Morphology, cytogenetics and classification of MDS.

Myelodysplastic syndromes are heterogeneous bone marrow diseases with a variable pathogenetic background. Cytomorphological alterations in peripheral blood films as well as bone marrow aspirates and histological findings in trephine biopsies result from cytogenetic and molecular abnormalities, epigenetic dysregulation and immune dysfunction and are key elements for setting the diagnosis of MDS. Whereas diagnosis can be made quite easily in advanced MDS this is much more difficult in early MDS, especially in cases with cytopenias or dysplasias of uncertain significance (ICUS and IDUS). Recommendations, illustrated by case reports for a stepwise annealing to the final diagnosis and exclusion of differential diagnoses are given. Furthermore, the problem of correct counting and identification of blasts is covered and features defining dysplasia in all three cell lineages are recapitulated thoroughly. Histopathology is not mandatory but has a distinct diagnostic and prognostic value especially in cases with hypoplasia or fibrosis and when the TP53 mutational status is of relevance. In up to 70% of patients with MDS clonal chromosome abnormalities can be identified which have a high impact on setting the correct diagnosis and estimation of prognosis. Incidence, type, molecular background and clinical relevance of distinct anomalies as well as cytogenetic subgroups are presented in detail and the development of the new cytogenetic prognostic scoring system as part of the IPSS-R is explained. The value of FISH-Analysis as a complementary tool for chromosome analysis in MDS is demonstrated with special emphasis on the possibility to perform frequent cytogenetic monitoring by CD34-FISH examination of peripheral blood. Finally the evolution of MDS-classification systems from FAB to WHO with a critical discussion of their shortcomings like degree of dysplasia, blast thresholds, inclusion/exclusion of CMML, and the lack of dynamic information is presented.