A 3-year-old boy presents with a palpable abdominal mass discovered incidentally during routine examination. Ultrasound shows a large intrarenal mass with heterogeneous echotexture. Which investigation is most appropriate to confirm the diagnosis of Wilms tumor and assess for metastatic disease?

## Investigation of Choice for Wilms Tumor Diagnosis and Staging **Key Point:** CT abdomen/pelvis with IV contrast is the gold standard for confirming Wilms tumor diagnosis, assessing local extent, and detecting metastases. Chest CT is mandatory because the lungs are the most common site of metastatic spread (10–15% at presentation). ### Why CT is Superior | Feature | CT | MRI | Ultrasound | |---------|----|----|-------------| | **Renal mass characterization** | Excellent | Excellent | Good (limited) | | **Lung metastases detection** | Gold standard | Poor (not used for lungs) | Not applicable | | **Vascular invasion assessment** | Good | Superior | Limited | | **Speed in acute setting** | Fast | Slow | Fast but limited | | **Staging accuracy** | 90–95% | Similar for abdomen | Insufficient alone | **High-Yield:** The COG (Children's Oncology Group) and SIOP (Société Internationale d'Oncologie Pédiatrique) protocols mandate: 1. **Abdominal/pelvic CT with IV contrast** — defines tumor stage, assesses renal vein/IVC involvement, evaluates contralateral kidney. 2. **Chest CT** — detects pulmonary metastases (present in ~10–15% at diagnosis; chest X-ray alone misses 30% of small nodules). **Clinical Pearl:** Bilateral Wilms tumors (5–10% of cases) must be identified preoperatively, as they change surgical strategy from nephrectomy to nephron-sparing approaches. CT is essential for this assessment. **Warning:** Biopsy is contraindicated in suspected Wilms tumor because of the risk of tumor rupture and peritoneal spillage, which upstages the disease and worsens prognosis. ### Role of MRI MRI is reserved for specific scenarios: - Assessment of vascular invasion (superior soft-tissue contrast) when CT is equivocal. - Patients with contrast allergy or renal insufficiency. - **NOT** used for lung staging (poor sensitivity for pulmonary nodules). ### Bone Scan Bone metastases are rare in Wilms tumor (~2% at diagnosis); bone scan is not part of routine staging.