Flow cytometry:ETP-ALL

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The ETP-ALL/LBL To diagnose acute undifferentiated leukaemia the cells need to meet specific criteria, and possible alternatives should be excluded. These are given in the table below:

Requirement 1

No lineage can be assigned by lineage-defining markers of myeloid, B-lymphoid or T-lymphoid lineage Review Criteria: Myeloid B-Lymphoid T-Lymphoid

Requirement 2

Ensure that other conditions that may resemble undifferentiated acute leukaemia are excluded: particularly consider whether marker patterns suggest any of the following: acute megakaryocytic leukaemia Non haematopoietic tumours Blastic plasma dendritic cell neoplasm Plasma cell leukaemia

Requirement 3

Consider whether the case may be better described as Acute Leukaemia of Ambiguous Lineage - not otherwise specified (ALAL-NOS) - see guidance Acute leukaemia of ambiguous lineage, not otherwise specified

The immunophenotype of ETP-ALL requires careful consideration – this reflects that the condition was identified using gene expression profiling rather than by immunophenotype. Using immunophenotype to establish the diagnosis is therefore challenging and may underestimate the number of true cases.* and may not be easily separated from ALAL T/my

Or T-ALL

The approach presently advocated by WHO requires: 1. Required expression: Cytoplasmic CD3 (may be heterogenous but should be expressed by ≥25% of blasts) 2. Required expression: One or more myeloid antigen (CD11b, CD13, CD33, CD65, CD117) and/or stem cell antigens (CD34, HLA-DR) 3. Required absence: CD3, CD1a and CD8 (<5% of blasts) 4. Required absent/dim CD5 expression (<75% positive blasts)

Note that CD7 is consistently positive in ETP-ALL and does not count as a stem cell antigen in this context

• CD4 may have the same pattern

The ETP-ALL immunophenotype may also be established by immunohistochemistry.

• T-ALL cases that have an immunophenotype similar to ETP-ALL but where CD5 is expressed (≥75% of blasts) may be designated as “near-ETP-ALL”, but the clinical implications of such a designation remain unclear.

Distinction from mixed-phenotype acute leukaemia (MPAL) requires that MPO is not expressed - in this context of ETP-ALL the WHO advocate a threshold of <3% to define negative myeloperoxidase expression (by cytochemistry or flow cytometry). This threshold is different from that of T/myeloid MPAL so may not be optimal but is retained at present.