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Background: Biphenotypic acute leukemia (BAL) is a rare type of acute leukemia that presents with a high degree of heterogeneity and is not well defined. Methods: We identified 51 cases (3%) of BAL from 1,693 newly diagnosed acute leukemia patients according to the EGIL scoring system. The immunophenotyping, cytogenetics, treatment, and outcome of 39 BAL patients were retrospectively analyzed. Results: There were 23 (59%) cases of the myeloid and B-lymphoid phenotype, 14 (36%) cases of the myeloid and T-lymphoid phenotype, and 1 case (3%) of the trilineage phenotype or B/T phenotype. Abnormal karyotypes were detected in 76% of the 37 validated patients and displayed a high degree of heterogeneity. Combined regimens for both acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL), as well as ALL type regimens, appeared to achieve a better complete remission rate than AML type regimens (71 and 64 vs. 33%, respectively). BAL patients with complex karyotypes or a rearrangement of chromosome 11 had a significantly reduced survival rate in comparison to patients with normal, t(8; 21), or t(9; 22) karyotypes. The probability of overall survival and disease-free survival at 2 years was 26 and 18%, respectively. Conclusions: Our findings indicate that BAL shows a high incidence of abnormal karyotypes and a poor prognosis. Combined-type regimens or ALL-based protocols are effective for the treatment of BAL.

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