CHRONIC LYMPHOCYTIC LEUKEMIA/SMALL LYMPHOCYTIC LYMPHOMA
Chronic lymphocytic leukemia is a neoplasm of mature B lymphocytes characterized by blood and marrow involvement and commonly associated with lymph node involvement (Chap. 92). Small lymphocytic lymphoma is the nonleukemic form of the disease. Lymph nodes involved by chronic lymphocytic leukemia show a diffuse infiltrate of small mature lymphocytes admixed with prolymphocytes and paraimmunoblasts, which characteristically form ill-defined nodules known as proliferation or growth centers (Figs. 96–10 and 96–11). The B cells have a characteristic immunophenotype, demonstrating CD5 and CD23 expression and dim expression of CD20 and clonal immunoglobulin light chain. Studies have divided chronic lymphocytic leukemia into two distinct subtypes with distinct clinical behavior (Chap. 92). The type with the more favorable prognosis expresses mutated variable regions of the immunoglobulin heavy-chain (IGH) genes, whereas the other subtype expresses unmutated IGH genes. The IGH gene mutation status is reflected in differences in gene expression.22,23 The gene encoding the zeta-associated protein of 70 kDa (ZAP-70) is one of these genes, which generally is expressed by leukemia cells that express unmutated IGH genes and hence can be used to discriminate between the two subtypes.24 Certain cytogenetic abnormalities also correlate with clinical aggressiveness.25
Small lymphocytic lymphoma with vague nodular appearance imparted by proliferation centers.
Small lymphocytic lymphoma, characterized by small lymphocytes with mature chromatin pattern. Note that individual lymphocytes in small lymphocytic lymphoma are morphologically indistinguishable from benign lymphocytes.
Some cases of chronic lymphocytic leukemia/small lymphocytic lymphoma demonstrate plasmacytic features but are distinct from an entity known as lymphoplasmacytic lymphoma, which is characterized by a prominent component of plasmacytic lymphocytes and plasma cells (Fig. 96–12). These cases typically do not express CD5, less often involve blood, and often are associated with a monoclonal immunoglobulin M serum protein that can cause hyperviscosity or cryoglobulinemia (Waldenström macroglobulinemia). Somatic mutations in MYD88 are a commonly recurring and highly specific feature of Waldenström macroglobulinemia.26
Imprint preparation of lymphoplasmacytic lymphoma demonstrating small lymphocytes and cells with plasmacytoid features (eccentric nuclei and bluish cytoplasm).
Mantle cell lymphoma most commonly involves lymph nodes, but it can involve extranodal sites, including the gastrointestinal tract, as a clinical variant known as lymphomatous polyposis (Fig. 96–13). It typically is composed of a uniform population of small lymphocytes with cleaved nuclei and a virtual absence of large transformed cells (Fig. 96–14).27,28 It most commonly has a diffuse growth pattern, but it can show a nodular or, more rarely, a mantle zone pattern (Fig. 96–15). The postulated cell of origin is the B cell of the inner mantle zone. The lymphoma cells coexpress CD5, as does chronic lymphocytic leukemia, but mantle cell lymphoma can be distinguished by lack of CD23 expression and expression of cyclin D1 (Fig. 96–16). Cyclin D1 expression results from the chromosomal translocation t(11;14)(q13;q32) characteristic of mantle cell lymphoma. Gene expression data have demonstrated a subset of mantle cell lymphomas that are cyclin D1–negative.29 Some of these cyclin D1–negative cases have chromosomal translocations involving the cyclin D2 gene.30 SOX11 expression is a highly specific immunohistochemical marker for mantle cell lymphoma, and can identify cases that are negative for cyclin D1.31 Overall, patients with mantle cell lymphoma have a median survival of approximately 3 years, but gene expression data that determine tumor cell proliferation are able to identify patient subsets that differ in median survival by more than 5 years29 (Chap. 100).
Large bowel involved with mantle cell lymphoma (multiple lymphomatous polyposis).
Mantle cell lymphoma with a diffuse pattern, characterized by a monomorphous infiltrate of small irregular lymphocytes with numerous mitotic figures.
Mantle cell lymphoma with a mantle-zone pattern, characterized by monomorphous small lymphocytes surrounding a benign germinal center.
Mantle cell lymphoma with mantle-zone pattern stained with antibody to cyclin D1.
