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Malignant lymphomas—primary neoplasms of lymphoid tissue derived from lymphocytes—occur as solid tumors, usually within lymph nodes and less often in extranodal lymphoid tissues such as the tonsil, gastrointestinal tract, and spleen.
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Malignant lymphomas are classified as non-Hodgkin's lymphomas, which are derived from lymphocytes, and Hodgkin's lymphomas. Hodgkin's lymphomas have retained their eponymous designation because the cell of origin remains uncertain.
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Lymphoma versus Leukemia
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Neoplastic lymphocytes circulate (mimicking normal lymphocytes) and frequently are found widely distributed throughout the lymphoid tissues. If bone marrow involvement and circulating cells predominate or if they constitute the first recognized manifestation of the disease, the process is termed leukemia. If the proliferation dominantly affects the lymphoid tissues or if a tissue mass is the presenting feature, the process is termed lymphoma. This distinction is arbitrary, and in children the term lymphoma-leukemia is sometimes used for two reasons. The two forms may coexist and lymphoma frequently evolves toward a leukemic state (Table 29-1).
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Malignant Lymphoma versus Benign Lymphoma
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The observation that neoplastic lymphocytes circulate extensively—coupled with widespread tissue involvement in many cases—has resulted in classification of all of these conditions as malignant lymphomas. This obscures the fact that many malignant lymphomas have relatively benign biologic behavior (ie, are slow-growing and compatible with long survival). The widespread distribution of the disease in the body represents mimicry of normal lymphocyte circulation rather than metastatic potential. Neoplastic proliferation of lymphocytes confined to one area of the body without the potential for dissemination—to which the term benign may truly be applied—occurs rarely, if at all.
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Incidence of Non-Hodgkin's Lymphomas
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Leukemias and lymphomas, including Hodgkin's lymphoma, account for approximately 8% of all malignant neoplasms and together represent the sixth most common type of cancer. Leukemia-lymphoma is the most common malignant neoplasm of children in the United States. About 25,000 cases of non-Hodgkin's lymphoma occur annually in the United States. The relative incidence of the subtypes of lymphoma varies greatly with age (Table 29-2) and to a lesser extent with sex and geographic factors.
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Etiology of Non-Hodgkin's Lymphomas
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In many animals, lymphoma and leukemia have a viral etiology. Murine, feline, and avian leukemias all are caused by retroviruses (type C), ...