Differential diagnosis of pediatric cystic renal mass

• Simple renal cyst
• Multicystic dysplastic kidney
• Hydronephrosis due to ureteropelvic junction obstruction, congenital megaureter, vesicoureteral reflux, duplicated kidney
• Renal abscess
• Pediatric cystic nephroma
• Cystic Wilms tumor

Approach to the differential diagnosis of cystic renal mass:

• US is the first step in imaging
• The most likely cause of a cystic renal mass is a congenital anomaly of renal formation
• In the clinical setting of infection, strongly consider the diagnosis of renal abscess. If this diagnosis is made, repeat the US in a month to ensure the abscess has resolved
• Otherwise you must move on to CT or MR and begin the workup for malignancy