Subtypes and Behavior: Understanding Your MDS Category

MDS is not a single disease, but a collection of different blood cancers that behave in very different ways. In the past, doctors grouped these based almost entirely on how your cells looked under a microscope. Today, the focus has shifted toward molecular classification—understanding the specific genetic “glitches” that drive your unique case ^[1][2].

Because the field is moving so fast, there are currently two major classification systems used by pathologists: the WHO 2022 and the ICC 2022 ^[3].

The “Blast” Percentage: MDS vs. AML

One of the most important numbers in your pathology report is the blast percentage. Blasts are immature, “baby” blood cells that have not matured. In a healthy person, they make up less than 5% of the bone marrow.

• Traditional Rule: Historically, if you had 20% or more blasts, you were diagnosed with Acute Myeloid Leukemia (AML).
• The WHO 2022 Update: This system generally keeps the 20% rule but will classify a patient as having AML regardless of the blast count if they have specific, high-risk genetic mutations ^[4][5].
• The ICC 2022 Update: This system lowered the AML threshold to 10% blasts if certain genetic markers are present ^[6]. It also created a new category called MDS/AML for patients with 10% to 19% blasts, acknowledging that these patients often need more intensive, AML-like treatment ^[7].

Behavior of Common Subtypes

Your subtype tells your doctor how the disease is likely to behave and which treatments might work best.

1. MDS with SF3B1 Mutation (Ring Sideroblasts)

This subtype is defined by a mutation in the SF3B1 gene ^[8].

• What it looks like: Under a microscope, your red blood cells may show a “ring” of iron, called a ring sideroblast.
• Behavior: This is generally considered a more favorable subtype that moves slowly ^[9]. The primary challenge is usually chronic, long-term anemia rather than rapid progression to leukemia ^[8].

2. MDS with del(5q)

This subtype occurs when a small piece of chromosome 5 is missing (deleted) ^[10].

• Behavior: It often causes severe anemia and a specific type of abnormal platelet cell ^[10].
• Treatment: This subtype is famous for being highly responsive to a drug called lenalidomide, which can often stop the need for blood transfusions for a long time ^[11].

Staging and Risk Scores

To determine your “stage” or “grade,” doctors use the International Prognostic Scoring System (IPSS).

• IPSS-R (Revised): The older standard, which uses blood counts, blast percentage, and chromosome changes ^[3].
• IPSS-M (Molecular): The new gold standard, which integrates your specific gene mutations ^[3].

The IPSS-M is much more precise. It can often identify that a patient who was previously labeled “low risk” actually has a higher-risk genetic profile, or vice versa, allowing for a much more tailored treatment plan ^[12].