Nephrogenic rests are abnormally retained embryonic kidney precursor cells arranged in clusters. Nephrogenic rests are found in about 1% of unselected pediatric autopsies, 35% of kidneys with unilateral Wilms tumors, and in nearly 100% of kidneys with bilateral Wilms tumors.[8,9] The term nephroblastomatosis is defined as the presence of diffuse or multifocal nephrogenic rests. There are two types: intralobar nephrogenic rests and perilobar nephrogenic rests. Diffuse hyperplastic perilobar nephroblastomatosis is defined as nephroblastomatosis forming a thick rind around one or both kidneys and is considered a preneoplastic condition. Patients with any type of nephrogenic rest in a kidney removed for nephroblastoma should be considered at increased risk for tumor formation in the remaining kidney. This risk decreases with patient age. Extrarenal nephrogenic rests rarely occur, but may develop into extrarenal Wilms tumor.
Clear Cell Sarcoma of the Kidney
Clear cell sarcoma of the kidney is not a Wilms tumor variant, but it is an important primary renal tumor associated with a significantly higher rate of relapse and death than favorable-histology Wilms tumor. In addition to pulmonary metastases, clear cell sarcoma also spreads to bone, brain, and soft tissue. The classic pattern of clear cell sarcoma of the kidney is defined by nests or cords of cells separated by regularly spaced fibrovascular septa. Previously, relapses have occurred in long intervals after the completion of chemotherapy (up to 10 years), however with current therapy relapses after 3 years are uncommon. The brain is a frequent site of recurrent disease.[14,15]
While little is known about the biology of clear cell sarcoma of the kidney, the t(10;17)(q22;p13) translocation has been reported in clear cell sarcoma of the kidney. As a result of the translocation, the YWHAE-FAM22 fusion transcript is formed; this transcript was detected in 12% of clear cell sarcoma of the kidney cases in one series.
Rhabdoid Tumors of the Kidney
Rhabdoid tumors are extremely aggressive malignancies that generally occur in infants and young children. The most common locations are the kidney and central nervous system (CNS) (atypical teratoid/rhabdoid tumor), although rhabdoid tumors can also arise in most soft tissue sites. Initially they were thought to be a rhabdomyosarcomatoid variant of Wilms tumor when they occurred in the kidney.