[HAIRY CELL LEUKEMIA]. Hairy cell leukemia (HCL) is a rare mature B-cell neoplasm which involves bone marrow and spleen but generally only few circulating cells are seen on peripheral blood smears. Bone marrow involvement can be interstitial, patchy or diffuse (as in this case) depending on the extent of involvement. Because of marrow involvement and splenomegaly most patients present with pancytopenia and splenomegaly. Monocytopenia is also common. Absolute lymphocytosis may occur in up to 25% of patients. Lymphadenopathy and hepatomegaly may also occur.

[HAIRY CELL LEUKEMIA]. The diffuse involvement of the marrow by hairy cell leukemia poses little diagnostic difficulty because of “pale” appearance of the marrow in contrast to the “blue” appearance of marrow seen in diffuse involvement of the marrow by chronic lymphocytic leukemia. Patients with diffuse involvement have significant pancytopenia and splenomegaly. Fatigue, weakness, malaise, fever, infections, and dragging sensations in the abdomen are common presenting complaints.

[HAIRY CELL LEUKEMIA]. In diffuse involvement of the marrow normal hematopoietic elements are almost totally replaced by leukemic cells. This pattern of involvement is easy to diagnose because of the markedly abnormal morphology of the marrow. Mild interstitial involvement and patchy involvement may be missed if careful consideration is not given to the possibility of “hairy cell leukemia” involvement in a pancytopenic patient.

[HAIRY CELL LEUKEMIA]. The “pale” appearance of the marrow is due the presence of abundant cytoplasm in leukemic cells. This may give rise to the typical “fried egg” appearance in some cells (arrow) and causes cells to appear “distant” from each other and not overlapping as seen in other leukemias and lymphomas. The leukemic cells in the marrow are slightly larger than normal lymphocytes and oval to oblong rather than round cells. The cells may also appear cleaved but cells do not have any paratrabecular location preference.

[HAIRY CELL LEUKEMIA]. Note the presence of abundant pink to pale cytoplasm in leukemic cells (arrow) and irregular nuclei in most cells (arrowheads). The cells in the marrow core biopsy are almost never overlapping as a result of abundant cytoplasm in normal thickness sections (3-4 microns).

[HAIRY CELL LEUKEMIA]. Immunohistochemical staining for a B-cell marker such as CD20 proves to be of significant value in identifying neoplastic cells in the marrow. A core biopsy may be unimpressive with respect to tumor burden on routine H&E stain but a CD20 stain highlights the true extent of marrow involvement and usually surprises the pathologist. This image highlights a vast number of HCL cells in the marrow otherwise not appreciated on a routine H&E stain. An immunohistochemical marker DBA.44 has also been used to identify hairy cells in paraffin sections.

[HAIRY CELL LEUKEMIA]. A bone marrow aspiration biopsy may yield a “dry tap” due to marrow fibrosis caused by leukemic cells. The neoplastic cells are known to synthesize fibronectin which contributes to diffuse marrow fibrosis. In patients with a “dry tap” physicians performing the procedure should prepare “touch imprints” of the core biopsy by gently touching and rolling the marrow core on a glass slide. In addition if the core biopsy is long enough (>2.0 cm) a portion of the core may be cut and put in RPMI culture medium for flow cytometric analysis. Alternatively, a 2nd core may be obtained for that purpose.

[HAIRY CELL LEUKEMIA]. In successful aspiration biopsies marrow smears may show abundant hairy cells admixed with other marrow elements. In this image numerous hairy cells are seen (arrowheads) which are larger than normal small lymphocytes (curved arrow). A nucleated erythrocyte is also shown (long arrow). Note the presence of abundant pale cytoplasm and “fuzzy” and hairy border in HCL cells. Although the typical morphology and clinical findings are enough to make a diagnosis of HCL immunophenotyping is routine performed tom exclude the possibility of mimics such as an artifact and marginal zone B-cell lymphomas with hairy cell projections. The neoplastic cells express CD20, CD19, CD11c, CD25, and CD103 in typical cases but may lack CD25 more often than CD11c and CD103. Histochemical staining for TRAP is not longer needed with the availability of flow cytometry markers.

[HAIRY CELL LEUKEMIA]. Hairy cells (multiple long arrows) are typically larger than small lymphocytes (arrowhead) and contain abundant pale blue cytoplasm on marrow aspirate smears. The diagnostic morphologic finding includes circumferential proliferation of long hairy projections responsible for the name of this disease. Other characteristic morphologic findings include the presence of oblong, reniform, or bean-shaped nuclei and small to indistinct nucleoli. However, in the “prolymphocytoid” variant of HCL nuclei may be round and nucleoli may be prominent.