Introduction

The T-cell non-Hodgkin lymphomas are a group of uncommon malignancies that in Western countries account for 15–20% of aggressive lymphomas and between 5% and 10% of all non-Hodgkin's lymphomas (NHL). Within the most current classification schemata, there are 21 distinct diseases that together constitute the mature T- and natural killer (NK)-cell lymphomas, diseases that range from indolent cutaneous lymphomas to aggressive malignancies that are often resistant to routine systemic therapy. Common to many of the types of T-cell lymphoma is a predilection towards extranodal disease, although sites of tropism vary among subtypes. Geographic frequencies of many subtypes, and of T-cell lymphomas as a group, are also highly variable: while only 1.5% of lymphomas are of T-cell lineage in Vancouver, Canada, 18.3% of lymphomas in Hong Kong show a T-cell phenotype. And in Asia, 47.4% of T-cell lymphomas are either NK/T-cell lymphoma, nasal type (NK/T-NT) or adult T-cell leukemia/lymphoma (ATLL), while in North America and Europe, these constitute only 7.1% and 5.3% of T-cell lymphomas, respectively. These variable incidence rates may in part represent differential exposure to risk factors for T-cell lymphoma, including Epstein–Barr virus (EBV) and human T-cell leukemia virus-1 (HTLV-1). As will be explored below, the epidemiologic variety across the subtypes of T-cell lymphoma is matched by the clinical heterogeneity of these diseases, confounding investigation (or understanding) of the class of diseases as a single entity.