NCCN Guidelines for Patients® | Myelodysplastic Syndromes

10 NCCN Guidelines for Patients ® : Myelodysplastic Syndromes, 2018 1 Myelodysplastic syndromes Types of MDS Inside of all cells are coded instructions for building new cells and controlling how cells behave. These instructions are called genes. The instruction for different genes is stored in the body in the form of DNA ( d eoxyribo n ucleic a cid). The DNA in our bodies is organized into long strands called chromosomes. See Figure 2. Changes (mutations) in genes can cause normal cells to become cancer cells. It is not known what exactly causes the genes to change. MDS happens when changes in genes cause blood stem cells to become abnormal. This damages the blood stem cells and bone marrow so they don’t work as well as they should. The abnormal blood stem cells have trouble making enough new blood cells for the body. They also make new blood cells that are abnormal (defective). The defective blood cells are different from normal blood cells in a few key ways: † † The cells have an abnormal size, shape, or look (appearance). This is called dysplasia. † † The cells do not grow into normal, mature blood cells and do not leave the bone marrow, as they should. † † The cells may die too early in the bone marrow or soon after they enter the bloodstream. Because of this damage, the bone marrow isn’t able to make enough healthy blood cells that the body needs. The defective cells can build up and overcrowd the bone marrow. As a result, even fewer healthy blood cells may be made or survive. This leads to a low number of red blood cells, white blood cells, and/or platelets in the bloodstream. MDS may continue to get worse over time. In some cases, it may progress to a fast-growing (aggressive) cancer called AML ( a cute m yeloid l eukemia). This can happen as more and more blast cells fill up the bone marrow. About a third of patients (1 out of 3) with MDS, who have other biological factors, may develop AML. Types of MDS MDS is divided (classified) into groups based on features of the bone marrow and blood cells. These groups are also called subtypes. There are two systems that are used to classify MDS. The first one was created in 1982 and is called the FAB ( F rench- A merican- B ritish) classification system. The FAB system groups MDS into five subtypes. This system is not used much today. The newer one is called the WHO ( W orld H ealth O rganization) classification system. The 2016 Revision to the WHO system groups MDS into seven main subtypes. The WHO system also has a category called MDS/MPN ( m yelo d ysplastic s yndromes/ m yelo p roliferative n eoplasms) with four other subtypes. The WHO system is most commonly used today. Most doctors use this system to diagnose MDS and classify the different MDS subtypes. The main factors that are used to define MDS subtypes in the WHO system include: † † The type and number of low blood cell counts (cytopenias). † † Which and how many types of blood cells in the bone marrow have an abnormal size, shape, or look (dysplasia). † † The number of blast cells found in the blood and bone marrow. † † The types of chromosome changes seen in bone marrow cells.