Diagnosis and Classification of Lymphomas
William R. Macon
Paul J. Kurtin
Ahmet Dogan
SPECIMEN EVALUATION
Tissue Sampling and Processing
Precise assessment of hematopathologic specimens depends, in large part, on adequate sampling and proper handling of tissues, both of which may be influenced significantly by clinicians. Therefore, effective communication between the clinician and the pathologist is imperative for obtaining pertinent patient history and the proper specimen. The largest lymph node or mass lesion generally provides the most useful material for accurate diagnosis and should undergo surgical biopsy. Fresh tissue, moistened in a balanced solution, such as normal saline, should be sent intact to the surgical pathology laboratory without delay to maximize the immunophenotypic, genotypic, and karyotypic studies that are available and to minimize irreversible tissue artifacts. Frozen sections should be discouraged on small specimens, as lymphoid hyperplasias may appear indistinguishable from lymphomas, and freezing permanently distorts the tissue. Touch imprints are generally satisfactory for initial evaluation and for directing specimen work-up.
Needle biopsy and aspiration cytology is playing an expanded role in the primary diagnosis and monitoring of patients with malignant lymphomas. The major advantages of these techniques include: (a) their relatively noninvasive nature and (b) the rapidity with which the cytology can be reviewed (minutes) and the aspirated cells immunophenotyped (2 to 3 hours by flow cytometry). With computed tomography guidance, lesions in the mediastinum and retroperitoneum or in any highly vascular organ or tissue can be sampled with minimal morbidity. Technical advances in flow cytometry and molecular biology continue to reduce the amount of tissue required to provide immunophenotypic and genetic data.
Just as with lymph node and bone marrow biopsies, however, optimal information can be obtained only by close coordination between the clinician, the person performing the aspirate (radiologist or pathologist), and the hematopathologist providing ancillary diagnostic services. If the material is put into fixative, it cannot be used for flow cytometric phenotypic analysis or standard karyotypic studies.
The limitations of fine-needle aspiration (FNA) include: (a) the possibility of missing focal lesions, (b) the difficulty in making a primary diagnosis of malignant lymphoma in which the tumor cells are a minor population (e.g., Hodgkin lymphoma [HL] and T cell/histiocyte-rich large B-cell lymphoma [THRLBCL]), and (c) the difficulty in precisely classifying many reactive processes in which architectural features are of prominent diagnostic importance. For example, in patients with mediastinal masses, it may be impossible to distinguish between normal thymus, thymoma, and T lymphoblastic lymphoma by the cytology of the lymphoid population. Furthermore, immunophenotypic studies may identify a common thymocyte phenotype that can be dominant in all three. Despite these limitations, FNA coupled with flow cytometry and fluorescence in situ hybridization (FISH) will play a major role in the diagnosis of many non-Hodgkin lymphomas (NHLs) in the future.
Morphologic Examination
Morphologic examination starts at low magnification to evaluate tissue architecture and patterns of infiltration. Lymphomas may cause partial or complete destruction of the normal architectural features of the lymph node. Growth patterns are generally described as nodular or diffuse. Lymphomas are often distributed within specific anatomic compartments of the lymph node, such as follicle (germinal) centers, follicle mantles, or the paracortical and medullary areas. The low magnification pattern of neoplastic cell distribution within the lymph node suggests the type of lymphoma present (Fig. 86.1). High magnification is then used to examine cytologic features, such as the neoplastic cell types (e.g., small cleaved cells [centrocytes], large transformed cells [centroblasts or immunoblasts], and plasmacytoid lymphocytes or plasma cells), because this information helps establish the classification and grade of the tumor. In some lymphomas, the composition of the reactive cell constituents also may be of prognostic significance.
Immunophenotypic and Genotypic Analysis
Immunophenotypic analysis uses antibodies of variable specificity to detect cellular antigens (surface, cytoplasmic, or nuclear) in cell suspensions (flow cytometry) or in frozen or paraffin-embedded tissue sections (Table 86.1). These studies are often invaluable, because they help in distinguishing subtle lymphomatous infiltrates from reactive hyperplasias, can demonstrate the lineage of the neoplastic cell (e.g., B cell, T cell, and natural killer [NK] cell), can provide data necessary for precise classification of some
lymphomas (e.g., mantle cell lymphoma vs. small lymphocytic lymphoma [SLL]) (Table 86.2), can identify important nonlineagerelated markers (e.g., CD15, CD30, and CD56), and can determine the proliferative rate of lymphomas. Immunoglobulin (Ig) light chain restriction is evidence of B-cell clonality, whereas aberrant B-cell or T-cell phenotypes infer clonality.1,2 As small monotypic (light chain-restricted) B-cell or genotypically clonal B-cell or T-cell populations may be seen in reactive processes, correlation of these studies with the morphologic features is essential to prevent misdiagnosis and clinical confusion.3, 4 and 5
lymphomas (e.g., mantle cell lymphoma vs. small lymphocytic lymphoma [SLL]) (Table 86.2), can identify important nonlineagerelated markers (e.g., CD15, CD30, and CD56), and can determine the proliferative rate of lymphomas. Immunoglobulin (Ig) light chain restriction is evidence of B-cell clonality, whereas aberrant B-cell or T-cell phenotypes infer clonality.1,2 As small monotypic (light chain-restricted) B-cell or genotypically clonal B-cell or T-cell populations may be seen in reactive processes, correlation of these studies with the morphologic features is essential to prevent misdiagnosis and clinical confusion.3, 4 and 5
 FIGURE 86.1. Sites of origin of malignant lymphomas in a lymph node according to anatomic and functional compartments of the immune system. CLL, chronic lymphocytic leukemia; F, follicles with germinal centers; M, medullary cords; PC, paracortex, or interfollicular areas; S, sinuses. (Adapted from Mann RB, et al. Malignant lymphomas: a conceptual understanding of morphologic diversity. Am J Pathol 1979;94:1.) |
TABLE 86.1 IMMUNOPHENOTYPIC MARKERS USED IN DIAGNOSIS OF MALIGNANT LYMPHOMAS | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 86.2 PATHOLOGIC FEATURES IN THE DIFFERENTIAL DIAGNOSIS OF SMALL B-CELL LYMPHOMAS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Each of the major immunologic techniques has its strengths and liabilities. Flow cytometry permits rapid analysis of large numbers of cells for virtually any surface or cytoplasmic antigen. Flow cytometry is the primary tool used for phenotyping most NHLs and leukemias and is a very sensitive and specific technique for detecting residual disease.6,7,8,9 Studies for coexpression of more than one marker are easily accomplished (e.g., CD5 on CD19+ B cells). Flow cytometry also allows correlation of phenotypic characteristics with cell cycle kinetics and other biologic features of cells. Data storage in list mode permits retrospective off-line multiparameter analysis of lymphocyte subpopulations. However, for most analyses, flow cytometry requires viable cell populations. The quality of the information produced by flow cytometry is directly related to the quality of the communication between the flow cytometrist and the pathologist. The basic question is, “Is the cell population of interest to the pathologist the same cell population analyzed by flow cytometry?”
