World Health Organization Classification of Tumours: Pathology and Genetics of Tumours of Haematopoietic and Lymphoid Tissues

Abstract

In the introduction to this remarkable and important publication it states that the World Health Organization (WHO) classification of tumours of haematopoietic and lymphoid tissuesis a collaborative project of the European Association of Haematopathology and the Society for Haematopathology. On the basis of the principles defined in the REAL classification of lymphoid neoplasms, a stearing group composed of members of both societies and 10 disease related committees, altogether more than 50 pathologists and more than 40 clinicians from all over the world, developed and agreed to accept the WHO as the standard classification for haematological neoplasms. Thus, the authors can claim that this classification represents the first truely world-wide consensus classification. Much has been written about the REAL classification of lymphoid malignancies which is based upon morphology, immunophenotype, genetics and clinical features. The reviewer believes that this was a useful and practical classification, for both pathologists and clinicians. In the WHO classification, the REAL classification has been modified and extended. Provisional entities of the REAL classification are now definite entities. Peripheral B and T neoplasms are now ‘mature’ neoplasms. New entities have been included, e.g. intravascular large B-cell lymphoma, aggressive NK-cell leukaemia and nodular lymphocyte predominant Hodgkin lymphoma. The strategy of classifying lymphoid neoplasms has also been applied to myeloid, as well as histiocytic/dendritic and mastocytic disorders. The principle criterion for classification is cell lineage. This implies knowledge about the cell of origin which is unknown in most lymphoid malignancies; instead, the stage of differentiation of the malignant cell at the time of diagnosis is used for classification. The situation is different in myeloid neoplasms, all of which are assumed to originate in a pluripotent haematopoietic stem cell, and it is the type of genetic defect which determines the maturation and differentiation potential of the affected cell. In contrast to many lymphoid malignancies in which immunophenotyping and to alesser degree genetics are indispensible for correct diagnosis, morphology is still the main diagnostic criterion for the classification of myeloid malignancies. Immunophenotyping is certainly needed to diagnose minimally differentiated acute myeloid leukaemia, monoblastic and megakaryoblastic acute leukaemias. However, the identification of typical chronic myeloid leukaemia, 5q syndrome or acute myeloid leukemia with t(8;21), t(15;17) and inv(16q) does not usually need chromosome analysis, but can be predicted by careful inspection of blood and bone marrow smears. Above all, and again in contrast to lymphoid malignancies, a new classification of myeloid neoplasms using immunophenotyping and genetics does not currently offer promising specific therapeutic strategies, except for acute promyelocytic leukaemia. Thus, the reviewer is sceptical about whether the WHO classification represents a convincing advantage over the old FAB classification for myeloid dysplasias and acute myeloid leukaemias. However, it is reasonable to exclude chronic myelomonocytic leukaemia from myelodysplasias and to create a new subgroup for myelodysplastic/myeloproliferative diseases comprising chronic and juvenile myelomonocytic leukaemias. Whether one appreciates the new WHO classification of myeloid disorders or not, this book is highly recommendedto all oncologists who take care of patients with tumours of lymphoid and haemotopoietic tissues, particularly those colleagues who are involved in cytological or histological diagnoses of these diseases. The book is carefully edited, concisely written, and beautifully illustrated with more than 800 colour photographs. It contains close to 1500 up-to-date references. Although written by a large number of experts the format of each chapter follows a strict order including definitions, ICD-O codes, synonyms, epidemiology, site of involvement, clinical features, prognosis and predictive factors, in addition to morphology, immunophenotyping and genetics. The book can be purchased from the IARC in Lyon, the WHO in Geneva or Oxford University Press, Oxford.