Pathology of Refractory Anemia With Excess Blasts

Updated: Jan 31, 2016
  • Author: Robert P Hasserjian, MD; Chief Editor: Cherie H Dunphy, MD, FCAP, FASCP  more...
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The 2008 World Health Organization (WHO) classification of myelodysplastic syndromes (MDS) recognized several distinct entities with particular clinical behavior and clinicopathologic features. [1] A number of factors influence prognosis in MDS, including number of cytopenias, degree of morphologic dysplasia, blast count, and cytogenetic findings.

Blast count represents one of the most important prognostic indicators in MDS: de novo MDS cases with increased blasts have a poorer survival, and an increasing blast count in patients carrying a diagnosis of MDS is often a harbinger of transformation to acute myeloid leukemia (AML). [2]

Refractory anemia with excess blasts (RAEB) is an MDS 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 2008 WHO classification recognized 2 specific diagnostic entities within RAEB: RAEB-1 and RAEB-2, with the latter representing the highest grade of MDS with the poorest prognosis (see the Table below). [3]

Table. Criteria Used to Classify MDS Cases as RAEB-1 or RAEB-2. (Open Table in a new window)

Criteria RAEB-1 RAEB-2
1. Bone marrow aspirate blast count (of at least 500 cells) 5-9% 10-19%
2. Peripheral blood blast count (of at least 200 cells) 2-4% 5-19%
3. Auer rods Absent Present

The presence of either criterion 1 or 2 plus criterion 3 classifies an MDS case as RAEB-1, whereas the presence of either criteria 1, 2, or 3 classifies an MDS case as RAEB-2.

Exceptions are as follows:

  • If nucleated erythroid elements accpount for 50% or more of all bone marrow cells and the blasts 20% or more of the nonerythroid cells, the diagnosis is AML, even if the bone marrow blast count as a percentage of all marrow cells falls within the RAEB-1 or RAEB-2 range.
  • If a t(8;21)(q22;q22), inv(16)(p13.1q22), t(16:16)(p13.1;q22), or t(15;17)(q22;q12) abnormality is present on cytogenetic analysis, the case should be classified as AML, even if the blast percentage falls within the RAEB-1 or RAEB-2 range.
  • If the patient has a history of cytotoxic chemotherapy or radiation therapy, the case should be classified as therapy-related MDS, not RAEB-1 or RAEB-2.

Go to Pathology of Refractory Anemia With Ring Sideroblasts, Chronic Anemia, Chronic Pediatric Anemia, and Emergent Management of Acute Anemia for complete information on these topics.


Epidemiology and Clinical Features


Like other types of myelodysplastic syndromes (MDS), refractory anemia with excess blasts-1 (RAEB-1) and RAEB-2 tend to occur in older adults with a male predominance. In a series of unselected MDS patients, RAEB-1 represented 15% and RAEB-2 represented 20% of all MDS cases. [4] Thus, RAEB is a relatively common form of MDS, accounting for over one third of cases.

Clinical features

As with other types of MDS, patients with RAEB usually present with symptoms related to 1 or more cytopenias. RAEB is often diagnosed in patients who carry a diagnosis of lower-grade forms of MDS (such as refractory cytopenia with unilineage dysplasia [RCUD] or refractory cytopenia with multilineage dysplasia [RCMD]) and in such cases is indicative of disease progression.

A diagnosis of RAEB 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 before rendering a diagnosis of RAEB.


Morphologic Features

The marrow in refractory anemia with excess blasts (RAEB) cases is usually hypercellular and shows variable dysplasia in 1, 2, or 3 lineages. The bone marrow biopsy exhibits architectural disorganization (see the images below): 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 sy 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.
CD34 immunohistochemical stain of case of rmyelody 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.

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 are highlighted by immunohistochemical staining for CD34. [5]

Abnormal localization of immature precursors (ALIP 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.
Another example of abnormal localization of immatu 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)

Obtaining an accurate blast count in RAEB 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 RAEB may be small and can be confused with lymphocytes in thickly spread or poorly stained smears.

Bone marrow aspirate from case of myelodysplastic 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
Bone marrow aspirate from case of myelodysplastic 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).

Only 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 RAEB-1 or RAEB-2; these 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. [6]

Most cases of MDS that manifest with a fibrotic marrow represent cases of RAEB, and 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 Immunophenotypic Features and Methods). [1] These cases exhibit markedly increased reticulin staining with coarse fibers, often show collagen fibrosis on trichrome stain, and usually show increased megakaryocytes, including many small forms with hypolobated nuclei. [7, 8]


Immunophenotypic Features and Methods

It is recommended that the blast percentage be based on counting of well-prepared smears (bone marrow aspirate and peripheral blood). Flow cytometry is now performed on most bone marrow samples, evaluating for myeloid neoplasms, and can give an estimate of the blast count based on light scatter and/or expression of precursor antigens CD34 and CD117.