Immunohistochemical studies on paraffin-embedded tissue permit direct visualization of antigens on the cell of interest. Leukocyte common antigen (CD45) is a reliable marker for identifying most hematopoietic or lymphoid neoplasms but can be negative in acute leukemias, plasma cell neoplasms, anaplastic large cell lymphoma (ALCL), and classical HLs.10,11 Two markers that work well in paraffin, such as CD20 (pan-B cell) and CD3 (pan-T cell), are adequate to categorize most NHLs as to their B-cell or T-cell lineage.12,13 Detection of light chain restriction is most easily achieved in B-cell lymphomas that have abundant cytoplasmic Ig (most lymphomas with plasmacytic differentiation and many large B-cell lymphomas). The major liability of paraffin immunohistochemistry is the loss of some lymphocyte antigens during tissue processing. However, the explosion of fixation-sensitive markers for lymphoma-associated antigens (e.g., CD2, CD4, CD5, CD8, CD10, CD23, cyclin D1, and anaplastic lymphoma kinase [ALK]) has allowed precise classification of most lymphomas in paraffin-embedded tissue, obviating the need to rebiopsy to obtain fresh tissue for flow cytometry and cytogenetics.14
Genotypic analysis using the Southern blot technique provides a sensitive, but time-consuming and expensive, means of detecting clonal lymphoproliferations and their lineage. Polymerase chain reaction (PCR) is more sensitive than Southern blot for detecting clonality and also can be performed more rapidly. PCR is particularly helpful for establishing B-cell or T-cell clonality in lymphoproliferations present in paraffin-embedded small biopsies, such as those obtained by endoscopy.15 These methods are most useful when immunophenotypic studies are inconclusive and are the only practical way of proving B-cell or T-cell clonality. Southern blot and PCR also can demonstrate significant chromosomal abnormalities (e.g., CCND1, BCL2, and MYC gene rearrangements) and viral nucleic acids that may be involved in lymphomagenesis (e.g., Epstein-Barr virus [EBV], human T-cell lymphotropic virus type 1 [HTLV-1], and human herpes virus 8). PCR also is proving valuable in the detection of minimal amounts of residual disease in treated patients.
More recently, FISH applied to paraffin-embedded tissue has allowed detection of critical translocations in NHLs with sensitivity that is much greater than classic cytogenetics on fresh tissue or PCR on fresh or fixed tissue.16,17,18 FISH can be performed on very small specimens (e.g., smears and touch imprints) and may be completed in less than 24 hours. FISH added to flow cytometry is greatly increasing the opportunity for making rapid primary diagnoses of NHLs on FNA material.19,20
In the coming 5 years, it is expected that next-generation whole genome/exome/transcriptome sequencing technologies will alter the way lymphomas are diagnosed, classified, risk stratified, and treated. Already a number of detailed genetic analyses have been published for diffuse large B-cell lymphoma (DLBCL), mantle cell lymphoma, Burkitt lymphoma, hairy cell leukemia, chronic lymphocytic leukemia (CLL), splenic marginal zone lymphoma, and ALCL. These studies have highlighted a marked heterogeneity within histologically defined lymphoma subtypes, but also have identified a range of genetic abnormalities shared between different tumor types. Translation of these findings to meaningful diagnostic, prognostic, predictive, and therapeutic tests and strategies will be the main challenge for the near future.
Classification of Non-Hodgkin Lymphomas
For the clinician, pathologist, and basic scientist working in lymphoid neoplasia, the classification of NHLs is a persistent, confusing, and controversial problem. Several competing classification schemes have evolved, all with their supporters and detractors. In the 1950s, Rappaport developed a classification system based on growth pattern (nodular or diffuse) and cytology of lymphocytes (well differentiated, poorly differentiated, undifferentiated, or histiocytic).21,22 This scheme enjoyed enormous popularity because of its simplicity and reproducibility, but was superseded by classification schemes that reflected advances in cellular immunology.
In the early 1970s, Lukes and Collins,23 in the United States, and Lennert,24 in Kiel, Germany, proposed NHL classifications that related morphology to lymphocyte lineage. Both recognized follicular structures as a histologic correlate of B-cell differentiation. Each subdivided follicular lymphomas by the cytologic appearance of the predominant follicle center cell type: small and large cleaved cells in the Lukes-Collins classification (centrocytes in Kiel classification) and small and large noncleaved (transformed) cells in the Lukes-Collins classification (centroblasts in Kiel classification). For NHLs with diffuse growth patterns, immunophenotypic studies facilitated and, in many cases, were essential for precise classification.25,26
In 1982, the Working Formulation (WF) was introduced in an attempt to provide a morphologic classification scheme with prognostic relevance.27 Although the WF was an improvement over the earlier Rappaport classification, it had the same limitations as all purely morphologic classification schemes, separating biologically closely related lymphomas and grouping together biologically unrelated entities. All consideration regarding immunophenotype was excluded, so that the WF did not foster recognition of new entities.
In 1994, the Revised European American Lymphoma (REAL) classification was proposed and listed well-defined, “real disease entities recognized and diagnosed in daily practice”.28 More recently, the World Health Organization (WHO) classification29 built on the REAL classification and corrected some of its deficiencies. The WHO classification has many strongpoints and is comprehensive, and its most recent edition includes virtually all lymphoid malignancies described at the time of its publication in 2008 (Table 86.3).30 It defines diseases by four features: morphology, immunophenotype, genetics, and clinical information. Accordingly, the major diagnostic criteria for each of the major groups of NHL and HL are presented in the following discussion.
