Nearly two thirds of infantile or fetal abdominal masses are of renal origin. Most are benign and related to hydronephrosis and multicystic dysplasia . Congenital renal tumors are rare, comprising 2.5 to 7 percent of all perinatal tumors. This category includes, in decreasing order of frequency, congenital mesoblastic nephroma, Wilms tumor, rhabdoid tumor, clear cell sarcoma, hamartomas (eg, angiomyolipoma), and ossifying tumor of infancy .
Congenital mesoblastic nephroma (CMN) arises from renal mesenchyma and is usually benign. It comprises less than 5 percent of all pediatric renal tumors. About 90 percent present in the first year of life, 50 to 75 percent of cases occur in young infants, and almost none occur after the age of 3 years . After 2 months of age, Wilms tumor is more common than CMN. (See "Prenatal sonographic diagnosis of cystic renal disease" and "Presentation, diagnosis, and staging of Wilms tumor".)
CMN is a solitary hamartoma. Mesoblastic nephroma is presumed to originate from proliferating nephrogenic mesenchyme. It is similar in gross and histologic appearance to a uterine leiomyoma with spindled cell sin bundles, but composed of immature renal stromal cells. The tumor lacks renal blastema and neoplastic metanephric elements, thereby differentiating it from Wilms tumor. In addition, it tends to infiltrate the kidney, rather than form the pseudocapsule of classic Wilms tumor.
There are two pathologic variants: classic CMN and atypical or cellular CMN. The classic form is characterized by rare mitoses and absence of necrosis. Dilated vessels and entrapped nephrons are sometimes seen at the periphery of the tumor. Atypical or cellular CMN is characterized by a high mitotic index, hypercellularity, and an atypical growth pattern with necrosis, hemorrhage, and invasion of adjacent structures other than connective tissue .
CMN is almost always unilateral and is rarely malignant . It may extend beyond the renal capsule, but rarely metastasizes to distant organs. Cellular CMN, older age, and positive surgical margins appear to increase the risk of recurrence and metastasis .