Extranodal lymphomas, namely, lymphomas which arise from tissue other than lymph nodes, represent at least one quarter of non-Hodgkin lymphomas (NHL).
These tumors are widely distributed throughout the body, even in sites which normally contain non lymphoid tissue. It has been noted that the incidence increases over the years in many sites, such as the central nervous system, the gastrointestinal tract and the skin. However, the most common form of extranodal lymphoma remains the gastrointestinal localization.
There are wide differences in the incidence among the countries worldwide and little is known about the incidence in developing countries.
Signs and symptoms at presentation depend on the localization. Outcome depends on many factors, some of which are the site of presentation, the histologic subtype, the age and the performance status. Age is a key factor and the incidence of the different sites varies accordingly. Some examples are testis and thyroid, which are more frequent in elderly patients, while hepatic and intestinal lymphomas usually appear at a younger age.
An improved understanding of the origin of primary extranodal lymphomas led to a first attempt to classify them in the REAL classification. Their role was better defined in the more recent WHO classifications.
Extranodal lymphomas are, therefore, a separate, very heterogeneous group of entities which differ greatly in their behavior and outcome, and therefore, require special attention in future clinical trials aiming to establish an adequate therapeutic approach.