The myelodysplastic syndromes (MDS) are classified according to features of cellular morphology, etiology, and clinical presentation. The morphological classification of the MDS is largely based on the percent of myeloblasts in the bone marrow and blood, the type and degree of myeloid dysplasia, and the presence of ringed sideroblasts. The clinical classification of the MDS depends upon whether there is an identifiable etiology and whether the MDS has been treated previously.
Work on the French-American-British (FAB) classification scheme for the MDS began in the late 1970s under the direction of the French-American-British Cooperative Group (see table below). The version published in 1982 was the first diagnostic classification scheme to clearly and reproducibly distinguish MDS from acute myelogenous leukemia (AML). According to the FAB scheme, the percentage of bone marrow blasts required for the diagnosis of MDS ranges from less than 5% to as much as 29%. The FAB scheme is still frequently used by clinicians to categorize the MDS.
Several weaknesses were identified in the FAB classification of MDS. The inclusion of chronic myelomonocytic leukemia (CMML) was problematic; CMML is a disease that combines features of both MDS and chronic myeloproliferative disorders. In addition, the FAB classification did not take cytogenetic findings into account. For example, the cytogenetically defined MDS subtype del(5q) represents a distinct clinical entity.
In 1997, under the auspices of the World Health Organization (WHO), a working group of pathologists and clinicians from around the world agreed to a new cellular classification scheme for hematopoietic and lymphoid malignancies. Significant changes to the FAB classification of these malignancies were made. For the classification of MDS, the new WHO classification lowered the threshold to 20% for the number of myeloblasts required to make the diagnosis of AML. This arbitrary threshold value for blast percentage eliminated the cellular type, refractory anemia with excess blasts in transformation (RAEB-t), found in the FAB classification scheme. In the WHO cellular classification scheme, RAEB-t is no longer considered a distinct clinicopathologic entity; instead, RAEB-t is included within the broader category, AML with multilineage dysplasia, and identified as AML with multilineage dysplasia following a myelodysplastic syndrome. (Refer to the PDQ summary on Adult Acute Myeloid Leukemia Treatment for more information.)
The elimination of RAEB-t from the WHO cellular classification scheme met some resistance. Some have argued that the biology of RAEB-t is distinct from AML and should be retained as a diagnostic category of MDS.[7,8] Others have emphasized the similar prognoses and responses to treatment for RAEB-t and AML with trilineage dysplasia.[9,10] The diagnosis of AML, which is based upon a threshold of 20% bone marrow or peripheral blood myeloblasts, does not represent a therapeutic mandate. The decision to treat must include other factors, such as patient age, prior history of MDS, clinical findings, disease progression, and most importantly, patient preference, in addition to the blast count. The same factors influence treatment options for patients with 30% or more myeloblasts in the blood or marrow. (Refer to the PDQ summary on Adult Acute Myeloid Leukemia Treatment for more information.)