Page 135 - Haematologica Atlas of Hematologic Cytology

CHAPTER 15 - Acute leukemias of ambiguous lineage
Chapter 15. ACUTE LEUKEMIAS OF AMBIGUOUS LINEAGE
In recent years, the routine use of a large number of monoclonal antibodies for acute leukemia phenotyping has highlighted the existence of cases in which distinct populations of blasts of more than one lineage (myeloid and lymphoid, or B- and T-lymphoid) are present at the same time or in which there is a single population of blasts co-expressing antigens of more than one lineage. According to the WHO, in general, the term mixed-phe- notype acute leukemia (MPAL) should be used to indicate any of these conditions, replacing the previous terms ‘bilinear leukemia’ and ‘biphenotypic leukemia’ (Borowitz et al., 2017). Cases in which there is a well-defined lineage involvement with inappropriate or unexpected expression of antigens from another line should instead be considered acute leukemias with aberrant antigen expression.
Mixed-phenotype acute leukemias belong to the group of acute leukemia of ambiguous lineage, that also in- cludes acute undifferentiated leukemias which do not express lineage-specific markers (Table 1).
Table 1. World Health Organization classification of acute leukemias of ambiguous lineage.

Acute undifferentiated leukemia
Mixed-phenotype acute leukemias with gene rearrangements
Mixed-phenotype acute leukemia with t(9;22)(q34.1;q11.2); BCR-ABL1
Mixed-phenotype acute leukemia with t(v;11q23.3); KMT2A-rearranged Mixed-phenotype acute leukemia
Mixed-phenotype acute leukemia, B-cell/myeloid, not otherwise specified Mixed-phenotype acute leukemia, T-cell/myeloid, not otherwise specified Mixed-phenotype acute leukemia, not otherwise specified, rare types
Acute leukemias of ambiguous lineage, not otherwise specified
Although in MPALs with distinct populations of blasts the morphological and cytochemical features of cells may suggest the correct diagnosis, identification of acute leukemias of ambiguous lineage requires the use of a large number of lineage-specific cytochemical and immunological markers. Multiparameter flow cytometry is the method of choice to identify the co-existence of distinct blast populations or co-expression by the same cells of markers specific for different lineages. Since many immunological markers are lineage-associated rather than lineage-specific, and many, such as TdT and CD7, are frequently expressed inappropriately, there is a need to establish criteria that distinguish true MPALs (which constitute distinct biological and clinical entities with unfavorable prognosis) from cases with minimal phenotypic deviations without prognostic significance. Table 2 shows the criteria for lineage assignment proposed by the WHO group for a diagnosis of MPAL.
It should be noted that cases that can be classified by genetic or clinical features in another category of acute leukemia, as well as cases of chronic myeloid leukemia in blast crisis, acute myeloid leukemia with myelodyspla- sia-related changes, and therapy-related acute myeloid leukemia, are excluded from the MPAL group.
Table 2. Criteria for lineage assignment for a diagnosis of MPAL.
Lineage
Assignment criteria
Myeloid
Myeloperoxidase ( ow cytometry, immunohistochemistry, or cytochemistry)
or
Monocy c di eren a on (at least 2 of the following: nonspeci c esterase cytochemistry, CD11c, CD14, CD64, lysozyme)
T-cell
Strong cytoplasmic CD3 (by ow cytometry with an bodies to CD3 chain) or
Surface CD3 (rare in MPAL)
B-cell
Strong CD19 with ≥1 of the following strongly expressed: CD79a, cytoplasmic CD22, or CD10 or
Weak CD19 with ≥2 of the following strongly expressed: CD79a, cytoplasmic CD22, or CD10
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