kidney cancer mesoblastic nephroma A solid, unencapsulated tumor of the kidney composed of spindle mesenchymal cells that resemble fibroblasts or muscle cells. The homogeneous mass typically extends into the renal parenchyma and replaces most of the kidney. In most cases, mesoblastic nephroma is benign and occurs in the fetus or newborn, and rarely in the older child or the adult. DOID:4772 ICD9:236.99 MESH:D018201 MONDO:0003130 SCTID:307604008 UMLS:C0206628 mesoblastic nephroma mesoblastic nephroma (morphologic abnormality)