Nephrogenic rests: their frequency and their fate.

Nephrogenic rests (NRs) are considered to be precursor lesions of Wilms tumor, the most common malignant neoplasm of the urinary tract in children. We have previously reported on 2 cases of NRs, incidentally diagnosed at 2 to 3 months of age during an ultrasound mass screening for urinary tract malformations between 1992 and 2006. As the screened population consisted of 17,065 infants, the observed prevalence of NRs in our area in the examined time period was of 1.17/10,000. This is the first reliable estimate of the frequency of clinically appreciable NRs in infants. Microscopic NRs have been found at autopsy in about 1% of infants. Our data are, therefore, helpful in the assessment of the proportion of NRs that disappear spontaneously in the childhood age group. To the best of our knowledge, no false-negative cases were found. Our observations indicate that our policy of "wait and see" is appropriate when NRs are identified incidentally during ultrasonographic screening done for whatever purpose.