B-CELL LYMPHOMAS
B-Cell Lymphoblastic Leukemia/Lymphoma
Lymphoblastic lymphoma is a malignant lymphoma of precursor lymphocytes. Approximately 10% of lymphoblastic lymphomas are of B-cell lineage.30 They are distinguished from extramedullary acute lymphoblastic leukemia (ALL) by (a) the absence of peripheral blood involvement at presentation, (b) absence or only patchy marrow involvement (<25%) by blast cells, and (c) a clinical picture dominated by extramedullary tumor. Although they
may occur at any age, the majority of these rare lymphomas have been described in children and young adults. These neoplasms present frequently in extranodal sites such as bone and soft tissue or as skin tumors of the scalp and face.31,32,33,34 They almost never present as mediastinal masses.
may occur at any age, the majority of these rare lymphomas have been described in children and young adults. These neoplasms present frequently in extranodal sites such as bone and soft tissue or as skin tumors of the scalp and face.31,32,33,34 They almost never present as mediastinal masses.
TABLE 86.3 WORLD HEALTH ORGANIZATION CLASSIFICATION OF MALIGNANT LYMPHOMAS | ||||||||||||||||||||||||||||||||||||||||
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Histologically, they exhibit a diffuse growth pattern with frequent mitoses and a “starry sky” appearance. Linear capsular infiltration and interfollicular involvement is characteristic in lymph nodes. Cytologically, the tumor cells are small to intermediate in size with irregular nuclear borders, dispersed chromatin, small nucleoli, and scant cytoplasm.33
Most B lymphoblastic lymphomas are CD19, CD79a, CD10, and terminal deoxynucleotidyl transferase (TdT) positive with undetectable cytoplasmic Ig and variable expression of CD45 (weak to absent) and CD20.33,34 Only a few surface Ig-positive cases have been described.35 Lymphoblastic lymphomas in tissue sections may be difficult to distinguish from nonhematopoietic small cell undifferentiated tumors, such as Ewing sarcoma, rhabdomyosarcoma, and neuroblastoma, as well as T lymphoblastic lymphoma and blastoid variants of mantle cell lymphoma. Therefore, immunohistochemical studies are essential for diagnosis.36,37
Studies of the genetics of B lymphoblastic lymphoma are limited. The blasts show clonal Ig gene rearrangements without somatic mutation consistent with a pregerminal center cell of origin.38 It appears that hyperdiploidy or the presence of many of the translocations seen in pediatric ALL, such as t(9;22), t(1;19), and t(4;11), are uncommon. Chromosome 21 additions, including trisomy and tetrasomy, have been described.34
Follicular Lymphoma
The WHO classification of follicular lymphomas requires at least a partial follicular growth pattern (Fig. 86.2A). It excludes lymphomas, such as extranodal marginal zone B-cell lymphomas of mucosa-associated lymphoid tissue (MALT lymphomas) and mantle cell lymphomas, which also may have a nodular growth pattern. The incidence of follicular lymphomas is second only to DLBCL in the United States and Western Europe. These usually indolent lymphomas commonly present as painless lymphadenopathy in middle-aged or elderly adults. They commonly involve the bone marrow early.30 Follicular lymphomas in young children and some extranodal follicular lymphomas (e.g., skin and gastrointestinal tract) tend to be more localized at presentation than nodal follicular lymphomas in adults.39,40,41,42
Background
The follicle center is the major site of B-lymphocyte differentiation and proliferation.43,44 It may also serve as the site of lymphomagenesis of most B-cell lymphomas and HLs.45 B-lymphocytes move into the follicle after they first encounter protein antigen in the paracortex, to begin a series of steps that ultimately produces plasma cells with high-affinity Ig and memory B cells. Morphologically, the follicular center reflects this biologic transformation by frequent mitoses and by its range of cell types, including cleaved cells (or centrocytes) and large noncleaved cells (or centroblasts). Differentiation results in the Ig heavy chain class switching from IgM to IgG, IgA, or IgE, and in enhanced Ig synthesis. Proliferation produces the clonal expansion that is the basis of immunologic memory and an effective humoral immune response.43,44
Somatic mutation in the Ig genes occurring during proliferation is followed by selection of B cells with surface Ig of higher affinity for antigen. Successful interaction of B cells with antigenbearing follicular dendritic cells triggers B-cell expression of BCL2 protein that saves the B cell from apoptotic cell death.46 Tingible body macrophages mark the passing via apoptosis of B cells not selected for survival. Somatic mutation may serve to identify those neoplasms that have arisen from B cells that have been exposed to antigen in the environment of the follicle, such as follicular lymphomas and multiple myeloma, versus those that have not, such as most mantle cell lymphomas. Most B-cell NHLs and HLs have extensive somatic mutations.45,47,48 Somatic mutation of non-Ig genes, such as BCL6, may participate in lymphomagenesis.49 The recognition of the contribution of somatic mutation, receptor editing, and class switching in lymphomagenesis supports a major role for the follicle in neoplastic transformation.45,50,51
 FIGURE 86.2. Lymph node: follicular lymphoma. A: Low magnification demonstrates effacement of architecture by a follicular proliferation producing a nodular pattern. B: In a higher magnification, the follicles are composed almost entirely of centrocytes (follicular lymphoma, grade 1), whereas, in (C), from another patient, the follicles are dominated by centroblasts (follicular lymphoma, grade 3). |
Grading
