What is the diagnostic and treatment approach for a patient with mixed phenotypic acute leukemia versus biphenotypic leukemia?

Mixed Phenotypic Acute Leukemia vs Biphenotypic Leukemia

Terminology and Classification

The term "biphenotypic acute leukemia" is outdated and has been replaced by "mixed phenotype acute leukemia" (MPAL) in the 2008 WHO classification, which remains current. 1 The terminology shift reflects more stringent diagnostic criteria that distinguish true lineage ambiguity from aberrant antigen expression. 1

MPAL encompasses two distinct patterns 2, 3:

• Bilineal MPAL: Separate blast populations of different lineages (e.g., distinct myeloid and lymphoid populations)
• Biphenotypic MPAL: Single blast population coexpressing markers of multiple lineages

Diagnostic Criteria

Lineage Assignment Requirements

For a diagnosis of MPAL, blasts must meet stringent criteria for assignment to more than one lineage using the WHO scoring system. 1

Myeloid lineage assignment requires 1, 4:

• Myeloperoxidase (MPO) expression (by flow cytometry, immunohistochemistry, or cytochemistry), OR
• At least 2 monocytic markers (NSE, CD11c, CD14, CD64, lysozyme)

B-lymphoid lineage assignment requires 1, 4:

• Strong CD19 expression with at least one of: CD79a, cytoplasmic CD22, or CD10

T-lymphoid lineage assignment requires 1, 4:

• Cytoplasmic or surface CD3

WHO-Defined MPAL Subtypes

MPAL is classified into specific categories 1:

• MPAL with t(9;22)(q34;q11.2)/BCR-ABL1 (requires exclusion of CML blast crisis)
• MPAL with KMT2A (MLL) rearrangements
• MPAL, B/myeloid, not otherwise specified
• MPAL, T/myeloid, not otherwise specified

Critical Diagnostic Pitfalls

The most important pitfall is misclassifying AML with aberrant lymphoid marker expression as MPAL. 4, 5 Many AMLs express CD7, CD56, or other lymphoid-associated antigens without meeting MPAL criteria—these remain AML and should be treated as such. 1, 4

MPO negativity does NOT exclude myeloid lineage if monocytic markers are present, but MPO-negative cases require at least 2 monocytic markers for myeloid assignment. 1, 4 Cases with only CD13 or CD33 positivity without MPO or monocytic markers are classified as ALL, not MPAL. 5

Recent evidence suggests that AML with mixed phenotype (AML-MP) represents a distinct entity from true MPAL. 6 AML-MP frequently harbors RUNX1 (44%) and TP53 (23%) mutations, shows stemness signatures, and responds poorly to ALL-directed therapy, whereas MPAL has better responses to ALL-directed treatment and may switch immunophenotype after therapy. 6

Mandatory Diagnostic Workup

When MPAL is suspected, perform the following immediately 1:

• Multiparameter flow cytometry (minimum 6 colors) on bone marrow and peripheral blood to definitively assign lineages 1, 2
• Conventional cytogenetics on bone marrow (minimum 20 metaphases analyzed) 1
• FISH or molecular testing for BCR-ABL1 and KMT2A rearrangements (mandatory for all MPAL cases) 1
• Molecular mutational analysis including FLT3-ITD, NPM1, CEBPA, TP53, and RUNX1 1, 6
• Bone marrow biopsy with immunohistochemistry if flow cytometry results are ambiguous 1

Consultation with an experienced hematopathologist is strongly recommended given the rarity of MPAL (represents <5% of acute leukemias). 1

Treatment Approach

Treatment selection for MPAL remains controversial, but specific genetic features guide therapy 1, 2:

• MPAL with BCR-ABL1: Treat with ALL-directed therapy plus tyrosine kinase inhibitors 1, 2
• MPAL with KMT2A rearrangements: Consider ALL-directed therapy, though outcomes are poor 2
• MPAL, B/myeloid or T/myeloid, NOS: Treatment choice depends on predominant lineage features and patient fitness 2, 6

Early allogeneic hematopoietic stem cell transplantation should be considered in first remission for adult patients with MPAL given poor prognosis. 2 HLA typing should be performed at diagnosis for all patients who are transplant candidates. 1

For cases meeting criteria for AML-MP (rather than true MPAL), AML-directed therapy is superior to ALL-directed therapy. 6 These patients have inferior responses to ALL regimens (HR 12.5) and rarely switch immunophenotype after treatment. 6

Key Distinction from Historical "Biphenotypic" Terminology

The shift from "biphenotypic acute leukemia" to MPAL reflects more than semantic change. 1, 5 The WHO 2008/2016 criteria eliminated cases with weak or single aberrant marker expression that were previously called biphenotypic, restricting MPAL to cases with definitive evidence of multiple lineages. 1, 3 This prevents overdiagnosis and inappropriate treatment of AML cases with minor lymphoid marker expression. 5