Composite lymphoma and related disorders.

Lymphomas evolve over time, usually from the small-cell to the large-cell category, and from follicular histologic characteristics to diffuse architecture. The histologic characteristics of lymphomas may be discordant in patients with multiple sites of involvement. When different types of lymphomas are encountered in a single organ or tissue, they are designated as CLs in the Working Formulation of NHL. The most common CLs consist of different subsets of follicular center cell lymphomas, usually one with low-grade follicular histologic characteristics and another with diffuse architecture and/or more aggressive cytologic features. These lymphomas now are considered to represent different phases of clonal evolution rather than representing a coincidental simultaneous occurrence of two unrelated lymphomas. Whether B-cell large-cell lymphoma coexisting with another B-cell lymphoma, namely, the nodular variant of lymphocyte-predominant Hodgkin's disease, also represents clonal evolution requires further study. Composite lymphomas consisting of a B-cell lymphoma and a T-cell lymphoma are extremely rare. The histogenetic implications for a clonal relationship between component subsets in these and other equally rare combinations remain uncertain. The CLs should continue to be recognized because (1) the component morphologic subsets may have entirely different natural histories, requiring different treatment modalities although they may be clonally related; and (2) the study of such cases may provide us with information regarding the complex interrelationship of the lymphoid system and its clonal evolution. The morphologic definition for CL of the Working Formulation should continue to be used whenever possible, with addition of appropriate immunologic and/or molecular-genetic data.