Origin of a central nervous system lymphoid neoplasm in an immunocompromised host with acute lymphoblastic leukemia.

We describe a case of relapsed pediatric pre-B acute lymphoblastic leukemia (ALL) with a simultaneous presentation of an intracerebral lymphoid mass. Cytogenetic, immunophenotypic and molecular analysis (immunoglobulin heavy chain and T-cell receptor gene rearrangements) revealed that the brain neoplasm was distinct from the relapsed leukemia. We discuss the etiology of this extremely rare event, and raise issues about the clonality of lymphoid neoplasms and the behavior of hematopoietic cells within the central nervous system (CNS).