ATYPICAL TERATOID RHABDOID TUMOR OF THE CENTRAL-NERVOUS-SYSTEM - A HIGHLY MALIGNANT-TUMOR OF INFANCY AND CHILDHOOD FREQUENTLY MISTAKEN FOR MEDULLOBLASTOMA - A PEDIATRIC-ONCOLOGY-GROUP STUDY/

Fifty-five patients with atypical teratoid/rhabdoid tumors of the cent ral nervous system were studied to define the clinical and pathologic features of this newly described neoplasm. The lesion occurred primari ly in children younger than 2 (mean age at diagnosis, 17 months). The neoplasms were located in the posterior fossa (36 patients) and the su pratentorial compartment (17 patients) or were multifocal in both comp artments (2 patients) at presentation. Histologically, the tumors were composed of small cells and large, pale cells in a jumbled architectu ral arrangement. The small cell component resembled medulloblastoma an d occasionally had cords of cells in a mucinous background, simulating chordoma. The cytoplasm of the larger cells was conspicuous with a so mewhat ''rhabdoid'' appearance, although rhabdoid features were not al ways prominent. Epithelioid features in the form of poorly formed glan ds or Flexner-Wintersteiner rosettes were noted in a minority of lesio ns. The neoplasms showed striking polyphenotypic immunoreactivity, inc luding that for vimentin, glial fibrillary acidic protein, epithelial membrane antigen, cytokeratins, synaptophysin, chromogranin, and smoot h muscle actin. Using a probe for chromosome 22, seven of eight scorab le cases showed a solitary signal by fluorescence in situ hybridizatio n (FISH) consistent with monosomy 22. The eighth scorable case showed three signals by fluorescence in situ hybridization and had a transloc ation involving chromosome 22 reported by conventional cytogenetics. I n contrast to patients with medulloblastoma, the neoplasm with which t hese lesions are often confused, the outcome of the patients was unifo rmly poor. The mean postoperative survival of patients with atypical t eratoid/rhabdoid tumors was only 11 months. Local recurrence, seeding of the cerebrospinal fluid pathways, or both, were common terminal eve nts. This study underscores the distinctive clinical, histopathologic, immunohistochemical, and cytogenetic character of this unusually aggr essive tumor.