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Pathology of Myelodysplastic Syndrome With Excess Blasts

Overview

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.
 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).
 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).

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

Criteria

MDS-EB1

MDS-EB2

1. Bone marrow aspirate blast count (≥500 cells)

5%-9%

10%-19%

2. Peripheral blood blast count (≥200 cells)

2%-4%

5%-19%

3. Auer rods

Absent

Present

 MDS = myelodysplastic syndrome; MDS-EB1, MDS-EB2 = MDS with excess blasts, subtype 1 or 2.

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.

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