Histopathology

Peripheral blood and bone marrow

Hairy cells can be identified in Wright's-stained blood smears from almost all patients with HCL, although the number of circulating hairy cells is usually low. The bone marrow is often inaspirable, resulting in a "dry tap". However, when aspiration is successful, hairy cells are morphologically similar to those in the blood.

The morphologic features of hairy cells are distinctive, Figure 30.1(a). They are approximately one to two

Figure 30.1 (a) Peripheral smear of a patient with hairy-cell leukemia, demonstrating classic feature of the hairy cells, including bean-shaped nucleus, homogenous, ground-glass chromatin, and abundant pale blue cytoplasm with "hairy" projections. Presence of leukocytosis is atypical for HCL. (b) Bone marrow trephine biopsy of a patient with hairy-cell leukemia, showing diffuse infiltration of hairy cells with abundant pale cytoplasm b

Figure 30.1 (a) Peripheral smear of a patient with hairy-cell leukemia, demonstrating classic feature of the hairy cells, including bean-shaped nucleus, homogenous, ground-glass chromatin, and abundant pale blue cytoplasm with "hairy" projections. Presence of leukocytosis is atypical for HCL. (b) Bone marrow trephine biopsy of a patient with hairy-cell leukemia, showing diffuse infiltration of hairy cells with abundant pale cytoplasm times the size of a small lymphocyte, with nuclei that appear round, oval, indented, monocytoid, and occasionally convoluted.34 The chromatin pattern is netlike in appearance and nucleoli are indistinct or absent. The amount of cytoplasm varies from scant to abundant and is pale blue-gray in color. The cytoplasmic borders are irregular and exhibit fine, hair-like projections or irregular borders. Occasionally, cytoplasmic granules are present. Examination of the bone marrow biopsy is important in the diagnosis of HCL because of its characteristic histopathology.35-38 In most patients, the bone marrow is hypercellular, although the cellu-larity may be normal or decreased, Figure 30.1(b). Hairy cell infiltration is diffuse, patchy, or interstitial, or a combination of these patterns. In patients with diffuse involvement, large areas of the bone marrow are replaced by hairy cells, with complete effacement of marrow in some patients. With patchy infiltration, subtle, small clusters of hairy cells are present focally or scattered throughout the bone marrow. Unlike most lymphomas, the hairy cells do not form well-defined, discrete aggregates; instead, they merge subtly with the surrounding residual hematopoietic tissue. In the interstitial pattern of involvement, variable numbers of hairy cells infiltrate between normal hematopoietic cells and fat, leaving the overall bone marrow architecture preserved. Hairy-cell nuclei in sections are round, oval, or indented and typically are widely separated from each other by abundant, clear, or lightly eosinophilic cytoplasm; rarely, the cells are convoluted or spindle shaped, Figure 30.2(a). The nuclear chro-matin is lightly condensed, nucleoli are inconspicuous, and mitotic figures are rare or absent. Extravasated red blood cells are often observed. Reticulin stains of the bone marrow trephine biopsy in HCL show a moderate to marked increase in reticulin fibers, Figure 30.2(b). Normal hematopoietic cells are usually decreased in HCL, with granulocytes being typically more severely reduced than erythroid precursors and megakary-

ocytes. In about 10-20% of patients with HCL, the bone marrow is hypocellular. The hypocellularity may be severe and may strongly resemble aplastic anemia.39

Spleen and other sites

The spleen in patients with HCL is usually enlarged, with a median weight of 1300 g.40 Splenic involvement in HCL is characterized by diffuse infiltration of the red pulp cords and sinuses, with atrophy or replacement of the white pulp. Blood-filled sinuses, referred to as "pseudosinuses", lined by hairy cells, are often present, but are not pathognomonic of HCL.41 The liver shows both sinusoidal and portal infiltration by hairy cells. Involved lymph nodes commonly exhibit partial effacement, with hairy cells infiltrating the paracortex and medulla in a leukemic pattern. The hairy cells often surround residual lymphoid follicles and extend through the capsule.

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