The variant form of hairy-cell leukaemia.

Hairy-cell leukaemia-variant (HCL-variant) is a rare B-cell disorder which accounts for 10% of HCL cases. It affects elderly or middle-aged males. The main features are splenomegaly, lymphocytosis and cytopenias without monocytopenia. The circulating cells have a morphology intermediate between prolymphocytes and hairy cells. The immunophenotype shows a mature B-cell phenotype with expression of the B-cell antigens CD11c and CD103-but unlike typical HCL the cells are CD25- and HC2-negative. The histology of bone marrow and spleen shows a pattern of infiltration similar to that in HCL. There is no recurrent chromosomal abnormality but complex karyotypes and monoallelic p53 deletion by fluorescence in situ hybridization are common. Patients are resistant to alkylating agents and interferon-alpha (IFN-alpha) and only half achieve partial responses to pentostatin and/or cladribine. Splenectomy results in long-lasting partial responses in over two-thirds of the patients and is a good palliative treatment. Despite the lack of response to most therapies, the clinical course of HCL-variant is chronic. The median survival is 9 years and 42% of patients die of unrelated causes. Transformation to large cell is seen in 6% of patients. The inferior survival in HCL-variant compared with typical HCL cases may reflect the chemotherapy resistance.