A major problem area within follicular lymphoma is grading. Virtually all follicular lymphomas are composed of a mixture of centrocytes and centroblasts. Numerous studies have correlated the number of centroblasts with prognosis. Most studies agree that follicular lymphomas composed predominantly of centrocytes have a more indolent course than those composed predominantly of centroblasts.52, 53 and 54 The most popular scheme for subclassification of follicular lymphoma is a modified version of that proposed by Berard, which bases categorization on numbers of centroblasts per standardized high magnification field (HMF) with fewer than six per HMF classified as grade 1 follicular lymphoma, 6 to 15 per HMF as grade 2, and greater than 15 per HMF as grade 3.55 In all of these schemes of grading, there is substantial subjectivity and often disappointing interinstitutional reproducibility.56,57
Morphology
Follicular lymphomas are recognized at low magnification by the effacement of nodal architecture by follicles that crowd one another and fill the cortex and the medulla (Fig. 86.2A). Tingible body macrophages are not seen in contrast to reactive follicles. Mantle zones are usually thin or absent.58 Occasional patients, usually with limited stage disease, have partial nodal involvement amidst residual reactive lymphoid follicles.59 A small number of follicular lymphomas show differentiation to marginal zone-like cells with a “halo-like” distribution around neoplastic follicles. These marginal zone cells, although often CD10 and BCL2 protein negative, are genetically identical to the neoplastic cells of the follicles.60 On high magnification, low-grade (WHO grades 1 and 2) follicular lymphomas are composed of a relatively homogeneous population of centrocytes (smaller than, or the same size as a macrophage nucleus) with twisted nuclei, condensed chromatin, and inapparent nucleoli (Fig. 86.2B). Centrocytes frequently infiltrate the interfollicular areas and capsule. A variable number of centroblasts with oval to round nuclei (greater in size than a macrophage nucleus), dispersed chromatin, and one to three nucleoli that often abut the nuclear membrane also are present. Grade 3 follicular lymphomas, which by definition have an increased number of centroblasts (Fig. 86.2C), have been provisionally subcategorized into 3a and 3b, with the former having a mix of centrocytes and centroblasts, and the latter consisting exclusively of centroblasts.61 Areas of diffuse growth composed predominately of centrocytes do not appear to affect prognosis.52 Low-grade follicular lymphomas with a purely diffuse growth pattern are uncommon and may be difficult to separate morphologically from mantle cell lymphoma without immunophenotypic or cytogenetic studies. Recently, an early phase of follicular lymphoma restricted to the B-cell follicles without altering the lymph node architecture has been recognized.62,63 These are often referred to as “follicular lymphoma in situ,” and are identified in lymph nodes removed for other reasons. These lesions appear to have an indolent course with a very low rate of progression to clinically significant follicular lymphoma.
Follicular lymphomas composed mostly of large centrocytes (large cleaved cell lymphoma) are not recognized as a separate entity in the WHO classification. Large centrocytes differ from small centrocytes not only in nuclear size (larger than macrophage nucleus), but also in chromatin pattern, which is more open with occasional small nucleoli. The nuclei are elongated and twisted like small centrocytes. Because of the abundant cytoplasm of tumor cells, the follicles appear pale. A minority of follicular
lymphomas of large centrocytes exhibit a predominantly diffuse growth pattern. Several studies suggest that these lymphomas have a course similar to that of low-grade follicular lymphomas composed predominantly of small centrocytes.64
lymphomas of large centrocytes exhibit a predominantly diffuse growth pattern. Several studies suggest that these lymphomas have a course similar to that of low-grade follicular lymphomas composed predominantly of small centrocytes.64
Follicular lymphomas, particularly those arising in the retroperitoneum, have a propensity for extranodal growth and interfollicular sclerosis.65 Occasionally, follicular lymphomas present with massive nodal necrosis. Immunopositivity for CD20 and clonal Ig gene rearrangements can sometimes be demonstrated in these necrotic lymph nodes without morphologically viable lymphocytes.66,67 A small number of follicular lymphomas contain plasma cells that may be polytypic or monotypic with the same Ig heavy chain and light chain as the follicle center cells.68 Rarely, accumulations of intracytoplasmic Ig give cells in follicular lymphomas a signet ring cell-like appearance.69 A leukemic phase may be present in as much as 10% of patients with low-grade follicular lymphomas. Circulating cells show nuclear irregularity and are immunophenotypically distinct from neoplastic cells of B-cell CLL/SLL or mantle cell lymphoma. Neoplastic transformation to a DLBCL occurs in approximately one-half of cases.70,71,72,73 Pathology reports should emphasize areas of diffuse growth of large cells, as they are sufficient for regarding the tumor as higher grade regardless of the grade of the tumor in any residual follicles.74 More recently, a rare form of transformation in which the tumor cells have a blast-like morphology has been recognized. These blastoid variants of follicular lymphoma are aggressive clinically.75 Another aggressive variant of follicular lymphoma has a low-grade histologic appearance with a high proliferation index on Ki-67 staining.76 Rare clinicopathological variants of follicular lymphoma include so-called pediatric-type follicular lymphoma77 and extranodal follicular lymphomas arising in the duodenum, skin, thyroid gland, and genitourinary tract. The pediatric-type follicular lymphoma is characterized by presentation in children and young adults, large cell cytology, lack of BCL2 gene rearrangement, limited stage disease, and indolent clinical behavior. Similarly, primary extranodal follicular lymphomas tend to have a more indolent behavior irrespective of the large cell component.
