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Perinatally detected abdominal masses are a common clinical finding, and two-thirds originate from the kidney.1 With the increased use of prenatal ultrasound evaluation, approximately 15% of congenital renal masses are detected prenatally. Of those diagnosed postnatally, nearly half present with a palpable abdominal mass. Congenital renal tumors, however, are exceedingly rare, representing only 7% of all neonatal tumors.2 Abdominal ultrasound, whether done prenatally or postnatally, will often give an accurate description of these renal lesions. Congenital renal masses can be broadly divided into 2 main categories based on its sonographic appearance. Solid and cystic renal masses can be further subcategorized as either benign or malignant (Table 43-1).

Table 43-1Differential Diagnosis of a Retroperitoneal Mass During the Neonatal Period

Congenital cystic renal masses are far more frequently encountered and more likely to be benign when compared to solid renal masses. The most common cystic lesions include hydronephrosis, multicystic dysplastic kidney, and hereditary polycystic kidney diseases (PKDs).3 The majority of congenital solid renal masses are also benign. Solid masses, however, can represent 1 of several rare neonatal neoplasms. During the first year of life, malignant Wilms tumor (WT) is the most common solid renal tumor, followed by congenital mesoblastic nephroma (CMN), rhabdoid tumor of the kidney (RTK), and clear cell sarcoma of the kidney (CCSK). However, during the neonatal period, two-thirds of all solid renal tumors are CMN, followed by WT, RTK, and CCSK4 (Table 43-1).



Fortunately, the great majority of congenital renal masses are benign. Forty percent of all neonatal abdominal masses are due to hydronephrosis and renal cystic disease.5 Depending on the onset and severity of hydronephrosis, a newborn child may warrant serial renal ultrasounds, voiding cystourethrograms (VCUGs), or renal scintigraphy in the early stages of life. The most common causes of antenatal hydronephrosis are ureteropelvic junction (UPJ) obstruction, ureterovesical junction obstruction, vesicoureteral reflux (VUR), primary megaureter, neurogenic bladder, posterior urethral valves, and prune-belly syndrome.3 These clinical entities are discussed in greater detail in the chapter 44 on obstructive uropathies.

Multicystic Dysplastic Kidney

Unilateral multicystic dysplastic kidney (MCDK) is the most common cystic renal disease of the newborn, with an incidence of 1 in 4300 live ...

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