Renal ultrasound in a 4-year-old boy shows a large unilateral heterogeneous mass replacing the kidney with areas of necrosis. CT confirms a well-encapsulated mass with a 'claw sign' of residual normal renal parenchyma. What is the most likely diagnosis and the staging investigation of choice for pulmonary metastases?

• A Neuroblastoma — crosses midline; chest X-ray for pulmonary involvement
• B Wilms tumor (nephroblastoma) — intrarenal mass; CT chest for pulmonary metastases ✓
• C Renal lymphoma — bilateral infiltrating masses
• D Clear cell sarcoma of the kidney — bone metastases on bone scan

Explanation

Wilms tumor (nephroblastoma) is the most common primary renal malignancy of childhood (peak 3–4 years). CT shows a large, well-encapsulated intrarenal mass with a 'claw sign' (residual normal renal tissue wrapping around the mass, confirming renal origin) and often areas of necrosis, hemorrhage, or fat. CT of the chest is required for staging as pulmonary metastases are the most common site (20% at diagnosis) and determine stage IV classification. Neuroblastoma arises from the adrenal gland/sympathetic chain, characteristically crosses the midline, and shows calcifications. Contralateral kidney evaluation is mandatory for synchronous Wilms.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.