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Incidence: 2% of all leukemia (approximately 600–800 new patients per year in the United States)
Median age: 52 years
Male to female ratio: Approximately 4:1

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Peripheral blood findings at diagnosis

  1. Pancytopenia: 50%

  2. “Leukemic” phase with a WBC >1000/mm3: 10–20%

  3. Monocytopenia

  4. Hairy cells identified in most patients, but the number is usually low and may be difficult to identify in the peripheral blood because of low numbers and staining technique


Bone marrow findings at diagnosis

  1. Hypercellularity

  2. Hairy cell infiltration: diffuse, patchy, or interstitial

    Diffuse infiltration: Often results in complete effacement of bone marrow

    Patchy infiltration: Subtle small clusters of hairy cells present focally or throughout the bone marrow

    Interstitial Infiltration: Hairy cells do not form well-defined discreet aggregates, but merge almost imperceptibly with surrounding normal hematopoietic tissue

  3. Hairy cell nuclei are usually round, oval, or indented, and are widely separated from each other by abundant, clear or lightly eosinophilic cytoplasm. Rarely hairy cells can be convoluted or spindle shaped

  4. Extravasated blood cells create blood lakes in the bone marrow similar to those observed in the spleen

  5. Mast cells are often numerous

  6. Reticulin stain of the bone marrow almost always shows moderate to marked increase in reticulin fibers

  7. Approximately 10–20% of patients show a hypocellular bone marrow


Immunophenotyping, cytogenetics, and molecular diagnostic studies

  1. Cytochemical studies: Tartrate-resistant acid phosphatase (TRAP) stain. However, TRAP is not specific for HCL

  2. Hairy cell immunophenotype: CD19(+), CD20(+), CD22(+), CD79B(+), CD5(−), CD10(−), CD11C(+), CD25 Sub(+), FMC(+), CD103(+), CD45(+)

  3. Clonal cytogenetics: Abnormalities in approximately two-thirds of patients. Chromosomes 1, 2, 5, 6, 11, 14, 19, and 20 are most frequently involved. Chromosome 5 is altered in approximately 40%, most commonly as a trisomy 5, pericentric inversion, and interstitial deletions involving band 5q13. However, the identification of cytogenetic abnormalities in a patient with a definite diagnosis of HCL is usually not important as it does not influence, as far as is currently determined, prognosis or therapy


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Sausville JE ...

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