Renal neoplasms account for approximately 10% of all malignant tumors in children (Breslow and Beckwith, 1982). Nephroblastoma (Wilms’ tumor) accounts for 80% of renal neoplasms in children, while other tumor types, such as anaplastic sarcoma, clear cell sarcoma, rhabdoid tumor, and renal cell carcinoma account for the rest. Wilms’ tumor has a peak incidence at 2 to 3 years of age, but it can present from fetal life to adulthood (Breslow et al., 1988a,b). However, neonatal Wilms’ tumor is unusual, with only 5 cases reported to the National Wilms’ Tumor Study among 3340 reported between 1969 and 1984 (Hrabovsky et al., 1986; Ritchey et al., 1995). Several cases now have been diagnosed in utero (Giulian, 1984; Ritchey et al., 1995; Applegate et al., 1999; Vadeyar et al., 2000; Cavicchioni et al., 2005).

Nephroblastoma is a tumor that arises within the kidney and consists of a variety of embryonic tissues such as glomeruli and tubules, spindle cells, smooth and skeletal muscle fibers, cartilage, and bone. Wilms’ tumor is associated with many genetic conditions, including Beckwith–Wiedemann, Denys–Drash, Klippel–Trenaunay syndromes and neurofibromatosis and the WAGR complex (Wilms’ tumor, aniridia, genito urinary malformations, and mental retardation) (King, 1993), suggesting a genetic predisposition to Wilms’ tumor (Jadresic et al., 1990). However, the most common presentation of Wilms’ tumor is an asymptomatic abdominal mass. Abdominal pain, hematuria or malaise, weakness, anorexia, and weight loss may also be presenting symptoms.

Fetal Wilms’ tumor may present as part of Perlman syndrome, which is characterized by familial nephroblastomatosis, fetal ascites, polyhydramnios, hepatomegaly, macrosomia, and Wilms’ tumor (Greenberg et al., 1986; Perlman, 1986). Wilms’ tumor in Perlman syndrome occurs in the absence of chromosomal abnormalities, enzymatic defects, or somatic conditions known to be associated with Wilms’ tumor. Nephroblastomatosis may present as either diffuse or discrete rests of abnormally persistent embryonic renal blastema. In some instances, this condition is believed to be a premalignant precursor of Wilms’ tumor (Kulkarni et al., 1980; Stone et al., 1990). Nephroblastomatosis has been defined by Ambrosino et al. (1990) as either persistent metanephric blastema in infants over 36 weeks of gestation or the presence of persistent metanephric blastema in an abnormal location and/or quantity in younger fetuses. Nephrogenic rests occur as two main types, perilobar and intralobar (Beckwith et al., 1990). Nephroblastomatosis occurs in four patterns: perilobar only, intralobar only, combined, and universal or panlobular (Beckwith et al., 1990). The diffuse, panlobar type is characterized by diffuse superficial blastemal tissue that surrounds and compresses the normal parenchyma. In contrast, in the multifocal, perilobar, or intralobar types, nodular renalblastoma is found in the subcapsular cortex and in the deep cortex above the columns of Bertin. In autopsy series of infants younger than 3 months of age, perilobular nephrogenic rests were observed in 0.87% and intralobular in 0.1% (Beckwith et al., 1990). In contrast, 41% of unilateral Wilms’ tumors are associated with nephrogenic rests and 99% of synchronous bilateral Wilms’ tumors were associated with nephrogenic rests (Beckwith et al., 1990). Nephroblastomatosis is considered a precursor of Wilms’ tumor, especially when it is of the multifocal type (Kulkarni et al., 1980; Ambrosino et al., 1990, Stone et al., 1990). Diffuse hyperplastic perilobar nephroblastomatosis is a distinct entity presenting as massively enlarged kidneys or kidneys that maintain their normal architecture and lack necrosis (Shamberger, 2005). A third of these will go on to develop Wilms’ tumor (Barbosa et al., 1998).

Relatively few cases of Wilms’ tumor have been diagnosed prenatally. However, our current understanding of the disease suggests that at least the predisposition to developing Wilms’ tumor is present before birth. The incidence of Wilms’ tumor is thought to be 1 in 8000 to 10,000 livebirths, with an estimated 8 cases in 1 million children younger than the age of 15 years (Kramer, 1985; Breslow et al., 1993; Miller et al., 1995). The fetal and neonatal incidence of the disease is unknown. In 50% of cases the left kidney is affected, in 45% of the cases the right is affected, and in 5% the tumor is bilateral. The tumor can be seen in association with specific anomalies, including aniridia (8.5 in 1000 cases), hemihypertrophy (25 in 1000 cases), cryptorchidism (28 in 1000 cases), and hypospadias (18 in 1000) (Breslow et al., 1988a,b).

The sonographic features of Wilms’ tumor may be indistinguishable from those of mesoblastic nephroma. Both present as complexmasses that arise from or may completely replace the normal kidney. These tumors are predominantly solid, but cystic areas also can be seen. There may be a well-defined pseudocapsule in Wilms’ tumor as opposed to mesoblastic nephromas (Figure 116-1A) (Giulian, 1984; Walter and McGahan, 1985). In mesoblastic nephroma, polyhydramnios is a feature of most cases that have been reported (Yambao et al., 1986). Few antenatally detected cases of Wilms’ tumor have been described thus far, so it is uncertain if polyhydramnios may also occur in association with Wilms’ tumor. Magnetic resonance imaging (MRI) may be used to enhance anatomic delineation of a renal mass (Figure 116-1B) (Toma et al., 1990). Fetal MRI provides a larger field of view with better appreciation of the impact ofa large Wilms’ tumor on adjacent structures (Figure 116-1A and 116-1B). Three cases of prenatally diagnosed nephroblastomatosis have been reported (Ambrosino et al., 1990; Gaulier et al., 1993). One was the stillborn product of a 28–week gestation complicated by polyhydramnios, ascites, pleural effusion, nephromegaly, and calcific foci in one kidney. The second case similarly showed polyhydramnios, homogeneous nephromegaly, and calcific foci within the kidney. The lungs of both fetuses were hypoplastic. The kidneys at autopsy showed diffuse nephroblastomastosis and were thought to represent Perlman syndrome. The third case occurred within a multicystic dysplastic kidney.

The differential diagnosis of a fetal renal mass includes hydronephrosis (see Chapters 80–83) and multicystic dysplastic kidney (see Chapter 78). While far more common, the typical cystic appearance of hydronephrosis and multicystic dysplastic kidney easily distinguishes them from Wilms’ tumor. However, mesoblastic nephroma (see Chapter 113) may be indistinguishable from Wilms’ tumor (Giulian, 1984; Walter and McGahan, 1985; Garmel et al., 1994). Focal renal dysplasia should also be considered in the differential diagnosis (Gordillo et al., 1987; Sanders et al., 1988) as well as masses extending from adjacent organs such as the adrenal gland or liver. In addition, nephroblastomatosis occurring in multicystic dysplastic kidney was detected by prenatal ultrasound examination as a renal mass may be difficult to distinguish from a Wilms’ tumor (Gaulier et al., 1993). Nephroblasto-matosis tends to maintain more normal renal architecture, and there are no cases of necrosis or cystic degeneration (Barbosa et al., 1998).