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Background
Pediatric abdominal masses range from benign to neoplastic. Renal neoplasms comprise 7% of all pediatric cancers and can be either solid or cystic. Ultrasound (US) is the first-line imaging modality for suspected abdominal masses, followed by contrast-enhanced CT or MRI renal mass protocol to further characterize such lesions and to guide patient management. The purpose of this exhibit is to describe benign and malignant pediatric renal neoplasms, as well as mimics of renal neoplasms, and emphasize characteristic imaging features that aid in their differential diagnosis.

Educational Goals / Teaching Points
1. Ultrasound is the first-line imaging modality for suspected abdominal masses, followed by contrast-enhanced CT or MRI renal mass protocol. 2. When a renal tumor is suspected, the patient’s age, imaging characteristics, distribution of metastases, and comorbid conditions are the most important factors to consider. 3. Synchronous ipsilateral and contralateral renal lesions, metastatic disease, lymphadenopathy, invasion of the renal vein/IVC, and signs of tumor rupture are important features to evaluate. 4. Suprarenal pathologies can mimic a renal tumor, such as neuroblastoma.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Solid renal neoplasms include mesoblastic nephroma, nephrogenic rests, Wilms tumor, rhabdoid tumor, clear cell sarcoma, angiomyolipoma, renal cell carcinoma, lymphoma, and metastases. Synchronous ipsilateral and contralateral renal lesions, metastatic disease, lymphadenopathy, invasion of the renal vein/IVC, and signs of tumor rupture are also important features to evaluate. In addition to key imaging characteristics, the patient’s age, distribution of masses and metastases, and comorbid conditions are important factors to consider. Solid neoplasms should also be differentiated from cystic renal neoplasms, such as cystic nephroma and cystic partially differentiated nephroblastoma, which may be distinguished by herniation of the cystic mass into the collecting system. A solid nodule component in addition to cysts is suggestive of a cystic Wilms tumor. It is also important to consider mimics of renal neoplasms, including hypertrophied column of Bertin, focal pyelonephritis and abscess, infarct, granulomatous diseases, fungal infection, suprarenal pathologies, and cystic renal diseases. For example, neuroblastoma, a suprarenal pathology, can be distinguished by Wilms tumor based on the presence of an ipsilateral adrenal gland, a mass that crosses the midline or extension into the spinal canal, and the presence of calcifications.

Conclusion
Imaging plays a central role in the management of pediatric renal tumors. In addition to narrowing the differential diagnosis, imaging is also invaluable in excluding important tumor mimics, delineating important prognostic factors such as vascular invasion, evaluating disease extent, and assessing treatment response.