MDS after exposure to radiation or alkylating agents such as busulfan, nitrosourea, or procarbazine, typically occurs 3–7 years after exposure and frequently demonstrates loss of chromosome 5 or 7. MDS after exposure to DNA topoisomerase II inhibitors occurs after a shorter latency of only 1–3 years and can have a 11q23 translocation. Other pre-existing bone-marrow disorders such as acquired aplastic anemia following immunosuppressive treatment and Fanconi anemia can evolve into MDS.MDS is thought to arise from mutations in the multipotent bone-marrow stem cell, but the specific defects responsible for these diseases remain poorly understood. Differentiation of blood precursor cells is impaired, and a significant increase in levels of apoptotic cell death occurs in bone-marrow cells. Clonal expansion of the abnormal cells results in the production of cells that have lost the ability to differentiate. If the overall percentage of bone-marrow myeloblasts rises over a particular cutoff (20% for WHO and 30% for FAB), then transformation to acute myelogenous leukemia (AML) is said to have occurred. The progression of MDS to AML is a good example of the multistep theory of carcinogenesis in which a series of mutations occurs in an initially normal cell and transforms it into a cancer cell.Although recognition of leukemic transformation was historically important (see History), a significant proportion of the morbidity and mortality attributable to MDS results not from transformation to AML, but rather from the cytopenias seen in all MDS patients.
Stem-cell transplantation offers possible cure, with survival rates of 50% at 3 years, although older patients do poorly.Indicators of a good prognosis:
Younger age; normal or moderately reduced neutrophil or platelet counts; low blast counts in the bone marrow (< 20%) and no blasts in the blood; no Auer rods; ringed sideroblasts; normal or mixed karyotypes without complex chromosome abnormalities; and in vitro marrow culture with a nonleukemic growth pattern
Indicators of a poor prognosis:
Advanced age; severe neutropenia or thrombocytopenia; high blast count in the bone marrow (20–29%) or blasts in the blood;
Auer rods; absence of ringed sideroblasts; abnormal localization or immature granulocyte precursors in bone marrow section;
completely or mostly abnormal karyotypes, or complex marrow chromosome abnormalities and in vitro bone marrow culture with a leukemic growth pattern
Karyotype prognostic factors:

Good: normal, -Y, del(5q), del(20q)
Intermediate or variable: +8, other single or double anomalies
Poor: complex (>3 chromosomal aberrations); chromosome 7 anomaliesThe IPSS is the most commonly used tool in MDS to predict long-term outcome.Cytogenetic abnormalities can be detected by conventional cytogenetics, a FISH panel for MDS, or virtual karyotype. The best prognosis is seen with RA and RARS, where some nontransplant patients live more than a decade (typical is on the order of 3–5 years, although long-term remission is possible if a bone-marrow transplant is successful). The worst outlook is with RAEB-T, where the mean life expectancy is less than 1 year. About one-quarter of patients develop overt leukemia. The others die of complications of low blood count or unrelated diseases.