Immunophenotype
Follicular lymphomas generally express bright surface Ig fluorescence, but a small number of large cell follicular lymphomas are surface Ig negative.1 Most follicular lymphomas are CD10+ (WHO grades 1 and 2), as are more than one-half of grade 3 lymphomas.78 Most follicular lymphomas are BCL6 positive as well. Follicular lymphomas are almost always CD5–, but rare CD5+/CD10+ cases have been described that often exhibit atypical histopathology such as floral variants or patterns that may be confused with progressive transformation of germinal centers.79,80 Expression of BCL2 protein may be helpful in differentiating some follicular lymphoid hyperplasia from follicular lymphoma; however, overexpression of BCL2 protein does not help differentiate follicular lymphomas from other NHLs.81 Overexpression of p53, detected by immunohistochemistry, may identify patients with increased risk for transformation.82,83 Low-grade follicular lymphomas differ from most other small B-cell lymphomas in having a significant admixture of T cells, and some authors have suggested that large numbers of T cells indicate a favorable prognostic feature.84 Immunohistochemical and gene expression data have correlated features of the tumor microenvironment such as number of infiltrating macrophages and composition of T-cell subsets with prognosis in follicular lymphomas.85, 86, 87 and 88
Genetics
More than 85% of nodal follicular lymphomas and 25% to 30% of DLBCLs have a t(14;18) (q32;q21) chromosomal abnormality. These numbers are lower in children and in patients with extranodal presentations. Molecular genetic studies and FISH may identify additional cases not recognized by conventional cytogenetics.89,90,91 Abnormalities of chromosomes 3q27, 5, 7, 12, and 13q are more common in a higher grade (WHO grade 3) or in transformation of follicular lymphoma. Abnormalities of chromosome 8q24 are associated with blastoid variants of follicular lymphoma.92,93 Recently a subset of follicular lymphomas occurring young adults and children (so-called pediatric follicular lymphoma) has been shown to have translocations involving the IRF4 gene.94 The pattern of somatic mutation in follicular lymphomas suggests a role for antigen selection in their clonal evolution.47,48
Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma
B-cell CLL/SLL is a neoplasm of small round B cells that usually have scant surface Ig and a CD5+/CD23+ phenotype. SLL is the term used to describe the disease when lymph node involvement is the dominant feature. Usually, these lymphomas present in elderly patients with a leukemic phase and generalized lymphadenopathy on routine physical examination. Bone marrow involvement is often extensive. Occasionally, patients present with bacterial infection related to hypogammaglobulinemia or with signs and symptoms secondary to anemia or thrombocytopenia that may have an autoimmune basis. CLL may occur in familial clusters with a decreasing age of onset from generation to generation.95
Lymph node architecture is totally effaced with loss of lymphoid follicles and obliteration of sinuses by an infiltrate of monomorphic small round lymphocytes with condensed chromatin and scant cytoplasm. Growth centers or proliferation centers (collections of intermediate-size round lymphocytes with open chromatin and small nucleoli called paraimmunoblasts) are dispersed throughout the lymph node and are thought to represent foci of cell proliferation (Fig. 86.3).96 Morphologic features that correlate with a worse prognosis include prominent proliferation centers in lymph nodes or increased numbers (>10%) of prolymphocytes (intermediate-size lymphocytes with prominent central nucleoli) in the blood.97,98 A clinically aggressive paraimmunoblastic variant of SLL has been described in which cells resembling those in growth centers predominate throughout the node.99
Richter syndrome, which is the evolution of CLL/SLL to a large cell lymphoma, occurs in less than 5% of patients and may arise as transformation of the neoplastic small B cell or as an unrelated B-cell clone.100,101,102 Less commonly, B-cell SLL may transform to prolymphocytic leukemia.103 Rarely, HL complicates B-cell SLL.104 Of interest is a description of 13 cases of B-cell SLL with Reed-Sternberg (RS)-like cells that contained EBV RNA as detected by in situ hybridization; three of these patients developed disseminated HL.105 The diagnosis of concurrent HL in a lymph node with CLL/SLL requires both RS cells and a polymorphic reactive cell background.
On immunologic typing studies, neoplastic lymphocytes have weak or scant monotypic surface Ig, which usually is IgM associated with IgD. Neoplastic lymphocytes coexpress the nominal T-cell antigen CD5 with B-cell markers CD19, CD20 (weak); CD21, CD11c (weak); and CD23. The tumor cells are negative for FMC7, CD10, and cyclin D1. Phenotypically, SLL and CLL are identical.106 Admixed T cells are usually few in number. CD23 expression and FMC7 and cyclin D1 negativity are helpful in the phenotypic separation of CLL/SLL from mantle cell lymphoma, another CD5+ small B-cell neoplasm.107 With the increased use of flow cytometry, small populations of circulating light chain-restricted B cells with a CLL/SLL phenotype are being detected in asymptomatic patients with no clinical features of neoplasia.108
Classic cytogenetic studies demonstrate specific chromosomal abnormalities in more than one-third of patients with CLL/SLL, with FISH detecting abnormalities of chromosomes 11, 12, 13,
and 17 in almost two-thirds of patients. Deletion of 13q is the most frequent abnormality and correlates with a stable clinical course in older patients. Trisomy 12, ATM deletions (11q), and TP53 deletions (17p) are associated with progressive disease.109,110 Molecular genetic studies divide CLL/SLL into two major groups based on the presence or absence of somatic mutation. The absence of somatic mutation is associated with a more aggressive clinical course and correlates with an increased expression of CD38 and the tyrosine kinase, ZAP-70.111,112,113,114,115 Risk stratification in CLL/SLL has therefore become an increasingly complex and controversial exercise with no clear consensus as to the relative roles of cytogenetics, molecular genetics for somatic mutation status, and use of surrogate markers such as CD38 and ZAP-70.116,117
and 17 in almost two-thirds of patients. Deletion of 13q is the most frequent abnormality and correlates with a stable clinical course in older patients. Trisomy 12, ATM deletions (11q), and TP53 deletions (17p) are associated with progressive disease.109,110 Molecular genetic studies divide CLL/SLL into two major groups based on the presence or absence of somatic mutation. The absence of somatic mutation is associated with a more aggressive clinical course and correlates with an increased expression of CD38 and the tyrosine kinase, ZAP-70.111,112,113,114,115 Risk stratification in CLL/SLL has therefore become an increasingly complex and controversial exercise with no clear consensus as to the relative roles of cytogenetics, molecular genetics for somatic mutation status, and use of surrogate markers such as CD38 and ZAP-70.116,117
 FIGURE 86.3. Lymph node: chronic lymphocytic leukemia/small lymphocytic lymphoma. A: Low magnification shows diffuse alteration of architecture, with pale areas corresponding to proliferation centers. B: A higher power of a proliferation center that is composed of intermediate-sized cells with small nucleoli that are surrounded by small round lymphocytes. |
Lymphoplasmacytic Lymphoma
Lymphoplasmacytic lymphomas (LPLs) are uncommon B cell neoplasms composed of small lymphocytes, plasmacytoid lymphocytes, plasma cells, and variable numbers of large lymphocytes. These lymphomas, like CLL/SLL, often have marrow involvement and a leukemic phase. They are often associated with high levels of an IgM paraprotein (Waldenström macroglobulinemia) or type II cryoglobulinemia.118,119 Some patients have antecedent systemic or localized abnormal immune reactions, such as Sjögren syndrome.120 Many patients have hepatitis C infection.121 LPL must be differentiated from other small B-cell lymphomas that frequently exhibit plasmacytic differentiation, including nodal and extranodal marginal zone B-cell lymphomas.
