Episode 116: AML Series, Pt. 2 - MDS/AML Diagnosis and Risk Stratification
In this week’s episode, we discuss the diagnostic criteria and risk stratification for myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML), building on our discussion from last week. Also, if you have not done so, please do check out our hemepath series to ensure you can more easily follow along with this conversation!
Key Components of a Bone Marrow Biopsy and their significance:
Bone marrow aspirate:
an aspirate smear is used to assess morphology and cell counts
flow cytometry is used to establish a potentially clonal abnormal immunophenotype
cytogenetics is used to look at larger chromosomal rearrangements, performed by FISH or conventional karyotyping
molecular studies (sequencing) are used to look for specific gene mutations. (cannot be performed on core biopsy since fixative for core biopsy sample denatures DNA).
Core biopsy:
Cell count and immunophenotype through immunohistochemical (IHC) staining of a specific protein like CD34 for blast counts
Morphology of the cells in the bone marrow
Cellularity of the bone marrow (overall picture of what percentage of the solid marrow sample is hematopoietic cells)
Touch imprint (touching the core biopsy to slides):
With this process, some of the cells can come off the core biopsy
This could allow for morphologic review of abnormal cells
We can potentially obtain flow or molecular studies from this sample if we are unable to get a good aspirate (depending on what fixative is used)
Notable findings on a bone marrow biopsy report in the context of myelodysplastic neoplasms:
Cellularity:
This is determined based on the percentage of fat cells to hematopoietic cells and is often 100 – age.
Dysplasia:
There are specific criteria for dysplasia in the granulocytic lineage, erythroid lineage, and megakaryocytic lineage. These are interpreted by the hematopathologist in the bone marrow biopsy and/or peripheral blood smear. While granulocytic dysplasia can be determined through the aspirate smear and peripheral blood smear, erythroid dysplasia is only diagnosed on an aspirate smear.
To call morphologic dysplasia, there must be >10% of cells in that lineage with dysplastic features.
Dysplasia is different from atypia, with the latter being a relatively benign finding. This distinction is made by hematopathologists and often discussed in tumor boards.
Morphologic dysplasia can be found as a reactive process and should not always be considered a myelodysplastic neoplasm (for example: reactive dysplasia can be seen during marrow recovery in patients who have recently received chemotherapy, copper deficiency can cause dysplastic features). Establishing clonality (as explained below), can help make the diagnosis of a myeloid neoplasm in such scenarios.
Blast count:
The gold standard to estimate blast count is the aspirate cell count with differential, which is the percentage of blasts out of 500 cells counted.
Sometimes the touch imprint from the core biopsy can be used if the marrow was very fibrotic and we are unable to get a good aspirate.
Flow cytometry is not supported by the WHO for diagnosis because there is a hemodilution process and lysis of erythroid precursors which can significantly alter the blast count.
Lastly, blast count can also be estimated by IHC on the core biopsy for a common blast marker like CD34 or CD117
Additional testing needed to make diagnosis of myelodysplastic neoplasms: molecular and cytogenetics
The key thing to know is that over 90% of MDS cases will have evidence of clonality with a cytogenetic abnormality or mutation identified on next generation sequencing (NGS).
Clonality can be established if a large chromosomal rearrangement is found on FISH or conventional karyotype, or if a somatic mutation is detected on NGS.
When interpreting NGS reports, note that Tier 1 and Tier 2 mutations are clinically significant and other Tiers have not been well described.
Prognostic scoring systems for MDS (such as IPSS-R and IPSS-M) incorporate cytogenetic and genomic findings for risk stratification (1). Link: https://mds-risk-model.com/
Diagnostic Criteria for myelodysplastic neoplasms (MDS):
To establish a diagnosis, the patient should have a cytopenia, morphologic evidence of dysplasia which is defined by >10% in that lineage, and evidence of clonality through a cytogenetic or molecular mutation.
Two main organizations have evolving diagnostic criteria: WHO and ICC. There is a significant variability between the two most contemporary classification systems which include the 2022 WHO Classification 5th edition and the 2022 International Consensus Classification (ICC) guidelines.
The WHO has defined MDS subcategories genetically and morphologically. These subcategories have prognostic relevance and are also important distinctions for clinical trial enrolment (2). Link: https://tumourclassification.iarc.who.int/login?redirecturl=%2Fchapters%2F63.
