2 locus (11p15.5): Contains cluster of imprinted genes (IGF2, H19, CDKN1C, KCNQ10T1). Epigenetic alterations (loss of imprinting) result in biallelic expression of IGF2.
• Wnt signaling pathway mutations: CTNNB1 (15%) and WTX (20%).
• miRNA processing gene mutations: DROSHA (10%), DICER1, DGCR8, XPO5, TARBP2 altered in 20-30% of tumors.
• TP53 mutations: Observed in 5% of tumors; strongly associated with anaplastic histology.
• SIX1 and SIX2 mutations: Alter progenitor proliferation.

Associated Congenital Syndromes

Syndrome	Genetic Lesion	Clinical Phenotype	Estimated Wilms Tumor Risk
WAGR Syndrome	11p13 deletion (WT1 and PAX6)	Wilms tumor, aniridia, genitourinary anomalies, delayed-onset renal failure, mental retardation.	45-57%.
Denys-Drash Syndrome	WT1 missense mutation (exon 8/9)	Ambiguous genitalia, pseudohermaphroditism, diffuse mesangial sclerosis, early renal failure.	~75% (>70%).
Beckwith-Wiedemann Syndrome	11p15.5 epigenetic alterations (WT2)	Hemihypertrophy, macroglossia, omphalocele, organomegaly, neonatal hypoglycemia.	~5%.
Frasier Syndrome	WT1 mutation (intron 9 splice site)	Ambiguous genitalia, streak gonads, focal segmental glomerulosclerosis.	8%.
Perlman Syndrome	DIS3L2 mutation	Fetal macrosomia, renal dysplasia, multiple congenital anomalies.	64%.
Simpson-Golabi-Behmel	GPC3 (Xq26)	Overgrowth, coarse facial features.	10%.

[Data derived from congenital anomalies and Wilms tumor associations].

Pathology And Histologic Classification

• Classic Triphasic Morphology: Composed of blastemal, stromal, and epithelial cells mimicking normal renal embryogenesis.
• Favorable Histology (FH): Absence of anaplastic features.
• Unfavorable Histology (Anaplasia): Characterized by extreme nuclear pleomorphism, nuclear enlargement, and irregular mitotic figures.
• Focal Anaplasia: Anaplastic cells strictly confined within a single region.
• Diffuse Anaplasia: Anaplastic cells in extrarenal sites, random biopsies, or multiple slides. Carries significantly worse prognosis.

Clinical Manifestations

• Abdominal Mass: Most common presentation (75-80%). Firm, non-tender, smooth mass rarely crossing midline.
• Abdominal Pain: Occurs in 28-30% of patients; typically follows trauma-induced intra-tumoral hemorrhage.
• Hypertension: Present in 25-26%; attributed to tumor-induced increased renin activity.
• Hematuria: Gross hematuria in 18%; microscopic hematuria in 24%.
• Constitutional Symptoms: Fever (20-22%), anorexia, vomiting, weight loss.
• Vascular Involvement: Tumor thrombus extends into inferior vena cava (IVC) in 4-10% of cases; rarely reaches right atrium. Poses fatal pulmonary embolism risk.
• Hematologic Derangements: Polycythemia (elevated erythropoietin), thrombocytosis, acquired von Willebrand disease (4% incidence), acquired Factor VII deficiency.

Diagnostic Evaluation

• Ultrasonography (US): Initial modality. Differentiates solid from cystic masses. Doppler imaging assesses renal veins and IVC for tumor thrombus.
• Computed Tomography (CT): Abdominal CT defines disease extent, assesses contralateral kidney, and identifies liver metastasis/lymphadenopathy. Chest CT essential for detecting pulmonary metastasis.
• Magnetic Resonance Imaging (MRI): Useful for delineating extensive IVC/atrial thrombus or distinguishing Wilms tumor from nephrogenic rests.
• Biopsy: Generally discouraged to avoid capsular rupture and subsequent tumor upstaging (COG approach). Indicated for atypical presentations (age >10 years, inflammation, unresectable tumors).
• Screening Protocols: Children with high-risk syndromes (WAGR, Denys-Drash, Beckwith-Wiedemann) require routine screening abdominal ultrasounds every 3 months until age 5-8.