LPLs are morphologically diverse and may cause diffuse or partial alteration of lymph node architecture. Many have an interfollicular distribution and abundant macrophages that may impart a mottled low magnification appearance, resembling HL. On high magnification, small lymphocytes are admixed with variable numbers of plasma cells, plasmacytoid lymphocytes, and admixed large lymphocytes. Other features indicating plasmacytic differentiation include Dutcher bodies (cytoplasmic Ig inclusions that appear to be intranuclear) and extracellular periodic acid-Schiff-positive material. Mast cells and hemosiderin-laden macrophages are often present. These lymphomas may transform to large cell lymphomas.122, 123 and 124 These higher grade lymphomas usually produce the same Ig heavy chains and light chains as the original tumor.
Plasmacytoid lymphocytes and plasma cells contain abundant intracellular Ig, which usually is monotypic IgM without IgD and can be detected using paraffin immunoperoxidase reactions. In cell suspension studies, neoplastic lymphocytes usually express more surface Ig than is usually seen in CLL/SLL.124 Tumor cells express pan-B-cell markers CD19 and CD20 and are usually CD5– and CD10–.125,126 As the tumor cells are actively secreting Ig, many patients have high levels of monoclonal IgM producing hyperviscosity (Waldenström macroglobulinemia). The IgM may have rheumatoid factor activity or function as cryoglobulin, or both.118 Less commonly, these neoplasms may be associated with production of other Ig heavy chain classes.127
LPLs lack translocations involving the Ig heavy chain gene locus but most have a 3p22.2 abnormality resulting in the somatic mutation MYD88 L265P, particularly in patients with Waldenström macroglobulinemia.128,129 This variant is present in few cases of other B-cell neoplasms that have features similar to LPL. Somatic mutations of Ig genes are commonly found in Waldenström macroglobulinemia.130,131
Mantle Cell Lymphoma
Mantle cell lymphoma is a B-cell lymphoma typically composed of small lymphocytes with irregular nuclear outlines that have a CD5+ and CD23– phenotype and overexpress cyclin D1. These lymphomas are usually widespread at diagnosis with generalized adenopathy and extensive bone marrow involvement. They may involve extranodal sites, such as the Waldeyer ring, or present as lymphomatous polyposis of the lower gastrointestinal tract.132,133,134 A few have a leukemic phase mimicking CLL, prolymphocytic leukemia, or acute leukemia (blastoid variants of mantle cell lymphoma).135 Recognition of mantle cell lymphoma is clinically important, as these lymphomas pursue a more aggressive clinical course than other small B-cell lymphomas.136,137
Mantle cell lymphomas are composed of small lymphocytes with irregular nuclear contours. Mitotic activity is often brisk, and large lymphocytes are usually few in number except in the pleomorphic form of the blastoid variant, in which they predominate. Mantle cell lymphomas usually have a diffuse growth pattern (Fig. 86.4A,B) or surround reactive germinal centers in a mantle zone pattern. Extension of the lymphoma into the capsule and perinodal fat is common.132,133,138 The blastoid variant of mantle cell lymphoma is characterized by nuclei with increased size, dispersed chromatin, small nucleoli, frequent mitoses, and an aggressive clinical course. There is now molecular genetic evidence that at least some blastoid variants of mantle cell lymphoma are morphologic transformations of typical mantle cell lymphoma.139 The differential diagnosis of blastoid transformation of mantle cell lymphoma includes acute leukemia involving lymph nodes and lymphoblastic lymphoma.37,140
The neoplastic cell shares immunophenotypic features with normal mantle zone lymphocytes, including moderate amounts of surface IgM, usually with IgD. Neoplastic cells are generally CD5+ and CD10–, but a subset of mantle cell lymphomas has CD5– tumor cells.141 Mantle cell lymphomas usually mark with
antibodies to pan-B-cell antigens CD19, CD20, and CD22. CD23 is negative or sometimes partially expressed, and FMC7 is positive in contrast to the tumor cells of CLL/SLL.107,142 Overexpression of cyclin D1 is almost universal in mantle cell lymphoma143 (Fig. 86.4C), is not seen in follicular lymphoid hyperplasia, and is uncommon in other small B-cell malignancies, with the exception of plasmacytic neoplasms and hairy cell leukemia.144 Recent gene expression studies have identified SOX11 expression as another specific marker for mantle cell lymphoma.145 Rare “in situ” mantle cell lymphomas have been recognized in which cyclin D1-positive cells fill unexpanded mantle zones without distortion of nodal architecture.146 The clinical significance of these cases is unclear at this time. High Ki-67 expression is associated with a poor prognosis.147,148
antibodies to pan-B-cell antigens CD19, CD20, and CD22. CD23 is negative or sometimes partially expressed, and FMC7 is positive in contrast to the tumor cells of CLL/SLL.107,142 Overexpression of cyclin D1 is almost universal in mantle cell lymphoma143 (Fig. 86.4C), is not seen in follicular lymphoid hyperplasia, and is uncommon in other small B-cell malignancies, with the exception of plasmacytic neoplasms and hairy cell leukemia.144 Recent gene expression studies have identified SOX11 expression as another specific marker for mantle cell lymphoma.145 Rare “in situ” mantle cell lymphomas have been recognized in which cyclin D1-positive cells fill unexpanded mantle zones without distortion of nodal architecture.146 The clinical significance of these cases is unclear at this time. High Ki-67 expression is associated with a poor prognosis.147,148
 FIGURE 86.4. Lymph node: mantle cell lymphoma. A: Low magnification shows diffuse architectural effacement, which, on higher magnification in (B), is composed of sheets of small irregular lymphocytes with few large cells. C: Small lymphocytes exhibit nuclear staining for cyclin D1. |
