Site Of Origin

Neither the Working Formulation nor the Kiel classification acknowledged that a significant percentage of lymphomas do not arise in lymph nodes. Extranodal lymphoma accounts for some 25% of all cases in the United States,8 while in the Far East the percentage is much higher amounting to 45% in Japan and 60% in Korea.9 The site of origin of lymphomas is of considerable importance and the distribution of lymphoma types shows a markedly different bias in different sites. Thus, extranodal Hodgkin's disease is altogether rare and fol-licular lymphoma, one of the commonest nodal tumors, occurs only infrequently as a primary tumor in the gastrointestinal tract despite its high content of native lymphoid tissue.10 In some organs and/or tissues, such as the skin, gastrointestinal tract and, to a lesser extent the spleen, lymphomas specifically characteristic of that site alone occur. Examples include cutaneous follicle center cell lymphoma,11 enteropathy-type T-cell lymphoma,12 and splenic marginal zone lymphoma (SMZL).13 For clinical purposes it is sometimes useful to group together the lymphomas that arise in specific sites and to approach the diagnosis of lymphoma arising in those sites in this way rather than in the purest sense of an overall lymphoma classification. This is best exemplified by lymphomas arising in the gastrointestinal tract10 and the skin.11 However, the use of entirely separate classifications for lymphomas arising at different extranodal sites is to be discouraged.14

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