Four patterns of histologic variation have been described for patients with non-Hodgkin's lymphoma (NHL).1 Mixed architectural pattern refers to a single biopsy which reveals areas with both diffuse and follicular patterns. Composite lymphomas contain more than one distinct type of lymphoma (or Hodgkin's lymphoma and NHL) in the same biopsy. Discordant lymphoma refers to different types of lymphoma that occur at different locations, simultaneously. Histologic transformation (or evolution, or conversion, or progression) refers to a change in lymphoma histology that occurs during the course of a patient's disease.

Transformation is frequently unrecognized until discovered at postmortem examination. Usually, transformation refers to a change from low-grade or indolent histology to more aggressive histology, although other definitions have been used. Frequently, progression from follicular to diffuse histology is used to define transformation, and at other times transformation may be defined as an increase in the number of large cells in a biopsy.

In 1928, Maurice Richter described a patient with "chronic lymphoid leukemia" and progressive lymph node enlargement.2 At autopsy, the lymph nodes, liver, and spleen were infiltrated by small lymphocytes and "endothelioid tumor cells" described as a "reticular cell sarcoma." Since then, the term Richter's syndrome has been used to describe the development of large-cell lymphoma in a patient with chronic lym-phocytic leukemia (CLL). Retrospective series have demonstrated the occurrence of Richter's syndrome in 2-3% of CLL patients.3 4 This syndrome has been described in CLL patients who are in remission, as well as those with active disease, and is classically associated with systemic symptoms, progressive lym-phadenopathy, extranodal disease, elevated serum lac-tate dehydrogenase (LDH), and poor prognosis.

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