Chapter 112
##### KEY POINTS

Key Points

• Most common congenital renal tumor.

• Sonographically, mesoblastic nephroma is most often a unilateral solid mass with nodular densities.

• Differential diagnosis includes hydronephrosis, multicystic dysplastic kidney, and nephroblastoma (Wilms’ tumor).

• Mesoblastic nephroma can be cystic in areas due to hemorrhage with cystic degeneration.

• There are 3 histologic variants: classic, cellular, and mixed.

• 50% deliver prior to 34 weeks of gestation and are associated with polyhydramnios; 23% are hypertensive.

• Surgical resection is curative - local recurrence and metastases usually associated with the cellular variant.

##### CONDITION

Although in general, congenital renal tumors are rare, mesoblastic nephroma is among the most common to present during the first few months of postnatal life, accounting for 50% of all renal masses in the neonate (Campagnola et al., 1998; Jones and Cohen, 2007). Mesoblastic nephroma is also known as fetal renal hamartoma, lyomyomatous hamartoma, and mesenchymal hamartoma (Wigger, 1975; Slasky et al., 1982). These tumors are composed of mesenchymal tissue, in the form of interlacing bundles of ovoid or spindle-shaped cells. Since this “classical” form of mesoblastic nephroma was reported, a cellular variant has been described that may account for a large proportion of cases. Both types can coexist in distinct areas in a given tumor; this is known as a mixed type (Joshi et al., 1986; Pettinato et al., 1989). The cellular type of mesoblastic nephroma accounts for 42% to 63% of all cases (Knezevich et al., 1998; Furtwaengler et al., 2006). The cause of mesoblastic nephroma is unknown, but it has been suggested that mesoblastic nephroma is a differentiated form of Wilms’ tumor (Bolande, 1974) or a derivative of secondary mesenchyme (Wigger, 1975). Current opinion favors classification of mesoblastic nephroma as a distinct, usually benign neoplasm arising from renal mesenchyme.

There is much debate as to the nature of mesoblastic nephroma and its biologic behavior. It is estimated that 95% of patients with mesoblastic nephroma do not relapse. The majority of those that either recur locally or metastasize have the cellular variant of mesoblastic nephroma (Bolande, 1974; Howell et al., 1982). The cellular variant has the strongest evidence, given the rarity of this tumor, for associated poor prognosis with a recurrence-free survival of 85% in contrast to 100% for the classic variant (Furtwaengler et al., 2006). This biologic behavior in the cellular variant may be due to a chromosome translocation, which results in the ETV6 sequence on chromosome 12p13 fusing with the NTRK3 sequence on chromosome 15q25 (Argani and Beckwith, 2000; Anderson et al., 2006). These genes are thought to result in constitutive dimerization of activating tyrosine kinase mediated growth signals. This translocation is not present in the classic variant and is variably present in the mixed type (Argani and Beckwith, 2000; Anderson et al., 2006; Ahmed et al., ...