Pathology
Peripheral blood findings at diagnosis
Pancytopenia: 50%
“Leukemic” phase with a WBC >1000/mm3: 10–20%
Monocytopenia
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
Hypercellularity
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
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
Extravasated blood cells create blood lakes in the bone marrow similar to those observed in the spleen
Mast cells are often numerous
Reticulin stain of the bone marrow almost always shows moderate to marked increase in reticulin fibers
Approximately 10–20% of patients show a hypocellular bone marrow
Immunophenotyping, cytogenetics, and molecular diagnostic studies
Cytochemical studies: Tartrate-resistant acid phosphatase (TRAP) stain. However, TRAP is not specific for HCL
Hairy cell immunophenotype: CD19(+), CD20(+), CD22(+), CD79B(+), CD5(−), CD10(−), CD11C(+), CD25 Sub(+), FMC(+), CD103(+), CD45(+)
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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