Malignant rhabdoid tumor of the pelvis.

An extrarenal malignant rhabdoid tumor (MRT) of the pelvis in a 14-year-old girl is described. The location of the tumor and age of the patient appear to be unique. The neoplasm is histologically and ultrastructurally identical to malignant rhabdoid tumor of the kidney seen in infants and very young children. The tumor had an aggressive clinical course, and the patient died with disseminated disease 13 months after diagnosis. The authors review previous reports of extrarenal MRT, discuss the occurrence of cytoplasmic filamentous inclusions, and contrast MRT with other pelvic sarcomas. Although the exact histogenesis is unknown, ultrastructural and immunohistochemical data suggest a mesenchymal origin.