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Background: Congenital mesoblastic nephroma (CMN) is generally considered to be a benign neoplasm requiring only total excision. Rare local recurrences have usually been related to incomplete removal, and distant metastases to the lung have been reported three times., Methods: The authors reported the first case of CMN metastatic to the brain, illustrating a comparison of the histopathology of the primary and secondary lesions., Results: The gross appearance of the renal tumor was that of a benign CMN. On histopathologic examination, there was a single nodule of more cellular tissue with focal necrosis. This area was diagnosed as within the designation "cellular" mesoblastic nephroma. The appearance of the brain metastasis corresponded to that of the cellular nodule., Conclusions: This report expands the spectrum of metastatic potential of CMN. Though a rare event, this case underscores the need for close follow-up of infants with CMN.

Background: Congenital mesoblastic nephroma (CMN) is a rare tumor of the neonatal kidney. It was once thought to be a variant of Wilms tumor that also arises from primitive renal cells., Methods: Molecular characteristics of two CMN were studied to clarify their potential relationship to Wilms tumors. Patterns of gene expression were assayed by Northern blot hybridization analysis. Tumors were tested for loss of heterozygosity (LOH) at chromosomes 11p13 and 11p15 using Southern blot analysis., Results: The CMN, like Wilms tumors, demonstrated high-level expression of insulin-like growth factor II. Unlike Wilms tumors, however, the CMN expressed neither the N-myc oncogene nor the putative Wilms tumor suppressor gene, WT1. Using a panel of probes spanning 11p13 and 11p15, no LOH was detected in the CMN, nor was there evidence of deletion or rearrangements of WT1., Conclusions: Although Wilms tumor and CMN both arise from the developing kidney, molecular characterization suggests that different factors are involved in the pathogenesis of these two tumors.

This study reported 10 cases of congenital mesoblastic nephroma (CMN) from Children's Hospital of Pittsburgh: five were the typical fibromatous lesion, two were the cellular variant, and three had both components. Six cases occurred in infants less than one week of age. The degree of cellularity had no correlation with patient or tumor size. Three- to 10-year follow-ups of nine patients showed neither local recurrence nor metastasis. One infant died of generalized bleeding post nephrectomy. Ultrastructural study of seven cases showed mesenchymal cells with varying degrees of fibroblastic and/or myofibroblastic differentiation.