Pathology of Myelodysplastic Syndrome With Excess Blasts

A number of factors influence prognosis in myelodysplastic syndromes (MDS), including the number of and severity of cytopenias, extent of morphologic dysplasia, blast count, and genetic findings. The 4th edition World Health Organization (WHO) classification of MDS, updated as a revised version in 2016, recognizes several distinct MDS entities with particular clinical behavior and clinicopathologic features. ^{[1, 2]}  Bone marrow and peripheral blood blast counts are strong prognostic indicators in MDS: Patients with increased blasts at diagnosis have a poorer survival, and an increasing blast count in patients already diagnosed with MDS is often a harbinger of transformation to acute myeloid leukemia (AML). ^{[3, 4, 5, 6, 7]}  Thus, blast counts of both the blood and bone marrow have been incorporated as critical features in the WHO classification of MDS.

MDS with excess blasts ([MDS-EB], previously termed refractory anemia with excess blasts [RAEB] in the 2008 WHO Classification) is a specific MDS entity characterized by increased myeloblasts in the bone marrow and/or blood or the presence of Auer rods. The thresholds of what constitutes an increase in blasts differ in the blood and bone marrow.

The WHO classification recognizes two specific subtypes of MDS-EB—MDS-EB1 and MDS-EB2—with the latter representing the highest grade of MDS with the poorest prognosis (see the Table below). ^{[2, 8]}

Table. Criteria Used to Classify MDS Cases as MDS-EB1 or MDS-EB2. (Open Table in a new window)

The presence of either 5%-9% blasts in the bone marrow (with <5% blasts in the blood and absence of Auer rods) or 2%-4% blasts in the blood (with <10% blasts in the bone marrow and absence of Auer rods) classifies an MDS case as MDS-EB1, whereas the presence of 10%-19% blasts in the bone marrow, 5%-19% blasts in the blood, or the presence of Auer rods classifies an MDS case as MDS-EB2.

Exceptions are as follows:

• If a t(8;21)(q22;q22.1);RUNX1-RUNXT1, inv(16)(p13.1q22)/t(16:16)(p13.1;q22);CBFB-MYH11, or PML-RARA rearrangement is detected, the case should be classified as AML, even if the blast percentage falls within the MDS-EB1 or MDS-EB2 range and/or Auer rods are present.

• If the patient has a history of cytotoxic chemotherapy or radiation therapy, the case should be classified as therapy-related MDS.

See Pathology of Myelodysplastic Syndrome With Ring Sideroblasts, Chronic Anemia, Pediatric Chronic Anemia, and Acute Anemia for more detailed information on these topics.

Epidemiology

As with other types of myelodysplastic syndromes (MDS), MDS with excess blasts-1 (MDS-EB1) and MDS-EB2 tend to occur in older adults with a male predominance. In a series of unselected MDS patients, MDS-EB1 represented 15% and MDS-EB2 represented 20% of all MDS cases. ^[9] Thus, MDS-EB is a relatively common form of MDS, accounting for over one third of cases.

Clinical features

Also as with other types of MDS, patients with MDS-EB usually present with symptoms related to cytopenias. MDS-EB is often diagnosed in patients who carry a previous diagnosis of lower-grade forms of MDS (such as MDS with single lineage dysplasia [MDS-SLD], MDS with multlineage dysplasia [MDS-MLD], or MDS with ring sideroblasts [MDS-RS]), and in such cases is indicative of disease progression.

Note that a diagnosis of MDS-EB should not be made in patients who have recently received granulocyte growth factors, as these may increase blasts in both the blood and bone marrow. Clinical follow-up and, if indicated, repeat marrow and/or blood sampling should be performed in such cases to confirm a diagnosis of MDS-EB as opposed to a transient increase in blasts due to the administration of growth factor.

Obtaining an accurate blast count in myelodysplastic syndrome with excess blasts (MDS-EB) is critical, and counts should be derived from at least 200 cells in the peripheral blood and at least 500 cells in the bone marrow aspirate smears (see the images below). Counting from several different areas of the aspirate smears and from multiple slides is important, because the distribution of the blasts may vary. The blasts in MDS are often small and can be confused with lymphocytes in thickly spread or poorly stained smears. If the aspirate smears are hemodiluted due to the presence of marrow fibrosis (see below), a bone marrow blast count may performed on touch preparations from the core biopsy.

It is important to note that in the revised 2016 World Health Organization (WHO) Classification, the bone marrow blast count is always a percentage of all nucleated cells (ANC) and is no longer calculated as a percentage of the non-erythroid cells, even in cases with over 50% erythroid cells. ^{[2, 10]}  Thus, the previous acute myeloid leukemiaa (AML) category of acute erythroid leukemia ([AEL]; erythroid/myeloid type) has been eliminated in the revised classification.

