Nephroblastomatosis is a complex abnormality of nephrogenesis. Normal developmental anatomy of the kidney occurs when the ureteric bud contacts aggregates of primitive nephroblasts (metanephric blastema) in the paraspinal region of the developing embryo. Renal development occurs as a result of the interaction of the advancing and bifurcating ureteric bud which branch and form the collecting system of the kidney and the metanephric blastema which give rise to form epithelial and stromal elements of the kidney. Metanephric blastema lies peripherally in the subcapsular and interlobular spaces. Nephrons and associated supporting structures arise from the metanephric blastema. Nephrogenesis is complete in the normal fetus at 34-36 weeks of gestation.
Nephroblastomatosis is defined as the persistence of metanephric blastema, also known as nephrogenic rests, into infancy and childhood. Beckwith classified nephrogenic rests as dormant (nascent), sclerosing (obsolescent) or hyperplastic/neoplastic. Thus, nephrogenic rests may regress, sclerose or become hyperplastic/mitotically active or give rise to larger and frankly neoplastic rests such as Wilm's tumor. Nephroblastomatosis can be differentiated from Wilm's tumor by its gross and microscopic appearance and imaging characteristics. Nephroblastomatosis is usually diffuse, involves the entire subcapsular portion, has no renal capsule, has a lobulated margin separating it from underlying uninvolved kidney and has a uniform pink-flesh appearance and firm consistency. On the other hand, Wilm's tumors are usually a bulky, spherical mass, occupies one portion of the kidney with areas of necrosis, hemorrhage and macroscopic cysts. On ultrasound, nephroblastomatosis may show enlarged kidneys and hypoechoic foci in a diffuse or mutifocal pattern or normal appearing enlarged kidneys. On CT, nephroblastomatosis may appear as a multifocal or diffuse subcapsular layer of abnormal hypodense tissue. Nephrogenic rests may not demonstrate enhancement with IV contrast material. Normal enhancing renal tissue is typically seen in the central portion of the kidney.
Treatment options for patients with nephroblastomatosis include follow-up with ultrasound or CT every 2-6 months. Chemotherapy in nephroblatomatosis is controversial. Beckwith recommends use of chemotherapy in stage 1 WT and histologically proven nephroblastomatosis to decrease the size of the lesion. Although potential for malignant transformation is low, the mass effect of growing nephrogenic rests may mechanically injure the kidney. These benefits may be outweighed against the potential long term side effects and unproven efficacy of chemotherapy.