Neoplastic hematopathology

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What is B-cell acute lymphoblastic leukemia, and how is it diagnosed?

B-cell acute lymphoblastic leukemia (B-ALL) is a neoplasm of B lineage lymphoid progenitors (B lymphoblasts). It includes multiple distinct subtypes characterized by constellations of genetic alterations, including aneuploidy, chromosomal rearrangements, DNA copy number alterations, and sequence mutations. The bone marrow (BM) usually shows extensive involvement by lymphoblasts at initial presentation. Rarely the marrow can show less than 20% involvement. The peripheral blood (PB) can be variably involved, ranging from minimal involvement, detectable only by flow cytometry (FC), to extensive involvement with hyperleukocytosis. Diagnostic work-up involves morphologic evaluation, immunophenotypic analysis by FC and immunohistochemistry (IHC), cytogenetics including conventional karyotype and fluorescence in situ hybridization (FISH), and molecular studies including next-generation sequencing (NGS).

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What is the role of flow cytometry in B-ALL at diagnosis and follow-up?

Because different leukemias have different therapy regimens and prognosis but can look similar on routine morphology, the phenotype of blasts needs to be established at diagnosis. FC analysis plays a crucial role in the diagnostic work-up and subsequent detection of measurable (formerly minimal) residual disease (MRD), critical in the follow-up and management of B-ALL. Leukemic cells are positive for the B-cell associated markers CD19 and CD79a, whereas CD20 expression is variable. In addition, leukemic cells show variable expression of the immaturity markers CD34, CD10, and terminal deoxynucleotidyl transferase (TdT).

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What is the role of cytogenetics in B-ALL diagnosis and classification?

B-ALL involves many subtypes associated with distinct gene expression profiles that have variable age-related prevalence, driven by three main types of initiating genetic alteration:

• Chromosomal aneuploidy

• Rearrangements that deregulate oncogenes or encode chimeric transcription factors

• Point mutations

High hyperdiploidy (> 50 chromosomes) is present in up to 30% of childhood B-ALL and is associated with mutations in the Ras pathway, chromatin modifiers such as CREBBP, and favorable outcomes. Low hypodiploidy (31–39 chromosomes) is present in approximately 1% of children and in more than 10% of adults with B-ALL. It is characterized by the deletion of IKZF2 and by near-universal TP53 mutations, which are inherited in approximately half the cases. Near-haploidy (24–30 chromosomes) is present in approximately 2% of childhood B-ALL and is associated with Ras pathway mutations (particularly NF1 ) and deletions of IKZF3. Both low-hypodiploid and near-haploid B-ALL are associated with unfavorable outcomes. B-ALL with intrachromosomal amplification of chromosome 21 (iAMP21) is most common in older children and has poor prognosis, but outcomes have improved with intensive treatment.

Of B-ALL subtypes characterized by translocations, the most common in childhood is t(12;21)(p13;q22) encoding ETV6-RUNX1, which is typically cryptic on cytogenetic analysis and associated with favorable prognosis. The t(1;19)(q23;p13) translocation and variants encode TCF3-PBX1, which is more common in African Americans and associated with frequent central nervous system (CNS) relapse and inferior outcomes with older, but not contemporary, treatment regimens. The t(9;22)(q34;q11.2) translocation results in the formation of the Philadelphia chromosome encoding BCR-ABL1 and is found in a subset of childhood B-ALL associated with unfavorable outcomes, although prognosis has improved with combined chemotherapy and tyrosine kinase inhibition. Rearrangement of KMT2A ( MLL ) at 11q23 to more than 80 partners, most commonly t(4;11) (q21;q23) encoding KMT2AAFF1, is common in infant B-ALL and is associated with a dismal prognosis.

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What is T-cell acute lymphoblastic leukemia and T lymphoblastic lymphoma?

T lymphoblastic lymphoma (T-LBL) is the second most common type of non-Hodgkin lymphoma (NHL) in childhood and adolescence, accounting for 25% to 35% of all cases. T-LBL often affects older males with a median age of onset around 9 years old and involves mediastinal and cervical lymph nodes in the vast majority of patients. Patients commonly present with an anterior mediastinal mass arising from the thymus that can cause airway compression or superior vena cava syndrome and is frequently accompanied with pleural or pericardial effusions. About 15% to 20% of patients exhibit BM infiltration. Peripheral blood and/or BM involvement by more than 25% T lymphoblast cells is required for the diagnosis of the leukemic phase of disease, acute T-cell lymphoblastic leukemia (T-ALL). Although debate exists whether T-ALL and T-LBL are distinct entities or a spectrum of the same disease, they do share many similar molecular alterations.

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What are the pathologic features of T-LBL?

Histologically, T-LBL shows an infiltrate of small- to medium-sized blast cells. Further evaluation either by FC (of malignant effusions and/or fresh tissue) or IHC analysis of biopsies is needed to confirm the diagnosis. T-LBL cells express cytoplasmic (or less frequently membrane-bound) CD3, with consistent expression of CD7. In addition, the majority of cases are positive for TdT and variably positive for CD1a, CD34, CD10, CD2, CD4, CD5, and CD8. They are further subclassified by the differentiation stage of T lymphoblasts on their passage through the thymus (pro, pre-, cortical and medullary).

More recently, a T-ALL phenotype of very early differentiation, termed early T-precursor acute lymphoblastic leukemia (ETP-ALL), has been recognized. It occurs in 10% to 15% of patients with T-ALL and is defined by expression of CD7 and low level CD5 and absence of CD1a, CD4, and CD8 expression. ETP-ALL is associated with increased incidence of acute myeloid leukemia (AML)-type mutations rather than T-ALL/LBL-associated NOTCH1 mutations. ETP-ALL was associated with an increased rate of induction failures, but with current treatment protocols there is no significant difference in prognosis.

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