CHILDHOOD Biphenotypic ACUTE LEUKEMIA with High Incidence of Basophilic Differentiation: The CONTRIBUTION of Ultrastructural Methods to Diagnosis.

Abstract 4698

The diagnosis of Biphenotypic Acute Leukemia (BAL) is still a challenge in clinical hematology. In spite of the use of the score formulation of the European Group for the Immunological Classification of Leukemias (EGIL) there are still cases difficult to define. We assessed the contribution of ultrastructural morphology and cytochemistry to the diagnosis of BAL in pediatric leukemic patients. Twenty six patients diagnosed at Schneider Children's Medical Center of Israel between the years 1989 to 2004 have been classified as BAL in a retrospective analysis evaluated by various combinations of light-microscopy (LM) morphology, immunophenotype flow-cytometry, cytogenetics and electron-microscopy (EM) including myeloperoxidase (MPO) and platelet peroxidase (PPO) reaction. The identification of myeloid features by EM was based on the presence of 1) MPO positive granules and MPO reactivity in the nuclear membrane, profiles of endoplasmic reticulum (ER) and in the membranes of the Golgi system; 2) PPO reaction in the nuclear membrane and profiles of ER, and 3) the presence of primary basophilic granules in the cytoplasm of the blasts. The EGIL scoring system has been used with the addition of the ultrastructural findings for the definition of BAL. In 24 cases the morphologic appearance and cytochemistry by LM of the leukemic cells were that of undifferentiated blasts. By cytogenetic evaluation abnormal karyotypes were detected in 16 patients, normal in 4 and unknown in 6 patients. Out of 26 patients 15 had the combination of T-ALL with myeloid phenotype and 11 had the B-lineage with myeloid features. Unexpectedly the blasts of 9 of the 26 BAL patients had basophilic differentiation as indicated by the presence of typical primary basophilic granules with MPO reactivity in a scattered pattern as previously reported in basophylic leukemia. Interestingly 7 of these 9 cases had a T lymphoid component. By contrast among 15 patients who had FAB M0, 9 had the B lineage markers. The remaining 2 patients had M1 and M7 features with T lymphoid phenotype. The contribution of EM studies enabled to establish the diagnosis of BAL in 11 out of 26 patients, in other 6 patients it allowed for the refinement of the AML subgroup such as basophilic leukemia and in the remaining 9 patients EM confirmed the diagnosis of BAL as defined by flow cytometry.