Malignant rhabdoid tumour of soft tissue. An ultrastructural and immunohistological study of a pelvic tumour.

A case of extrarenal malignant rhabdoid sarcoma arising in the pelvic soft tissues of a 12-year-old girl is described. By routine light microscopy the tumour resembled, in some areas, an embryonal rhabdomyosarcoma and, in other areas, a neuroblastoma. Electron microscopy revealed characteristic cytoplasmic aggregates of intermediate filaments, often with central clusters of organelle membranes surrounded by these filaments. Immunohistochemical stains showed strong cytoplasmic reactivity for vimentin. Staining for cytokeratin, myoglobin, desmin, neurofilaments, neurone specific enolase, S-100 protein and leucocyte common antigen was negative. A histogenetic origin from primitive mesenchymal cells is favoured. We strongly support the use of electron microscopy for the definitive diagnosis of small round cell undifferentiated sarcomas of childhood.