Ultrastructure of malignant rhabdoid tumor of the kidney. A distinctive renal tumor of children

An unusual and highly malignant childhood renal tumor has been noted among the specimens of the National Wilms' Tumor Study. Prominent nucleoli, PAS positive cytoplasmic inclusions, and light microscopic features suggestive of rhabdomyoblastic differentiation are hallmarks of this tumor. Ultrastructural examination of 11 specimens revealed the filamentous nature of the cytoplasmic inclusions, some features of epithelial cells, but no diagnostic rhabdomyoblastic elements. The similarity of the filamentous cytoplasmic inclusions to those of some APUD tumors may suggest a neural crest origin of the renal tumor. Alternatively the filamentous masses may have no histogenetic significance but may serve as a useful marker for this prognostically unfavorable tumor. Because of its striking light microscopic resemblance to rhabdomyosarcoma but lack of acceptable rhabdomyoblastic ultrastructural features, we refer to this tumor as malignant rhabdoid tumor of the kidney.