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INTRODUCTION

DEFINITION

Neoplasms of lymphocytes usually represent malignant counterparts of cells at discrete stages of normal lymphocyte differentiation. When bone marrow and peripheral blood involvement dominate the clinical picture, the disease is classified as a lymphoid leukemia. When lymph nodes and/or other extranodal sites of disease are the dominant site(s) of involvement, the tumor is called a lymphoma. The distinction between lymphoma and leukemia is sometimes blurred; for example, small lymphocytic lymphoma and chronic lymphoid leukemia are tumors of the same cell type and are distinguished arbitrarily on the basis of the absolute number of peripheral blood lymphocytes (>5 × 109/L defines leukemia).

CLASSIFICATION

Historically, lymphoid tumors have had separate pathologic classifications based on the clinical syndrome—lymphomas according to the Rappaport, Kiel, or Working Formulation systems; acute leukemias according to the French-American-British (FAB) system; Hodgkin's disease according to the Rye classification. Myelomas have generally not been subclassified by pathologic features of the neoplastic cells. The World Health Organization (WHO) has proposed a unifying classification system that brings together all lymphoid neoplasms into a single framework. Although the new system bases the definitions of disease entities on histology, genetic abnormalities, immunophenotype, and clinical features, its organization is based on cell of origin (B cell vs. T cell) and maturation stage (precursor vs. mature) of the tumor, features that are of limited value to the clinician. Table 73-1 lists the disease entities according to a more clinically useful schema based on the clinical manifestations and natural history of the diseases.

TABLE 73-1CLINICAL SCHEMA OF LYMPHOID NEOPLASMS

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