Cyclin D1–positive lymphocytes in mantle zones can be an incidental finding in reactive lymphoid follicles, a condition referred to as in situ mantle cell lymphoma. These appear to have an indolent behavior and do not require treatment.32
Follicular lymphoma is a proliferation of cells that correspond to normal germinal center cells,33 retaining expression of germinal center markers (BCL6, CD10), and demonstrates a follicular architecture (Fig. 96–17) imparted by nodular aggregates of CD21-positive follicular dendritic cells. Follicular lymphomas are composed of a variable mixture of centrocytes (small cleaved cells) and centroblasts (large noncleaved cells). They can be divided into three grades (grades 1 to 3) based on the number of centroblasts present. The most common is grade 1 (0–5 centroblasts per high-power microscopic field), previously known as follicular small-cleaved-cell lymphoma (Fig. 96–18). Both grade 1 and grade 2 tumors are indolent, and distinguishing between them is not required. Grade 3 follicular lymphoma (>15 centroblasts per high-power microscopic field) can be further divided into grade 3A (mixture of centroblasts and centrocytes) (Fig. 96–19) and grade 3B (solid sheets of centroblasts). Data have shown some molecular genetic differences between 3A and 3B cases, but further study is required because no significant clinical impact has been demonstrated.34,35 Follicular lymphoma can have an accompanying diffuse component, and identification of a diffuse area of large cells (diffuse large B-cell lymphoma) indicates transformation to a more aggressive disease. Approximately 90 percent of follicular lymphoma demonstrate the t(14;18)(q32;q21) involving rearrangement of the BCL2 gene, leading to the constitutive expression of the antiapoptotic BCL2 protein. Although BCL2 protein expression does not help distinguish follicular lymphoma from other lymphomas, it is a helpful feature in distinguishing it from reactive follicles that are BCL2-negative (Fig. 96–20).
Grade 2 follicular lymphoma (low-power magnification), characterized by crowded follicles throughout the entire lymph node.
Center of a neoplastic follicle in grade 1 follicular lymphoma with almost exclusively small centrocytes.
Grade 3A follicular lymphoma with >15 centroblasts per high-power field.
Positive BCL2 immunostain of a follicular lymphoma (contrast with Fig. 96–5).
Follicular lymphoma in situ is an entity where BCL2-positive germinal centers are present in an otherwise reactive lymph node. When it is distinguished from partial involvement by follicular lymphoma, follicular lymphoma in situ has a very low rate of progression to overt follicular lymphoma.36
MARGINAL ZONE B-CELL LYMPHOMAS
Marginal zone lymphomas are characterized by a proliferation of small lymphocytes, commonly with abundant pale cytoplasm (called monocytoid B cells) and plasmacytic features. The postulated cell of origin of these lymphomas is the postgerminal center B cell of the marginal zone at various anatomic sites. Marginal zone lymphomas can be divided into three distinct types based on site of presentation: (1) extranodal marginal zone lymphomas of mucosa-associated lymphoid tissue (MALT), (2) splenic marginal zone lymphomas,37 and (3) nodal marginal zone lymphomas (Fig. 96–21).38 This classification is supported by distinctive cytogenetic abnormalities in each entity. Extranodal lymphomas of the MALT type are the most common and arise in mucosal sites subject to longstanding chronic inflammation (Fig. 96–22), including chronic infection, the prototypical example being chronic Helicobacter pylori infection of the stomach.39 At early stages of development, many of these lymphomas respond to treatment with antibiotics to eradicate H. pylori, whereas later changes, including cases with chromosomal translocations activating genes involved in nuclear factor-κB (NF-κB) signaling,40 lead to antigen-independent growth (Chap. 101).
Lymph node involved by marginal zone B-cell lymphoma, in which the benign germinal centers and mantle zones are surrounded by expanded pale marginal zones.
Salivary gland involved by mucosa-associated lymphoid tissue (MALT) lymphoma, showing a diffuse infiltrate of small lymphocytes with pale cytoplasm, infiltrating an enlarged salivary gland duct (lymphoepithelial lesion).
DIFFUSE LARGE B-CELL LYMPHOMA
Diffuse large B-cell lymphoma (DLBCL) is characterized by a diffuse infiltrate of large B cells that can resemble centroblasts or immunoblasts (Figs. 96–23 and 96–24). The 2008 WHO classification identifies several types of large B-cell lymphoma, the most common type being DLBCL not otherwise specified, which constitutes 25 to 30 percent of all non-Hodgkin lymphomas.
Diffuse large B-cell lymphoma.
Diffuse large B-cell stained with antibody to CD20 (B-cell marker).
Gene-expression data show that DLBCL is a heterogeneous disease consisting of at least three entities having distinct gene-expression profiles based on cell of origin: (1) cases with an expression profile similar to germinal center B cells (GCBs), (2) cases expressing genes typical of activated B cells (ABCs), and (3) cases with a different pattern referred to as “unclassifiable” that are neither GCB-type nor ABC-type (Fig. 96–25).41 Importantly, clinical differences were apparent, with GCB-type cases having a significantly better prognosis compared to the other two types, even when clinical prognostic markers are considered (Chap. 98). Further studies confirmed these differences in the current era of therapy (including anti-CD20 antibody therapy), and identified nonneoplastic cells in the microenvironment as important contributors to patient survival.42 New therapies with selective activity in these subtypes of DLBCL are under development.43 It has been suggested that division of DLBCL into clinically distinct groups may be determined by the expression profile of a limited number of genes using routine immunohistochemistry.44 However, such an approach to classification is limited by problems of reproducibility of immunohistochemical staining and interpretation.45 The study of gene expression patterns of a small number of genes using formalin-fixed paraffin-embedded material may provide a rapid and accurate method to classify DLBCL.46
Gene-expression profiling diffuse large B-cell lymphoma (DLBCL), showing the subgroup discriminator used divide cases into germinal center B-cell–like (GCB) and activated B-cell–like (ABC). Each vertical column represents an individual patient and each horizontal row a unique gene. Red is relative overexpression of a gene and green relative underexpression. Using a probability of subgroup assignment of 90%, approximately 15% of cases are left unclassified (cases between the vertical yellow bars that are neither GCB or ABC). This approach allows one to analyze thousands of genes from a single patient in one experiment, and forms the basis of the new molecular classification of lymphoma. (Reproduced with permission from Wright G, Tan B, Rosenwald A, et al: A gene expression-based method to diagnose clinically distinct subgroups of diffuse large B cell lymphoma, Proc Natl Acad Sci U S A. 2003 Aug 19;100(17):9991–9996.)