On classic cytogenetics, most cases of mantle cell lymphoma show evidence of t(11;14) (q13;q32) involving the cyclin D1 gene (CCND1).149,150 FISH analysis of paraffin-embedded tissue extends the sensitivity of t(11;14) detection to greater than 90% and does not require fresh or frozen tissue.16 Rare cyclin D1-negative mantle cell lymphomas exhibit a gene expression profile similar to cyclin D1-positive tumors including expression of SOX11.151 The former cases show overexpression of cyclin D2 or cyclin D3 without translocations involving these genes. Almost one-third of patients with mantle cell lymphoma have somatic mutation of their Ig heavy chain genes. The presence of somatic mutation is associated with nonnodal disease, lack of SOX11 expression, and a more indolent clinical course.152,153,154
Nodal Marginal Zone B-cell Lymphoma
Nodal marginal zone B-cell lymphomas represent a rare, clinically and biologically heterogeneous entity.155, 156 and 157 The diagnosis often rests on exclusion of other possible lymphomas, in particular, nodal involvement by splenic marginal zone B-cell lymphoma (SMZL) and extranodal marginal zone B-cell lymphoma of MALT lymphoma. Clinically, these lymphomas appear more aggressive than other marginal zone lymphomas.158 They have a tendency to early relapse, and a small minority transform to large cell lymphoma.157
Nodal marginal zone lymphomas characteristically involve the interfollicular areas of lymph nodes and are composed predominantly of “monocytoid” small- and medium-sized lymphocytes of uniform size with distinct cell membranes that surround an abundant amount of pale cytoplasm. Nuclei are bland and oval to coffee bean in shape. In some cases, plasma cells are prominent.159,160 Admixed large lymphocytes are present, and mitotic activity is variable.155
Nodal marginal zone B-cell lymphomas express pan-B-cell antigens (CD19, CD20, and CD79a) and monotypic Ig (IgM without IgD) and are typically negative for CD5, CD10, and cyclin D1. A variable number are positive for BCL2 protein and CD43.155,156
Genetic studies are limited and have not identified any common cytogenetic abnormalities. Somatic mutation is usually, but not always, present in the Ig genes.156
Splenic Marginal Zone B-cell Lymphoma
SMZL is a rare disease accounting for approximately 1% of all lymphomas and approximately 20% of lymphoproliferative disorders in diagnostic splenectomy specimens.161,162 SMZL is a small B-cell lymphoma of the white pulp of the spleen that often involves the splenic hilar lymph nodes, bone marrow, and peripheral blood.163 The patients typically present with splenomegaly, B symptoms (fever, weight loss, and night sweats), abdominal pain, and cytopenias.164,165 A low level of monoclonal paraprotein can be detected in approximately one-half of the patients.166,167
The spleen is massively enlarged, weighing well over 1,000 g in most cases. Microscopically, the most striking feature is a nodular infiltrate centered on the preexisting white pulp lymphoid follicles.168 The tumor has a biphasic pattern with an inner core of small lymphocytes and an outer margin of medium-sized lymphocytes. The inner central zone of small lymphocytes resembles normal mantle zone cells with scant cytoplasm and small irregular nuclei, clumped chromatin, and indistinct nucleoli. The lymphocytes occupying the marginal zone appear similar to normal splenic marginal zone lymphocytes. These have well-defined clear cytoplasm and round/oval nuclei with a more open chromatin pattern and indistinct nucleoli. Variable numbers of large lymphocytes with prominent nucleoli are also present in the marginal zone. Occasionally, plasmacytic differentiation within the marginal zone or in the center of the nodules can be observed. Bone marrow involvement is nodular or interstitial and frequently intrasinusoidal. Tumor cells in peripheral blood often have short villous projections.163,166 This cytologic appearance is the reason why some cases of SMZL were called splenic lymphoma with villous lymphocytes. Transformation to large B-cell lymphoma is seen in as many as 15% of cases.169
Neoplastic cells are CD45, CD20, and BCL2 protein positive with monotypic IgM, with or without IgD. Tumor cells are negative for CD10, CD21, CD43, and cyclin D1, and in most cases, for CD5.163,164,168 Unlike hairy cell leukemia, they are usually CD11c, CD103, and DBA.44 negative.
Allelic loss of 7q21-32 is seen in slightly less than one-half of patients.170 Chromosomal translocations that are seen in other small B-cell lymphomas involving CCND1, BCL2, and MALT1 are not present. On molecular genetic studies, approximately one-half of cases show somatic mutation, and one-half have unmutated Ig genes.171 The latter group more frequently has deletions and allelic loss of 7q31-32 and may have a worse prognosis.
Recently, a primary low-grade B-cell splenic lymphoma showing morphologic features distinct from splenic marginal lymphoma has been described. This entity is often referred to as “splenic diffuse red pulp small B-cell lymphoma.” It is characterized by diffuse red pulp involvement in the spleen, intrasinusoidal involvement in the bone marrow, more frequent expression of IgG and DBA.44 by immunohistochemistry, bright expression of CD11c and CD22 by flow cytometry, and an indolent clinical behavior.172,173
Extranodal Marginal Zone B-cell Lymphoma of Mucosa-associated Lymphoid Tissue
Extranodal marginal zone B-cell lymphoma of MALT lymphoma is the third most common type of NHL, accounting for approximately 6% to 8% of all NHLs in the Western hemisphere.174 Although MALT lymphomas are clinically indolent, the disease is typically chronic, requiring long-term clinical surveillance and, often, repeated biopsies.175,176
Most MALT lymphomas arise in mucosal sites devoid of organized lymphoid structures. Development of MALT lymphoma is often preceded by a chronic inflammatory process that leads to acquisition of lymphoid tissue.177,178, 179, 180, 181, 182, 183, 184, 185, 186 and 187 This forms the background for the emergence of the lymphoma. The examples of chronic inflammatory processes that are associated with MALT lymphoma development are listed in Table 86.4.
The infectious process not only creates the microenvironment for development of the tumor but also the microenvironment necessary to sustain the tumor progression. Eradication of the infectious etiology has been shown to lead to remission in gastric, orbital, and cutaneous MALT lymphomas and is recognized as the first line of therapy, particularly for gastric MALT lymphoma.
