T-cell blast crisis in chronic myelogenous leukemia is rare. We examined three patients (ages 35 to 72 years) in whom T-cell blast crisis developed 11 to 36 months (mean, 25 months) after diagnosis of chronic myelogenous leukemia and who died 4 to 12 months (mean, 7 months) thereafter. Two patients had diffuse lymphadenopathy, and the third had marked lymphocytosis (white blood cell count 217,000/uL, with 90% circulating blasts). In all three patients, neoplastic cells had the appearance of lymphoblasts and were immunoreactive for T-cell markers by immunohistochemical or flow cytometric analysis or both. Chronic myelogenous leukemia (CML), one of the chronic myeloproliferative disorders, typically is diagnosed in an indolent phase of disease of 3 to 4 years' duration, which, after conventional therapy, is followed by an accelerated phase of 12 to 18 months' duration and a fatal, blast phase (blast crisis) of 3 to 6 months' duration.1,2 Blast crisis is defined as the presence of 30% or more blast cells in the blood or bone marrow or a focus of blast cells at an extramedullary site.3 ' 4 Blast cells in CML exhibit a myeloid phenotype in 70% of cases and a lymphoid phenotype in 30%. 4 In most lymphoid blast crises, the blast cells have LI or L2 lymphoblast morphologic features, are positive for terminal deoxynucleotidyl transferase (TdT), and have the immunophenotypic profile of pre-B cells4 Only rare cases of CML blast crisis w i t h a T-lymphoblast immunophenotype have been reported.
5-18From the Department of Pathology, Brigham and Women's Hospital, and Harvard Medical School, Boston The characteristic cytogenetic finding in CML, observed in 90% to 95% of cases, is the Ph' chromosome, representing t(9;22)(q34;qll) translocation of the abl oncogene on chromosome 9 to the breakpoint cluster region (bcr) on chromosome 22.