Rearrangement of the MYC gene is present in 5 to 10 percent of DLBCLs, and is associated with an inferior prognosis.47 Approximately half of these cases will also have a rearrangement involving the BCL2 gene, referred to as a “double-hit” lymphoma. Such double-hit lymphomas have a very poor prognosis.48 MYC protein expression is present in approximately 30 percent of DLBCLs, which may be independent of gene rearrangement. Concurrent expression of MYC and BCL2 in DLBCL is associated with an inferior prognosis.49
Mediastinal large B-cell lymphoma is a distinct subtype of DLBCL that has been separately identified in the WHO classification.50 Patients with mediastinal lymphomas typically are younger than those with conventional DLBCL. The histology shows large cells with abundant cytoplasm associated with diffuse fibrosis (Fig. 96–26). Gene expression studies have demonstrated an expression profile that is distinct from conventional diffuse large B-cell lymphoma and shares some features with classic Hodgkin lymphoma (Fig. 96–27).51,52 Indeed, the 2008 WHO classification recognizes that some cases of mediastinal lymphomas can have features that are intermediate between DLBCL and classical Hodgkin lymphoma.14
Primary mediastinal large B-cell lymphoma with sclerosis.
Gene-expression profiling of primary mediastinal large B-cell lymphoma (PMBCL), contrasting the expression profile with nodal diffuse large B-cell lymphomas (DLBCL). This figure shows numerous genes that are overexpressed in PMBCL (red). Many of these genes are shared with classical Hodgkin lymphoma, suggesting a biologic overlap between these two diseases. Cases listed as “Other Mediastinal” refer to those cases of DLBCL with mediastinal involvement, but not felt to be typical of PMBCL. This is borne out by the gene-expression data, showing that these cases are more closely related to DLBCL rather than PMBCL. ABC, activated B cell; GCB, germinal center B cell. (Reproduced with permission from Rosenwald A, Wright G, Leroy K, Yu X, et al: Molecular siagnosis of primary mediastinal B cell lymphoma identifies a clinically favorable subgroup of diffuse large B cell lymphoma related to Hodgkin Lymphoma. J Exp Med 15;198(6):851–862, 2003.)
Burkitt lymphoma is a highly aggressive lymphoma characterized histologically by a diffuse infiltrate of intermediate-size cells with a high mitotic rate. The lymphomas commonly have significant spontaneous cell death (apoptosis), which results in a “starry sky” appearance caused by numerous macrophages that have engulfed the apoptotic debris (known as tingible body macrophages) (Figs. 96–28 and 96–29). The postulated cell of origin is the early follicular B blast cell of the germinal center. Virtually all cases of Burkitt lymphoma are characterized by chromosomal translocations involving the MYC gene on chromosome 8. The MYC gene most commonly is translocated to the IGH gene on chromosome 14, resulting in t(8;14)(q24;q32), but it also can involve the light-chain genes on chromosomes 2p12 (κ) and 22q11 (λ). A diagnosis of Burkitt lymphoma can be suggested based on morphologic examination alone but should be supported by immunophenotypic data (positive for CD20, CD10, and BCL6; negative or focally weakly positive for BCL2; growth fraction near 100 percent as determined by Ki67 stain) and confirmed by molecular testing for MYC translocations whenever possible.
Burkitt lymphoma with starry-sky appearance, imparted by macrophages that have engulfed apoptotic debris of dying tumor cells.
Burkitt lymphoma, characterized by a diffuse infiltrate of medium-size cells with small nucleoli and a high mitotic activity.
Gene-expression studies have shown that Burkitt lymphoma has a consistent gene-expression signature, but that there is not always correlation between the diagnosis based on gene-expression profiling and the diagnosis based on standard diagnostic testing.53,54 To reflect this, the 2008 WHO classification recognizes a provisional entity of B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and Burkitt lymphoma.14 Many of these cases represent “double-hit” lymphomas, which carry a MYC gene rearrangement and another chromosomal rearrangement, often involving the BCL2 gene.48