MDS-RS (SLD & MLD)

Myelodysplastic Syndrome with Ring Sideroblasts (MDS-RS)

2016 WHO revision effectively replaces the category of refractory anemia with ringed sideroblasts (RARS) as MDS-RS with single lineage dysplasia (MDS-RS-SLD), and is expanded to include cases of MDS with ring sideroblasts and multilineage dysaplasia (MDS-RS-MLD).

Diagnostic Criteria: MDS-RS-SLD

Single lineage erythroid dysplasia (>10% for each affected lineage)
1-2 cytopenias
Increased ring sideroblasts (≥15% or ≥5% with SB3F1 mutation, the presence which constitutes dysplasia irrespective of the presence or absence of other features of erythroid dysplasia)
<5% bone marrow blasts
<1% peripheral blood blasts
No Auer Rods
Any abnormalities, except for isolated 5q deletion (MDS with isolated 5q-)

Diagnostic Criteria: MDS-RS-MLD

Multi lineage (2-3) dysplasia (>10% for each affected lineage)
1-2 cytopenias
Increased ring sideroblasts (≥15% or ≥5% with SB3F1 mutation, the presence which constitutes dysplasia irrespective of the presence or absence of other features of erythroid dysplasia)
<5% bone marrow blasts
<1% peripheral blood blasts
No Auer Rods
Any abnormalities, except for isolated 5q deletion (MDS with isolated 5q-)

Immunohistochemistry

The use of immunohistochemistry (IHC) in many of the MDS syndromes is limited. Identifying an increased blast population is one of the most useful, and may indicate a more aggressive course or transformation to acute myelogenous leukemia (AML). Helpful IHC markers may include:

Stain	Comment
CD34	CD34 marks immature cells including myeloblasts. In the setting of AML, it is ~70% sensitive. A subset of lymphoblasts may express CD34.
CD117	CD117 is a specific myeloid marker, and marks a subset of myeloblasts. The expression is dim, and one often must look at 20-40X to clearly see expression. Mast cells (fried egg looking cell) will have very strong expression.
CD71	CD71 marks nucleated erythroid cells. This may be helpful in quantitating and differentiating erythroid cells from myeloid cells. This marker may be set-up as a double stain with CD34.
E_Cadherin	In the setting of hematopoietic cells, E-Cadherin marks immature erythroid cells. Like CD71, E-Cadherin may be useful to differentiate immature erythroid cells from immature myeloid cells.
TdT	TdT is a sensitive lymphoblast (~95%) marker. It is not entirely specific for lymphoblasts, but other markers can help clarify diagnostic difficulties (B and T-cell markers).

References

Swerdlow SH, Campo E, Harris, NL, Jaffe ES, Pileri SA, Stein H, Thiele J (Eds): WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues (Revised 4th edition). IARC: Lyon 2017