Frequency of additional clonal populations detected by high sensitivity flow cytometry in patients with hairy cell leukemia

Abstract

Samples from 115 patients with hairy cell leukemia (HCL) were analyzed by high sensitivity flow cytometry for evidence of an additional B cell or plasma cell clone. We found that 10.4% of HCL patients harbored an additional clonal population in either peripheral blood or bone marrow. Among the patients with additional clones, 58% had the second population present at the time of HCL diagnosis and 42% developed the second clone in a subsequent sample. Half of the clonal populations identified represented true disease entities rather than a monoclonal B lymphocytosis (MBL) or monoclonal gammopathy of undetermined significance (MGUS). The frequency of MBL and MGUS was similar to that reported in the general population, but the frequency of B cell-derived malignancies appears increased in the HCL cohort. Compared to patients without additional clones, the patients with second clones were older (median age 66 vs. 55 years old). Patients over age 60 were at higher risk for a second clone (18.2% vs. 5.6%, OR 3.7, P < 0.05). Among the patients with a second clone, 75% had second B cell clone, 42% had a plasma cell clone, and 17% had both. We have also identified a clear case of bi-clonal HCL; a finding not previously reported using contemporary diagnostic criteria. We conclude that second clonal expansions are relatively common in patients with HCL, in particular in older patients, and may represent clinically significant neoplasms.