Introduction

The term leukemia covers a group of malignant diseases of the blood. As a whole, these constitute less than 5% of all cancers. Despite extensive studies over the last several decades, relatively little is known regarding causation. Childhood leukemias are discussed in Chapter 58.

Histology, classification and diagnosis

Leukemias have been classified traditionally according to the cytology and origin of the leukemic cells. The common types are lymphocytic and myelocytic in origin. The rarer varieties are monocytic, basophilic, eosinophilic, plasma cell, erythro- and hairy-cell leukemias. Each cell type can be further divided into two major groups: acute and chronic depending on the degree of cellular differentiation.

Modern classification is dependent on a growing number of histological, cytochemical and immunological techniques to identify specific cellular biomarkers. None the less, the classification of acute leukemia by cell type can be difficult and a number are described as stem cell. From an epidemiological point of view, it is sufficient to discuss leukemias as acute lymphocytic (ALL), chronic lymphocytic (CLL), and chronic myelocytic (CML).

In certain cases of CLL, there is a close association with lymphomas and a number of lymphomas terminate in a leukemic phase whether or not the bone marrow is initially involved.