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Findings: Ultrasound: Large inhomogeneous mass arising from the upper right kidney. Multiple areas of decreased echogenicity are present compatible with necrosis.

MRI: The mass demonstrates heterogeneous low T1 and high T2 signal characteristics with areas of high T1 signal corresponding with hemorrhage. A pseudocapsule is evident on T2 imaging. There is no involvement of the renal vein or IVC.

Diagnosis: Wilm's tumor (nephroblastoma).

Discussion: Wilm's tumor (nephroblastoma) is the most common abdominal malignancy, and the most common solid renal mass in childhood. The incidence is 1 in 10,000 with a bimodal peak at ages 2.5 and 3.5 years (1). There are both hereditary and sporadic forms, and approximately 6% are bilateral(1). Most are unicentric and arise from the kidney. Extra-renal Wilm's tumor is rare.

Associated conditions include congenital genitourinary malformations, hemihypertrophy, sporadic aniridia, as well as Beckwith-Weidemann and Drash syndromes. nephroblastomatosis (residual metanephrogenic blastema beyond 36 weeks gestation) is also associated with Wilm's tumor and is considered by many its precursor.

The most common clinical presentation is an asymptomatic abdominal mass of insidious growth. Secondary hypertension may be seen in up to 25% due to increased renin levels(1).

Initial ultrasound imaging is commonly performed, demonstrating a smooth, well defined mass of renal origin with uniform echogenicity. Hemorrhagic or cystic areas can be present but are uncommonly seen. CT demonstrates mixed, low attenuation with bands of enhancing tissue surrounding cystic/necrotic areas. T1 and T2 prolongation is present with MR imaging. Capsular invasion is difficult to predict with imaging, however, CT and MRI are useful in demonstrating IVC extension, or tumor thrombus(1). Metastatic disease seen to liver and lung can be identified by CT. MR has been utilized in screening for the nephrogenic rests of nephroblastomatosis, and multicentric Wilm's tumors.

Surgical resection is mandatory for treatment and staging with all patients receiving chemotherapy. Radiation therapy is reserved for residual abdominal tumor or hematogenous metastatic disease. There is an overall cure rate of >85%(1).

References:
1. Silverman, FN and Kuhn JP. Caffey's Pediatric X-ray Diagnosis. St. Louis: Mosby,
1993; 9th ed:1244-46.

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