Extranodal hematopoietic/lymphoid disorders. An introduction.

Extranodal lymphomas are increasing in frequency owing in part to a true increase, but also as a consequence of AIDS and the recognition by pathologists that many cases formerly regarded as extranodal lymphoid hyperplasias or as pseudolymphomas are low-grade lymphomas of mucosa-associated lymphoid tissue (MALT) type. However, most primary extranodal lymphomas are aggressive lymphomas, usually of large B-cell type. Extranodal T-cell lymphomas also occur, such as the NK/T-cell lymphomas of the sinonasal area, the subcutaneous panniculitic T-cell lymphomas, and anaplastic large cell lymphomas. The aggressive lymphomas need to be distinguished from extranodal myeloid leukemias, whereas the low-grade lymphomas of MALT need to be distinguished from other extranodal lymphomas composed of small lymphocytes, such as mantle cell and follicular lymphomas. MALT lymphomas also must be differentiated from the various forms of extranodal lymphoid hyperplasia. Caution is required when determining this differentiation, since the discovery of a clonal cell population in a clinically and pathologically innocuous extranodal lymphocytic infiltrate does not necessarily equate with a diagnosis of malignant lymphoma. In this setting, a more appropriate designation might be a neoplasm or disorder of "uncertain malignant potential."