However, although substituting this apparently objective, quantitative measurement of blasts may be tempting, flow cytometry blast count should not substitute for a visual count of the bone marrow aspirate and peripheral blood smears. [9]

The flow cytometry blast count may artifactually decrease as a result of hemodilution, loss of blasts during specimen processing, unusual light scatter, CD45 expression characteristics that may cause blasts to fall outside the typical gate, or lack of staining of blasts for precursor antigens. Conversely, the blast count may be artifactually elevated as a result of loss of nonblasts (eg, erythroid elements) during specimen processing or the presence of nonblast 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 the criterion standard. Of course, pathologists should use their judgment, because on some occasions the aspirate smears may be very poor and the flow cytometry sample may be more representative.

In situations in which the bone marrow is fibrotic, a poor aspirate smear may be obtained, precluding an accurate blast count. In such cases, the core biopsy is likely to be more representative. Because assessing 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 may be helpful in these situations.

CD34 immunostaining is preferable to myeloid markers such as MPO, CD68, lysozyme, or CD117, which stain nonblast cells. In the interpretation of a CD34 immunostain, it is important to count only cells morphologically consistent with blasts; endothelial cells and often dysplastic, small megakaryocytes stain with CD34.

Another caveat is that CD34 may not stain the entire blast population and may be entirely negative. 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 refractory anemia with excess blasts (RAEB). [5]


Molecular/Genetic Features and Methods

About one half of cases of refractory anemia with excess blasts (RAEB) show cytogenetic abnormalities, with a spectrum of changes that is in keeping with other types of myelodysplastic syndromes (MDS). The most common abnormalities are del(5q), -7, del(7q), +8, and del(20q); about 15% of cases have a complex karyotype (3 or more cytogenetic abnormalities). [4, 10] However, nearly 50% of RAEB cases have a normal karyotype at diagnosis. Mutations in isocitrate dehydrogenase-1 and -2 (IDH1 and IDH2) genes, which occur in a significant subset of acute myeloid leukemia cases, occur in 14-23% of RAEB cases and are most often seen in cases with normal karyotype. [11]

Although RAEB is the most clinically aggressive MDS subtype and likely represents progression from MDS cases without excess blasts, 1 study found no significant difference in the incidence of MDS International Prognostic Scoring System (IPSS) poor-risk karyotypes between RAEB and refractory cytopenia with multilineage dysplasia (RCMD). [4] However, among RAEB cases, cytogenetics is a predictive factor: patients with an IPSS favorable karyotype have better survival than patients with an intermediate- or poor-risk karyotype. [4] .

In RAEB affecting pediatric patients, complex karyotype (at least 3 chromosomal aberrations, including at least one structural aberration) was found to be a strong predictor of adverse outcome. [12, 13] A revised international prognostic scoring system (IPSS-R) has recently been published that confirms the important prognostic role of karyotype in MDS, including RAEB. [14]

Note that MDS cases with a del(5q) cytogenetic abnormality and excess blasts must be classified as RAEB, but they may still respond to therapy with lenalidomide (a drug that is highly effective in treating MDS with isolated del(5q)). [15]

Malcovati et al performed a comprehensive mutation analysis in 293 patients with myeloid neoplasm and 1% or more ring sideroblasts. The study concluded that SF3B1 mutation identifies a distinct myelodysplastic syndrome subtype that is unlikely to develop detrimental subclonal mutations and is characterized by indolent clinical course and favorable outcome. [16]


Differential Diagnosis

The differential diagnosis includes the following:

Acute Myeloid Leukemia With Myelodysplasia-Related Features

Acute Myeloid Leukemia, Not Otherwise Categorized

Myelodysplastic Syndromes Associated With Isolated Del (5q)

Myelodysplastic Syndromes, Unclassifiable

Refractory Cytopenia With Multilineage Dysplasia

Other conditions to be considered include the following:

  • Acute erythroid leukemia
  • Myelodysplastic/myeloproliferative neoplasms

Prognosis and Predictive Factors

As a group, patients with refractory anemia with excess blasts (RAEB) 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 RAEB-1, whereas it is 9 months for RAEB-2. [17]

The prognostic relevance of bone marrow blast count in myelodysplastic syndrome (MDS) has been extensively validated. [2] Increased peripheral blood blasts in RAEB cases convey a particularly poor prognosis: The survival of RAEB-2 patients with 5-20% circulating blasts is similar to that of AML, [18] and patients with greater blasts in the blood than in the marrow also have an aggressive course. [6]

Irrespective of blast count, Auer rods in MDS are associated with a poor survival and a high risk of transformation to AML, much as in cases classified as RAEB-2 on the basis of the bone marrow blast count. [18, 19]

Cases previously classified in the French-American-British classification (FAB) as RAEB in transformation (RAEB-T) on the basis of a bone marrow blast count of 20-30% are now considered to represent AML; indeed, these patients have a survival similar to that of patients with AML and worse than that of patients with RAEB-2. [18]

Although cases of acute erythroid leukemia (erythroid/myeloid type) are defined by blast count as a percentage of the nonerythroid marrow cells, in cases of RAEB, the World Health Organization (WHO) recommends that the blast count be taken as a percentage of all marrow cells, even if there is erythroid hyperplasia. [9]

However, 1 study suggested that cases of MDS with relative erythroid hyperplasia (>50% bone marrow erythroid elements) may be better stratified by blast count as a percentage of nonerythroid cells. [20] In cases of RAEB with relative erythroid hyperplasia, cytogenetics, rather than blast count, appears to be the most important prognostic determinant. [20]