13-31: Hairy Cell Leukemia

Hairy cell leukemia is a rare malignancy of hematopoietic stem cells differentiated as mature B-lymphocytes with hairy cytoplasmic projections. The V600E mutation in the BRAF gene is recognized as the causal genetic event of hairy cell leukemia, since it is detectable in almost all cases at diagnosis and is present at relapse.

A. Symptoms and Signs

The disease characteristically presents in middle-aged men. The median age at presentation is 55 years, and there is a striking 5:1 male predominance. Most patients present with gradual onset of fatigue, others complain of symptoms related to markedly enlarged spleen, and some come to attention because of infection.

Splenomegaly is almost invariably present and may be massive. The liver is enlarged in 50% of cases; lymphadenopathy is uncommon.

Hairy cell leukemia is usually an indolent disorder whose course is dominated by pancytopenia and recurrent infections, including mycobacterial infections.

B. Laboratory Findings

The hallmark of hairy cell leukemia is pancytopenia. Anemia is nearly universal, and 75% of patients have thrombocytopenia and neutropenia. The “hairy cells” are usually present in small numbers on the peripheral blood smear and have a characteristic appearance with numerous cytoplasmic projections. The bone marrow is usually inaspirable (dry tap), and the diagnosis is made by characteristic morphology on bone marrow biopsy (eFigure 13–32). The hairy cells have a characteristic histochemical staining pattern with tartrate-resistant acid phosphatase (TRAP). On immunophenotyping, the cells coexpress the antigens CD11c, CD20, CD22, CD25, CD103, and CD123. Pathologic examination of the spleen shows marked infiltration of the red pulp with hairy cells. This is in contrast to the usual predilection of lymphomas to involve the white pulp of the spleen.

Hairy cell leukemia should be distinguished from other lymphoproliferative diseases such as Waldenström macroglobulinemia and non-Hodgkin lymphomas. It also may be confused with other causes of pancytopenia, including hypersplenism due to any cause, aplastic anemia, and paroxysmal nocturnal hemoglobinuria.

Treatment is indicated for symptomatic disease, ie, splenic discomfort, recurrent infections, or significant cytopenias. The treatment of choice is a nucleoside analog, specifically pentostatin or cladribine for a single course, producing a complete remission in 70–95% of patients. Treatment is associated with infectious complications, and patients should be closely monitored. The median duration of response is over 8 years and patients who relapse a year or more after initial therapy can be treated again with one of these agents. Rituximab can be used in the relapsed setting either as a single agent or in combination with a nucleoside analog. The BRAF inhibitor vemurafenib exhibits ∼100% overall response rate in patients with refractory/relapsed hairy cell leukemia, with 35–40% complete remissions. The median relapse-free survival is ∼19 months in patients who achieved complete remission and 6 months in those who obtained a partial response. Moxetumomab pasudotox is a recombinant CD22-targeting immunotoxin recently approved for patients with refractory disease. It has shown a durable complete response rate of 31% in the pivotal trial. However, it can be associated with capillary leak and hemolytic-uremic syndrome attributable to the diphtheria toxin moiety.

More than 95% of patients with hairy cell leukemia live longer than 10 years.