Evolution of Sézary syndrome in the course of hairy cell leukemia

A patient with a history of "leukemia" for 19 yr and documented hairy cell (HC) leukemia for 10 yr developed mycosis fungoides and the Sézary syndrome. The manifestations of both diseases were diagnostic on clinical and pathologic grounds. Ultrastructural, immunohistochemical, and surface marker techniques proved the HC to have phenotypic characteristics of the T-helper subset of lymphocytes to which the Sézary cells (SC) also belonged. Both types of cells contained tartrate-resistant acid phosphatase. HC did not infiltrate the skin. SC did not contain ribosome lamellar complexes. Because of otherwise overlapping morphology and the apparent replacement of HC by SC, it is likely that the Sézary cells constituted a genetic variant of the original neoplastic clone represented by the hairy cells. Since the biologic and therapeutic implications of such clonal evolution may be important, subtle phenotypic changes should be looked for repeatedly in patients with these diseases.