Staging Classification (Children’s Oncology Group)

Stage	Pathologic And Surgical Criteria
Stage I	Tumor confined to kidney. Completely resected with negative margins. Renal capsule intact. No prior biopsy or rupture. Regional lymph nodes negative.
Stage II	Tumor extends beyond kidney but completely resected with negative margins and nodes. Includes penetration of renal capsule or invasion of renal sinus vessels.
Stage III	Residual tumor confined to abdomen. Includes gross/microscopic residual, preoperative/intraoperative spillage, prior biopsy, positive regional lymph nodes, or peritoneal implants.
Stage IV	Hematogenous metastases (lung, liver, bone, brain) or lymph node metastases outside abdominopelvic region.
Stage V	Bilateral renal involvement at time of initial diagnosis.

Prognostic Factors

• Favorable Predictors: Young age (<24 months), low tumor weight (<550 g), low stage, favorable histology.
• Adverse Predictors:
  • Diffuse anaplasia (associated with TP53 mutations).
  • Loss of heterozygosity (LOH) at chromosomes 1p and 16q (associated with inferior relapse-free and overall survival).
  • Gain of chromosome 1q (independent predictor of inferior survival).
  • Incomplete lung nodule response after 6 weeks of chemotherapy.

Management Strategies

Surgical Approach

• Children’s Oncology Group (COG) Paradigm: Recommends upfront radical nephrectomy and thorough lymph node sampling prior to chemotherapy. Facilitates accurate histological diagnosis and risk-adapted staging.
• International Society of Pediatric Oncology (SIOP) Paradigm: Recommends preoperative chemotherapy without prior biopsy to shrink tumor, followed by surgery.
• Inoperable Or Bilateral Tumors: Preoperative chemotherapy (Vincristine, Actinomycin D, Doxorubicin) administered for 6 weeks. Followed by definitive surgery (renal-sparing partial nephrectomy preferred for bilateral disease).

Risk-Adapted Chemotherapy Regimens

Risk Category	Chemotherapy Regimen	Duration
Very Low Risk (Stage I FH, <24 months, <550g)	Nephrectomy alone (Observation)	N/A
Stage I & II FH (No LOH)	Regimen EE4A: Vincristine + Actinomycin D	18 weeks
Stage III FH (No LOH)	Regimen DD4A: Vincristine + Actinomycin D + Doxorubicin	24 weeks
Stage I-III FH with LOH 1p/16q	Regimen DD4A (Stage I/II) or Regimen M (Stage III)	24-31 weeks
Stage IV FH	Regimen DD4A (Rapid lung responders) or Regimen M (Slow responders/LOH)	24-31 weeks
Diffuse Anaplasia (Stage II-IV)	Regimen UH-1 / UH-2: Vincristine, Doxorubicin, Cyclophosphamide, Carboplatin, Etoposide (+ Irinotecan)	30-36 weeks

[Data synthesized from COG clinical trial protocols].

Radiation Therapy

• Specifically omitted for Stage I and II Favorable Histology Wilms tumor.
• Flank/Tumor Bed Irradiation: Indicated for Stage III FH (1080 cGy) and all stages with focal/diffuse anaplasia (up to 1980 cGy for diffuse anaplasia).
• Whole-Abdominal Irradiation: Indicated for gross tumor spillage, peritoneal seeding, or diffuse intraperitoneal rupture (1050 cGy).
• Whole-Lung Irradiation: Indicated for metastatic pulmonary disease (1200 cGy; reduced to 1050 cGy if <12 months old). May be omitted if rapid complete response achieved with initial chemotherapy.
• Preoperative chemotherapy for bilateral Wilms tumor does not constitute an independent indication for radiotherapy unless positive margins persist post-surgery.

Relapse And Late Effects

• Recurrence: Occurs in ~15% of favorable-histology and 50% of anaplastic tumors. Most relapses manifest within 2 years of diagnosis.
• Salvage Therapy: Agents include Ifosfamide, Carboplatin, Etoposide (ICE), Topotecan, and Irinotecan. High-dose chemotherapy with autologous stem cell rescue considered for very high-risk relapse.
• Overall Survival: Exceeds 90% with modern multimodal therapy.
• Late Complications: Survivors face significant risks including doxorubicin-induced cardiotoxicity, radiation-induced pulmonary toxicity, progressive renal failure, fertility impairment (secondary to alkylating agents or pelvic radiation), and secondary malignant neoplasms. Long-term surveillance remains essential.