Myeloid neoplasms with more than 50% bone marrow erythroid cells in which blasts comprise over 20% of the non-erythroid cells, but less than 20% of all cells, are now classified as MDS, with the vast majority of these falling into the MDS-EB category. ^[11]  In such cases, cytogenetics appears to be the most important prognostic determinant. ^[12]

Bone marrow aspirate from case of myelodysplastic syndrome with excess blasts-2 (MDS-EB2). Megakaryocytes are small with hypolobated nuclei and myeloid forms are hypogranular. Frequent small blast forms are scattered among the maturing elements

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Bone marrow aspirate from case of myelodysplastic syndrome with excess blasts-2 (MDS-EB2). An erythroblast with deeply basophilic cytoplasm can be seen in center of image; myeloblasts are smaller, with scant pale cytoplasm. A pseudo-Pelger-Huet cell is also present (center of image).

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The bone marrow biopsy in MDS-EB cases is usually hypercellular and shows variable dysplasia in one, two, or three lineages. The bone marrow biopsy exhibits architectural disorganization (see the images below) in that the normal clustering of erythroid and myeloid elements is disrupted, and erythroid elements and megakaryocytes may be inappropriately located adjacent to bone trabeculae.

Bone marrow biopsy from case of myelodysplastic syndrome with excess blasts-2 (MDS-EB2. The marrow shows marked architectural disorganization, with disruption of erythroid islands.

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CD34 immunohistochemical stain of case of rmyelodysplastic syndrome with excess blasts-2 (MDS-EB2). Myeloblasts, difficult to enumerate on routine histology, are shown to be increased by presence of numerous CD34+ mononuclear cells.

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In addition, abnormal localization of immature (myeloid) precursors (ALIP) often occurs in clusters away from their normal paratrabecular location. These can be identified on routine histology in well-prepared sections, but they are highlighted by immunohistochemical staining for CD34. ^[13]

Abnormal localization of immature precursors (ALIP) in case of myelodysplastic syndrome with excess blasts-2 (MDS-EB2) is revealed by CD34 immunostaining, showing a cluster of CD34+ blasts (center of image), located away from bone trabeculae.

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Another example of abnormal localization of immature precursors (ALIP)(cluster of CD34+ blasts in lower right corner of image), occurring in case of hypocellular myelodysplastic syndrome with excess blasts-2 (MDS-EB2)

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A minority of MDS cases (13% in one study) have a higher blast count in the blood than in the bone marrow, and most of these cases fulfill criteria for MDS-EB; affected patients have a poor survival, underscoring the importance of taking into account the peripheral blood blast count as well as the bone marrow blast percentage. ^[4]

Many cases of MDS that manifest with a fibrotic marrow represent cases of MDS-EB. If the aspirate smears are dry or nonrepresentative, the increased blasts can often be demonstrated by CD34 staining of the bone marrow core biopsy (see the next section, Immunophenotypic Features and Methods). ^[1] These fibrotic MD-EB cases exhibit moderately or markedly increased reticulin staining and usually show increased megakaryocytes, including many small forms with hypolobated nuclei. ^{[14, 15]}  CD34 immunostaining also may be useful in hypocellular MDS cases, in which excess blasts may be underrpresented in the bone marrow aspirate smears.

It is recommended that the blast percentage in myelodysplastic syndromes (MDS) be based on counting of well-prepared smears from the bone marrow aspirate and peripheral blood. Immunophenotyping is an accurate method for quantitative and qualitative evaluation of hematopoietic cells. ^[16]

Flow cytometry can not only give an estimate of the blast count based on light scatter and/or expression of precursor antigens CD34 and CD117, but it can also detect bone marrow dysplasia. ^[16] However, although the flow cytometry blast count does correlate with prognosis in MDS, ^[17]  flow cytometry blast count should not substitute for the "gold standard" of a manual count of the bone marrow aspirate and peripheral blood smears. The flow cytometry blast count may be artifactually decreased as a result of hemodilution, loss of blasts during specimen processing, lack of expression of CD34 by blasts, or unusual light scatter and CD45 expression characteristics that may cause blasts to fall outside the typical gate.

Conversely, the blast count may be artifactually elevated as a result of the loss of non-blast cells (eg, erythroid elements) during specimen processing or the presence of non-blast cells (eg, dysplastic myeloid elements) within the blast gate. Thus, in instances of noncorrelation between the morphologic and flow cytometric blast counts, the former should be considered as the standard. Of course, the pathologist should use his/her judgment, because on some occasions the aspirate smears may be very hemodiluted and, in such cases, the flow cytometry sample may be more representative.

In situations in which the bone marrow is fibrotic, a poor aspirate smear may be obtained, thereby precluding an accurate blast count. A blast count may often be obtained from a Wright-Giemsa stained touch preparation prepared from the unfixed core biopsy in such cases. However, in cases with a poor aspirate and lacking, or with, a poor touch preparation, the core biopsy is likely to be more representative. Because assessing the blast percentage in bone marrow trephine biopsies on routine histology is often difficult (particularly when numerous early myeloid precursors and erythroid elements are present), immunohistochemistry is very helpful in these situations.

