Hairy cell leukemia is a mature B-cell neoplasm. This rare disease occurs in only 2% of all leukemia cases. The average age at onset of the disease is 50 years. Men are 5 times more often affected than women. seen most often in older men. Malignant cell clones are B-lymphocytes. The characteristic cytoplasmic projections are responsible for the name of the disease. The disease can often be diagnosed or suspected by looking at the blood film.
Patients with hairy cell leukemia suffer typically from a severe pancytopenia. this explains the symptoms including fatigue (anemia), recurrent infections (neutropenia), often with unusual pathogens, and bleeding (thrombocytopenia). Splenomegaly is almost always present without enlargement of the lymph nodes. Hepatomegaly also occurs in approximately 50% of the cases. The disease progresses slowly and can simply be treated with anti-purines (2-CDA = Cladribine). Since the response only occurs two to three weeks after the start of treatment, patients are prone to infections with often lethal complications.
Pancytopenia exists where the neutropenia is most severe. Typically, monocytes are completely absent. If monocytes are found in the blood film, the diagnosis of hairy cell leukemia is unlikely. A variant with increased white blood cell counts exists. Hairy cells are characteristic in the peripheral blood with cytoplasmic projections (left image). These cells show in contrast to normal lymphocytes a positive reaction to the tartrate-resistant acid phosphatase staining (right image). This staining is nowadays less used and replaced by flow cytometry immunophenotyping and cytogenetics.
Bone marrow aspiration often yields no material (Punctio sicca), since myelofibrosis often exists. The biopsy shows interstitial infiltration by lymphoid elements with large cytoplasm margins. This leads to a wide nucleus to nucleus gap which is typical and not seen in other lymphoproliferative infiltrations. Cytoplasmic projections are not visible.
Hairy cells demonstrate a typical but not unique pattern of antigens. Since the discovery of the mutation BRAF V600E, immunophenotyping has become of lesser importance in diagnosing hairy cell leukemia.
The mutation BRAF V600E is found in almost all cases of hairy cell leukemia but not in hairy cell leukemia-variant or other lymphoproliferative neoplasms. Therefore. the demonstration of this mutation is a central element in diagnosing hairy cell leukemia.