A role for fluorescence in situ hybridization detection of chromosome 22q dosage in distinguishing atypical teratoid/rhabdoid tumors from medulloblastoma/central primitive neuroectodermal tumors

It has been postulated that infants with medulloblastomas/central primitive neuroectodermal tumors (MB/PNET) may fare worse than older patients becaus e some of them harbor unrecognized atypical teratoid/rhabdoid tumors (AT/RT ), rare intracranial neoplasms that are typically unresponsive to therapy a nd rapidly fatal. Although small primitive cells are common to both entitie s, chromosome 22q11.2 deletions are common only in AT/RTs. Using fluorescen ce in situ hybridization (FISH) on archival, paraffin-embedded biopsy tissu e with commercially available probes to 22q11.2, the region associated with RTs, we studied 8 cases of AT/RT, 12 cases of MB/PNET, and 4 cases of prim itive central nervous system (CNS) neoplasms, which were difficult to class ify. 22q Deletions were identified in 6 of 8 (75%) conventional AT/RTs and 0 of 12 (0%) children with classic MS/PNET. Of the 4 originally "difficult to classify'' cases, 3 had deletions of 22q. In light of the FISH results, review of the morphology and immunophenotype resulted in 3 tumors being rec lassified as AT/RTs and 1 as a large cell MB. These 4 cases highlight the p otential diagnostic use of FISH for selected cases of primitive CNS maligna ncies in children and substantiate the notion that misdiagnosed AT/RTs may, in part account for the worse prognosis associated with "MB/PNET" in child ren younger than 2 years of age. HUM PATHOL 32:156-162. Copyright (C) 2001 by W.B. Saunders Company.