A reappraisal of Richter's syndrome. Development of two phenotypically distinctive cell lines in a case of chronic lymphocytic leukemia.

Richter's syndrome, the development of a malignant lymphoma in a patient with preexisting chronic lymphocytic leukemia (CLL) is an infrequent but well-documented phenomenon generally thought to represent a monoclonal proliferation of B-lymphocytes arising from the CLL. A heterogeneous population of cells consisting of sheets of transformed lymphocytes in combination with clusters of bizarre, atypical histiocytes developed in a patient with a history of longstanding CLL. Immunocytochemistry using a panel of monoclonal and polyclonal antibodies by immunoperoxidase techniques identified the presence of both B-lymphocytic and monocytic-histiocytic cell lines of differentiation. A mechanism of multiple differentiation is proposed to account for the dual cell population observed in this patient. Review of the literature appears to indicate that this phenomenon often may be involved in cases diagnosed as Richter's syndrome. The demonstration of cellular heterogeneity in the current case underscores the need for establishing a more precise definition for the histologic characterization of the terminal malignancy in Richter's syndrome.