Malignant rhabdoid tumor of the central nervous system.

Originally described and most frequently reported in association with the kidney, the malignant rhabdoid tumor (MRT) is a highly aggressive neoplasm with distinctive morphologic features. Extrarenal sites reported for this neoplasm include the liver, thymus, and various soft tissue sites. Young infants are affected with rare exceptions. We report the case of a 3-month-old boy who presented with hyperirritability and increasing head size over several weeks. The patient died following a two-week hospital stay marked by development of seizures, paralysis, and apnea. At autopsy, significant findings were limited to the central nervous system. The subarachnoid space contained neoplasm throughout, with multiple areas of parenchymal invasion. A predominating intraparenchymal mass was present in the inferior cerebellum contiguous with the neoplasm in the subarachnoid space and probably represented the site of origin. Microscopically, the neoplasm was composed of a highly cellular monomorphic population of polygonal cells with roughly ovoid vesicular nuclei and conspicuous nucleoli. Variable amounts of cytoplasm were present, and many cells contained a single, well-demarcated eosinophilic hyaline globule adjacent to the nucleus. Ultrastructurally, the cytoplasmic globules were composed of whorled aggregates of intermediate filaments. Immunoperoxidase studies confirmed that the filaments were composed, at least in part, of vimentin. The morphologic and immunohistochemical features are diagnostic of MRT, an entity of unknown histogenesis that has not been reported previously as a primary neoplasm of the CNS.