B Lymphoblastic Leukemia With a Novel t(11;15) (q23;q15) and Unique Burkittoid Morphologic and Immunophenotypic Findings in a 9-Year-Old Boy.

B lymphoblastic leukemia is a B progenitor cell neoplasm with a range of immature immunophenotypes and several associated cytogenetic lesions. In contrast, Burkitt leukemia/lymphoma is a mature B lymphocyte neoplasm with a characteristic germinal center immunophenotype and MYC rearrangement. With modern immunophenotyping and cytogenetic methods, the distinction between these 2 entities is seldom ambiguous. Herein, we report a case of a 9-year-old white boy with circulating leukemic cells that demonstrate morphologic overlap between Burkitt leukemia and B lymphoblastic leukemia. Flow cytometry and immunohistochemical stains demonstrated expression of sets of markers with overlap between immature and mature immunophenotypes. While the leukemic cells tested positive for terminal deoxynucleotidyl transferase (TdT), they expressed CD20, BCL6 (in a subset), and lambda-restricted surface light chain. Molecular studies confirmed a true clonal light chain rearrangement, whereas fluorescent in situ hybridization (FISH) results were negative for MYC rearrangement. Metaphase cytogenetics identified a novel gene rearrangement, t(11;15)(q23;q15), that does not involve the MLL gene. This unique cytogenetic abnormality involves the loss of INO80, an adenosine triphosphatase (ATPase) with DNA binding ability. This cytogenetic abnormality may represent a unique feature of this overlap entity of B lymphoblastic lymphoma that expresses markers of maturity and demonstrates Burkitt-like morphology. Copyright© by the American Society for Clinical Pathology (ASCP).