CD34 immunostaining is preferable to using myeloid markers such as myeloperoxidase (MPO), CD68, lysozyme, or CD117, which stain non-blast cells. In the interpretation of a CD34 immunostain, it is important to count only cells morphologically consistent with blasts; endothelial cells stain for CD34, and sometimes dysplastic, small megakaryocytes in MDS can express CD34. However, megakaryocytic expression of CD34 does not necessarily establish malignancy; CD34+ megakaryocytes may be seen in some nonneoplastic conditions such as megaloblastic anemia. ^[18]

Another caveat is that CD34 may not stain the entire blast population or it may be entirely negative in rare MDS cases. Although it may be difficult to provide a precise blast percentage on the basis of the CD34-stained core biopsy, clusters of blasts away from bone trabeculae (ie, abnormal localization of immature precursors [ALIP]) are present in most cases of MDS with excess blasts (MDS-EB). ^[13]

About one half of cases of myelodysplastic syndrome with excess blasts (MDS-EB) show cytogenetic abnormalities, with a spectrum of changes that is in keeping with other types of MDS. Cytogenetics is an important prognostic factor within MDS-EB, as with other MDS subtypes. ^[9]  The most common abnormalities are del(5q), -7, del(7q), +8, and del(20q); about 15% of cases have a complex karyotype (≥3 cytogenetic abnormalities). ^{[9, 19]} However, nearly 50% of MDS-EB cases have a normal karyotype at diagnosis, and one study found no significant difference in the incidence of poor-risk karyotypes between MDS-EB and MDS with multilineage dysplasia (MDS-MLD). ^[9] The Revised International Prognostic Scoring System (IPSS-R) has confirmed the important prognostic role of karyotype in MDS, including MDS-EB. ^[5]  In MDS-EB affecting pediatric patients, complex karyotype (≥3 chromosomal aberrations, including at least one structural aberration) was found to be a strong predictor of adverse outcome. ^{[20, 21]}  

Relatively recent data have identified a broad spectrum of mutations in MDS-EB, including mutations in TP53, NRAS//KRAS, ASXL1, CBL, RUNX1, splicing genes including SRSF2 and SF3B1, and cohesion complex genes. ^{[22, 23, 24, 25]}  Mutations in the IDH1 and IDH2 genes, which occur in a significant subset of acute myeloid leukemia (AML) cases, occur in 14%-23% of MDS-EB cases, and they are most often seen in cases with normal karyotype. ^[26]  FLT3 and NPM1 mutations, also associated with AML, are rarely found in MDS-EB cases and may be harbingers of rapid progression to AML. ^{[27, 28, 29]}  

It is important to note that MDS cases with an isolated del(5q) cytogenetic abnormality and excess blasts must be classified as MDS-EB, but these patients may still respond to therapy with lenalidomide (a drug that is highly effective in treating MDS with isolated del(5q)). ^[30]

The differential diagnosis for myelodsysplastic syndrome (MDS) with excess blasts includes the following:

• Acute myeloid leukemia with myelodysplasia-related changes
• Acute myeloid leukemia, not otherwise categorized
• MDS associated with isolated del (5q)
• MDS, unclassifiable
• MDS with multilineage dysplasia
• Myelodysplastic/myeloproliferative neoplasms
• Erythroleukemia ^[31]

As a group, patients with myelodysplastic syndrome with excess blasts (MDS-EB) have a high risk of progression to acute myeloid leukemia (AML), which develops in up to one third of patients. The median survival is 16 months for MDS-EB1, whereas it is 9 months for MDS-EB2. ^[32]

The prognostic relevance of bone marrow blast count in MDS has been extensively validated. ^{[3, 5]}  Excess blasts in MDS are associated with a poorer prognosis that is independent of the mutation status or karyotype ^[25] ; thus, accurate counting of bone marrow and blood blasts remains highly relevant in risk-stratifying MDS, even in the current era of advanced genetic characterization.

The presence of peripheral blasts in MDS is an independent adverse factor. ^[6] Increased peripheral blood blasts in MDS-EB cases convey a particularly poor prognosis: The survival of MDS-EB2 patients with 5%-20% circulating blasts is similar to that of AML, ^{[10, 33]} and patients with greater blasts in the blood than in the marrow also have an aggressive course. ^[4]  Irrespective of blast count, Auer rods in MDS are also associated with a poor survival and a high risk of transformation to AML, similar to the more frequent cases classified as MDS-EB2 on the basis of the bone marrow blast count. ^{[33, 34]}

Although cases previously classified in the French-American-British classification (FAB) as refractory anemia with excess blasts in transformation (RAEB-T) on the basis of a bone marrow blast count of 20%-30% are included in some MDS risk stratification schemes such as the revised International Prognostic Scoring System (IPSS-R), ^[5]  they are considered to represent AML in the revised World Health Organization (WHO) Classification. These patients appear to have a survival that is worse than that of patients with MDS-EB2, ^[33]  although one study found that they have a more favorable survival compared to AML patients with more than 30% bone marrow blasts when presenting with de novo disease. ^[35]