INTRODUCTION

Lymphomas, a heterogeneous group of malignancies arising in the lymphoid tissue, account for over 3% of cancers occurring worldwide. Most lymphomas are B-cell in origin, with a minority being T-cell. These cancers are primarily divided into Hodgkin's (HL) and non-Hodgkin's lymphomas (NHL), where HLs are B-cell malignancies distinguishable by the presence of Reed–Sternberg cells, and NHLs are of either B- or T-cell origin. A few inherited disorders, immunosuppressive drug therapies and certain viruses are known to be associated with specific types of lymphoma. However, for the most part, little is currently known about the etiology of lymphomas. The heterogeneous nature and inconsistent definitions of the specific lymphomas has hindered the identification of potential risk factors, but with the introduction of the Revised European–American Lymphoma (REAL) classification in 1994 and its 2001 successor, the World Health Organization Classification of Tumours of the Haematopoietic and Lymphoid Tissues, lymphomas are more consistently segregated on the basis of morphology, immunophenotype, and genetic and clinical features.

DESCRIPTIVE EPIDEMIOLOGY

With a view to elucidating potential causes of disease, descriptive epidemiological studies are routinely concerned with measures of disease incidence, prevalence, mortality and survival in well-defined populations and/or subgroups. For cancer, disease occurrence is commonly estimated from national, or specialist, cancer registries and the “population at risk” of disease from national, or local, census data.