Extranodal lymphomas

The assessment of extranodal lymphomas poses considerable challenges to general surgical histopathologists and specialist haematopathologists alike. The breadth of lymphoma subtypes that may occur at extranodal sites, the variation in morphology, immunophenotype, genetics and clinical features within and between these entities, the difficulties in distinguishing these lymphomas from reactive lymphoid infiltrates or other neoplasms, and the different clinical significance of lymphomas with primary versus secondary involvement of extranodal sites, all contribute to these challenges. In this review, I will discuss several lymphomas defined by extranodal origin (in particular extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue/MALT lymphoma, and enteropathy-associated T cell lymphoma) and aspects of selected others likely to be encountered at extranodal sites, with an emphasis on practical histopathological assessment. Lymphomas of the spleen, skin, bone marrow and thymus are not covered, as these are comprehensively reviewed in companion articles.