Chapter 131:  Diagnosis of hairy cell leukemia

Morphology

In peripheral blood smears, hairy cells are lymphoid cells of small to medium size (10–20 mm in diameter). They have pale blue or blue-gray cytoplasm with circumferential, shaggy, irregular projections (Figure 131-1).^15,16 The nucleus is often eccentrically placed and is oval or indented, with loose, spongy chromatin. Occasionally, the nucleus may be bilobate or dumbbell-shaped, a feature that is particularly evident in patients who have a true leukemic phase. Nucleoli are absent or inconspicuous.

Because hairy cells are frequently scarce in the blood and because increased bone marrow reticulin fibers usually prevent aspiration of marrow spicules, the bone marrow biopsy specimen is often the most reliable for confirming the diagnosis of hairy cell leukemia.^2,15,17 In virtually all patients with hairy cell leukemia, the bone marrow biopsy specimen demonstrates a diffuse or patchy interstitial infiltrate that is characterized by cells with a wide rim of pale-staining cytoplasm that surrounds and separates the relatively monotonous, bland hairy cell nuclei.^2,17 This pattern of infiltration is apparent even at low magnification and is in contrast to the focal nodules or aggregates of lymphoid cells with closely packed nuclei found in most other mature B-cell lymphoproliferative disorders (Figure 131-2). In tissue sections, the nuclei of hairy cells are usually round, ovoid, or slightly indented, but small numbers of cells may demonstrate irregular or even convoluted nuclei or can sometimes assume a spindled, fusiform appearance. Mitotic figures are scarce. Although silver stains demonstrate increased stromal reticulin fibers, overt collagen fibrosis is rarely observed.⁷ Extravasation of red blood cells into the interstitial marrow spaces and between the hairy cell infiltrate is commonly observed.¹⁷ The biopsy specimen usually reveals hypercellularity of the marrow, but hypocellular specimens that mimic aplastic anemia have been reported as well.^7,18,19 Varying amounts of normal marrow elements may persist. The myeloid-to-erythroid ratio is usually less than 1:1, and islands of erythropoiesis with immature erythroid precursors may be prominent.^7,20 Hematopoiesis may be reduced, even when hairy cells minimally involve the bone marrow. Plasma cells and mast cells are often numerous.^20,21

The splenic enlargement in hairy cell leukemia is a result of diffuse infiltration of the red pulp cords and sinuses by hairy cells.^1,2 The white pulp is often atrophic. Blood-filled lakes lined by hairy cells (pseudosinuses) are often present.²² Mild to moderate hepatomegaly may be identified in 20% to 40% of patients and is most usually a result of an infiltration by hairy cells that is present in both the portal areas and the hepatic sinusoids.² Angiomatous lesions, similar to the pseudosinuses described in the spleen, are commonly found in the liver as well.²² Although palpable lymphadenopathy is uncommon, when lymph node involvement is discovered it is characterized by a diffuse hairy cell infiltrate in the subcapsular sinuses, cortex, and the medullary cord regions, while the lymphoid follicles are spared.²

Cytochemical and Immunophenotypic Features

Although the morphologic features of hairy cell leukemia in the blood and marrow are usually characteristic, determination of the immunophenotype of the neoplastic cells is essential to confirm the diagnosis and to distinguish hairy cell leukemia from other lymphoproliferative lesions with which it can be confused. Monoclonal antibody and gene rearrangement studies have indicated that the hairy cell is a postgerminal B cell.^23–28 There is no single antigen yet identified that is specific for hairy cell leukemia. Rather, a combination of a number of antigens presents a profile that not only aids in the identification of hairy cells, but also provides a clue about the place of the hairy cell in B-cell ontogeny. The most complete immunophenotypic data are acquired by flow cytometry analyses, which can be performed on the peripheral blood even when there is a low number of hairy cells in the peripheral blood.²⁹ However, the bone marrow biopsy provides additional tissue for immunohistochemical studies that can provide a partial, if not entirely diagnostic, immunophenotypic profile.

Studies with flow cytometry show that hairy cells express the pan-B-cell antigens CD19, CD20, CD22, and CD79a, although they usually lack CD21 (Table 131-1). They also brightly express surface immunoglobulin (Ig), which may be IgM, with or without IgD, in about one-half of the patients and IgG or IgA in the rest.^23,25,26,30 Hairy cells also express an early plasma cell marker, PCA-1. This finding has prompted some authors to suggest that the hairy cell is a relatively mature B cell, perhaps at a preplasma cell stage of development.^23,25,27,31 In most reported series, virtually all hairy cell leukemia patients also strongly express CD103, CD11c, FMC7, and CD25.^25,26,32,33 Less than 5% of cases are reported to express CD5 or CD10, and CD23 and CD24 are usually absent as well.²⁵ However, aberrant and unexpected phenotypes have been reported, so careful correlation of the phenotype with the morphologic and clinical findings is always necessary.^25,34,35 Unfortunately, at the current time, monoclonal antibodies that detect many of the antigens described above are not available for routine use on paraffin-embedded biopsy specimens, but antibodies for CD20 and for DBA.44 do work well in routinely processed tissue sections. Although neither of these is specific for hairy cell leukemia, they do provide the advantage of simultaneous morphologic and immunophenotypic assessment, and thus can be very helpful at arriving at the correct diagnosis. In addition, they aid in the recognition of small numbers of residual hairy cells present following therapy.^36–38

Tartrate-resistant acid phosphatase (TRAP) activity is present in the leukemic cells of most patients with hairy cell leukemia, and its detection with cytochemical techniques has been a traditional test for substantiating the diagnosis.^39,40 However, up to 5% of patients with otherwise typical hairy cell leukemia lack TRAP activity, and the enzyme has occasionally been found in mature B-cell lymphoproliferative disorders other than hairy cell leukemia.^2,18,39,41 Furthermore, interpretation of TRAP study results is often hampered by the scarcity of neoplastic cells in the blood and marrow aspirate specimens that are used for the cytochemical studies. Consequently, as the availability of immunophenotyping studies has increased, TRAP studies are less frequently used. Recently, however, monoclonal antibodies to TRAP that work well in biopsy sections have become available, and when used in conjunction with antibodies for CD20 and/or DBA.44, they have proved to be useful in the diagnosis of hairy cell leukemia.^42,43