Gold Standard Test for Wilms' Tumor
The gold standard for diagnosing Wilms' tumor is imaging with ultrasound as the initial screening tool, followed by CT or MRI for staging—not biopsy, which is generally contraindicated in the North American approach to avoid tumor spillage and upstaging. 1, 2
Initial Diagnostic Approach
Ultrasound is the optimal first-line imaging modality for suspected Wilms' tumor because it is widely available, lacks ionizing radiation, can be performed without sedation, and has high sensitivity for detecting renal masses. 1 The ultrasound should:
- Confirm the renal origin of the mass 1
- Evaluate the inferior vena cava for tumor extension using Doppler 1
- Assess the bladder for additional lesions 1
Advanced Imaging for Staging
CT scanning is superior to ultrasound for complete tumor characterization and staging, detecting tumor necrosis, pseudocapsule, perinephric extension, lymph node involvement, and bilateral tumors with 77% diagnostic accuracy compared to only 23% for ultrasound alone. 3
MRI may be considered as an alternative to CT, particularly for:
- Evaluating bilateral tumors 1
- Detecting nephrogenic rests 1
- Pre-operative planning for potential nephron-sparing surgery 1
Why Biopsy is NOT the Gold Standard
In the North American Children's Oncology Group (COG) protocol, pre-operative biopsy is generally avoided because:
- Biopsy risks tumor spillage, which upstages the disease and necessitates more intensive chemotherapy and radiation 2
- Imaging characteristics combined with clinical presentation are typically sufficient for diagnosis 4, 2
- The COG approach involves upfront nephrectomy for unilateral tumors without pre-operative biopsy 2
This contrasts with the European SIOP approach, which uses pre-operative chemotherapy based on imaging alone, but even in that protocol, biopsy is not routinely performed. 2
Critical Diagnostic Considerations
Genetic testing should be pursued in patients with:
- Epithelial-predominant histology (test for TRIM28 variants, present in 86% of these tumors) 5
- Bilateral tumors (often represent genetic predisposition syndromes) 5
- Clinical features suggesting predisposition syndromes 6, 5
The diagnostic workup should distinguish Wilms' tumor from other pediatric renal masses by recognizing that elevated AFP and hepatic mass suggest hepatoblastoma, while bone involvement and elevated urinary catecholamines indicate neuroblastoma. 1