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There are several classification systems that are used to determine the severity of myelodysplastic syndromes (MDS). The first classification model was developed in 1982 by a group of physicians called the French-American-British [FAB] Cooperative Group. Using this model, the primary criterion for classifying MDS is the percentage of blasts (early or embryonic leukemic cells)in the bone marrow. The presence of more than 15% ringed sideroblasts for marrow with less than 5% blasts is an important indicator for classification. Sideroblasts are red blood cells that contain iron. Ringed sideroblasts are an abnormality.

The FAB classification further characterizes MDS into 5 distinct types:

• RA (refractory anemia)
• RARS (refractory anemia with ringed sideroblasts)
• RAEB (refractory anemia with excess blasts)
• RAEB-T (refractory anemia with excess blasts in transformation)
• CMML (chronic myelomonocytic leukemia)

The main difference between the classifications is the cells that are affected. In RA and RARS, bone marrow cells that develop into red blood cells are negatively affected. In RAEB and RAEB-T, there is an excessive number of myeloblasts (cells that develop into white blood cells). In CCML, there is an excessive production of blood monocytes by myeloblasts.

There has been some controversy surrounding the FAB classification of defining chronic myelomonocytic leukemia (CMML) as a type of MDS. In 1999, the World Health Organization (WHO) proposed classification changes. In the WHO classification, CMML is considered to be a myelodysplastic/myeloproliferative disease and is not associated with MDS. It also does not include the RAEB-T classification.

The WHO Classification of myelodysplastic syndromes is as follows:

• RA
• RARS
• Refractory cytopenia with multilineage dysplasia
• RAEB
• 5q- syndrome (interstitial deletion on the long arm of chromosome 5)
• MDS, unclassifiable

Some members of the medical community have criticized the WHO classification modifications for perceived deficiencies in clinical and prognostic relevance.

The most recent system for determining the severity of myelodysplastic syndromes that is more widely accepted is the International Prognostic Scoring System (IPSS). The IPSS helps determine a patient's prognosis and the risk of MDS developing into AML. The IPSS is the single most important guide that doctors use for predicting the outcome (prognosis) as well as for determining the course of treatment of MDS.

The IPSS provides a sum of three scores: marrow blast percentage, the degree of cytopenia (a deficiency in the numbers of blood cell elements), and karyotype (this is determined by testing for chromosomal abnormalities in bone marrow blood cells).

After scoring, the patient is assigned to one of four IPSS risk groups:

• Low (score = 0) - The median survival is 11.8 years for all patients and 5.7 for patients over the age of 60.

• Intermediate-1 [INT-1] (score = 0.5-1.0) - The median survival for all patients is 5.2 years and 3.5 years for patients over the age of 60.

• Intermediate-2 [INT-2] (score = 1.5-2.0) - The median survival for all patients is 1.8 years and 1.2 years for patients over the age of 60.

• High (score = greater than or equal to 2.5) - The median survival for all patients is 0.4 years and 0.3 for patients over the age of 60.