Genetic subcategories in WHO classification:
MDS with isolated del 5q
MDS with SF3B1 mutation → this is the most favorable MDS mutation
MDS with biallelic TP53 mutation (two mutations in TP53 on NGS with VAF > 10% or loss of heterozygosity meaning VAF > 50%)- bad prognosis (loss of heterozygosity lowers the total number of alleles, so higher VAF)
The morphological categories in WHO classification:
MDS IB-1 (increased blasts category 1) with 5-9% blasts
MDS IB-2 (increased blasts category 2) with 10-19% blasts
MDS-f (MDS with fibrosis)
MDS-h (hypoplastic MDS)
The ICC notably has the same genetic classifications, but the main difference is the morphological categories (3). Link: https://ashpublications.org/blood/article/140/11/1200/485730/International-Consensus-Classification-of-Myeloid
The main difference here is that there is only one increased blast category which is called MDS-EB (or excess blasts) and is defined by 5-9% blasts.
MDS with > 10% blasts and a characteristic MDS genetic (cytogenetic abnormality or somatic mutation) abnormality are defined as MDS/AML and can be essentially considered diagnostic for AML. (Note: this is the major difference, since per WHO classification, AML is diagnosed only if blast count is >20%).
So essentially patients with blast counts between 10-19% are categorized as having AML per ICC classification and MDS IB-2 per WHO classification, which has huge management implications.
Evaluation of patients with unexplained cytopenia and evidence of dysplasia with normal cytogenetics and NGS:
Rule out secondary causes. Some hematopathologists do not believe that these patients have a diagnosis of MDS.
Consider obtaining NGS on bone marrow biopsy, if already performed. Although peripheral blood NGS is adequate in most cases (all downstream mature cells from the myeloid hematopoietic stem cell that picked up the mutation would also carry the mutation), but there is an approximately 5% false negative rate (4).
Using clinical NGS platforms covering a large panel of genes, approximately 10% patients with de-novo MDS may lack detectable pathogenic mutations (one-third of these would lack a cytogenetic abnormality or increased blasts). Patients with “mutation-negative” MDS had a better overall prognosis than patients with MDS with clonal genetic abnormalities, when treated with MDS-directed therapies, based on a multi-center retrospective study (5).
Diagnostic of Acute myeloid leukemia will be made in the following scenarios:
20% blasts in the blood or marrow
Characteristic cytogenetic abnormality regardless of blast count
The ones to remember are t(8;21) and inv(16) which are core binding factor AML and t(15;17) which is characteristic for acute promyelocytic leukemia
Characteristic molecular abnormalities regardless of blast count
NPM1 mutation or
CEBPA mutations in the bZIP region
Biopsy proven myeloid sarcoma, which is a solid mass of myeloblasts (previously called chloroma)
Per ICC classification, MDS-defining cytogenetic or molecular abnormalities and 10-19% blasts are sufficient for diagnosis of MDS/AML (managed as AML).
AML risk stratification systems will be described in the next section.
References:
https://evidence.nejm.org/doi/10.1056/EVIDoa2200008
https://tumourclassification.iarc.who.int/login?redirecturl=%2Fchapters%2F63.
Arber DA, et al. International Consensus Classification of Myeloid Neoplasms and Acute Leukemias: integrating morphologic, clinical, and genomic data. Blood. 2022 Sep 15;140(11):1200-1228. doi: 10.1182/blood.2022015850. PMID: 35767897; PMCID: PMC9479031.
Shanmugam V, Parnes A, Kalyanaraman R, Morgan EA, Kim AS. Clinical utility of targeted next-generation sequencing-based screening of peripheral blood in the evaluation of cytopenias. Blood. 2019 Dec 12;134(24):2222-2225. doi: 10.1182/blood.2019001610. PMID: 31697812.
Wang SA, Ok CY, Kim AS, Lucas F, Morgan EA, Thakral B, Patel S, Nardi V, Patel KM, Weinberg OK, Hasserjian RP. Myelodysplastic syndromes with no somatic mutations detected by next-generation sequencing display similar features to myelodysplastic syndromes with detectable mutations. Am J Hematol. 2021 Nov 1;96(11):E420-E423. doi: 10.1002/ajh.26325. Epub 2021 Aug 30. PMID: 34416041.
The crew behind the magic:
Show outline: Vivek Patel
Production and hosts: Ronak Mistry, Vivek Patel, Dan Hausrath
Editing: Resonate Recordings
Shownotes: Agrima Mian
Social media management: Ronak Mistry
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