MALT lymphomas are cytologically low-grade lymphoid neoplasms and are thought to arise from the marginal zone B-cell compartment of the mucosal lymphoid follicles.188 The cytologic features and the architecture of the tumor often mimic the features of normal mucosal organized lymphoid tissues such as intestinal Peyer’s patches. For this reason, many of the cases have been mistakenly diagnosed as pseudo-lymphomas before the application of clonality and genetic studies. MALT lymphomas tend to remain localized at the site of origin for many years. When they disseminate they tend to go to other mucosal sites, a phenomenon thought to be a result of homing programming. Transformation to large cell lymphoma occurs in a minority of cases.176
TABLE 86.4 RISK FACTORS FOR EXTRANODAL MARGINAL ZONE B-CELL LYMPHOMAS OF MUCOSA-ASSOCIATED LYMPHOID TISSUE (MALT LYMPHOMAS) | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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The lymphoma cells assume varied cytologic appearances, often within individual specimens. Characteristic are the centrocyte-like cells, small to medium-sized lymphocytes with irregular nuclei. Alternatively, the neoplastic cells may have a monocytoid appearance with abundant pale cytoplasm and distinct cell borders. The tumor cells also may resemble mature small lymphocytes. Scattered large cells are usually dispersed throughout the lymphoma. Variable numbers of plasma cells are frequently present, often adjacent to epithelium. In approximately one-third of cases the plasma cells are part of the neoplastic clone and may show atypical features such as Dutcher bodies. Regardless of the neoplastic cells’ appearance, they produce a diffuse infiltrate that invades epithelial structures, producing lymphoepithelial lesions (Fig. 86.5A,B) and subsequent epithelial disruption.189 Reactive lymphoid follicles are generally present, and the neoplastic lymphocytes may infiltrate and colonize them.190
MALT lymphomas express B-cell antigens (CD19 and CD20) and monotypic surface Ig (usually IgM without IgD). They may be CD43+ but usually lack other antigens expressed by small B-cell lymphomas, such as CD5, CD10, CD23, and cyclin D1. Cytoplasmic Ig is present in cases exhibiting plasmacytic differentiation.190 These lymphomas demonstrate clonal Ig gene rearrangements and show extensive somatic mutations characteristic of post-germinal center B cells.
The three major chromosomal translocations seen in MALT lymphomas are t(11;18)(q22;q21)/BIRC3 (API2)-MALT1, t(14;18) (q32;q21)/IGH@-MALT1, and t(1;14)(p22;q32)/IGH-BCL10.175,176 Remarkably, these appear to promote lymphoma development by a common mechanism. This concordance results from the roles of both the adapter protein BCL10 and the caspase-like protein MALT1 in antigen receptor-mediated activation of nuclear factor kappa B (NFκB), a transcription factor that regulates the expression of genes involved in lymphocyte proliferation and survival.191,192 Importantly, MALT lymphomas do not carry t(11;14)(q13;q32)/IGH@-CCND1 or t(14;18)(q32;q21)/IGH@-BCL2 chromosomal abnormalities typical of mantle cell lymphoma or follicular lymphoma, respectively.
T(11;18) identifies gastric MALT lymphomas that (a) present with advanced-stage disease, (b) do not respond to Helicobacter pylori eradication, and (c) are unlikely to transform to large cell lymphoma.193,194
 FIGURE 86.5. Stomach: extranodal marginal zone B cell lymphoma of mucosa-associated lymphoid tissue. A: The submucosa contains a diffuse infiltrate of small lymphocytes. B: Centrocyte-like cells with moderate amounts of clear cytoplasm invade gastric glands, producing lymphoepithelial lesions. |
Diffuse Large B-cell Lymphoma
DLBCLs are a morphologically, phenotypically, and genetically heterogeneous group of mature B-cell malignancies unified by the presence of large (nuclear size greater than the macrophage nuclei) neoplastic B cells. Comprising 30% to 40% of all lymphomas, they are the most common lymphoma type in the United States and Europe.174,195 A greater appreciation for their complexity has followed from recent clinical, pathologic, and biologic studies, so that the most recent WHO classification recognizes 13 subtypes/DLBCL clinical entities.30 The more common DLBCL entities are discussed below.
Diffuse Large B-cell Lymphoma, Not Otherwise Specified
DLBCL, not otherwise specified (NOS), occurs in all age groups, with an incidence that increases with age, and with a sight male predominance.196,197 Though most DLBCL, NOS cases arise de novo, risk factors for DLBCL, NOS include immunosuppression and prior low-grade B cell lymphoma. The most common presentation is as a bulky mass of lymph nodes, but up to 40% of cases arise in extranodal sites, including spleen or bone marrow. Staging bone marrows are involved in approximately 10% to 25% of cases. The pattern of bone marrow involvement may be concordant or discordant—constituted by a lower grade lymphoma. The former is associated with an adverse outcome; the latter with response to initial therapy similar to the cases without bone marrow involvement, but with increased risk for late recurrence.198
 FIGURE 86.6. Lymph node: diffuse large B cell lymphoma. A: On low magnification, a neoplastic large lymphocyte population diffusely effaces lymph node architecture. B: On high magnification, round nuclei, partially clumped chromatin, small nucleoli and modest amounts of pale cytoplasm characterize the tumor cells. Note the apoptotic bodies and mitotic figures. |
Morphology
Regardless of the anatomic site involved by DLBCL, NOS, the basic pathologic features are similar. The large neoplastic lymphoid cells grow in a purely diffuse pattern and efface the underlying tissue architecture (Fig. 86.6). Tingible body macrophages, apoptotic bodies, mitotic figures, and zones of necrosis and/or fibrosis variably accompany the neoplastic cells. Three cytologic variants are recognized by the WHO: centroblastic, immunoblastic (Fig. 86.7), and anaplastic.197 Rounded nuclei, dispersed chromatin, multiple small nucleoli, and modestly abundant basophilic cytoplasm characterize centroblastic DLBCL. These cells resemble the centroblasts of normal germinal centers. Round nuclei, with dispersed or marginated chromatin, prominent single centrally located nucleoli, and abundant cytoplasm usually eccentrically distributed relative to the nucleus are the cytologic features of immunoblastic DLBCL. Patients whose DLBCL contains 90% of cells with immunoblastic cytology may have an adverse outcome.196,199 A pleomorphic cell population that includes multinucleated tumor giant cells constitutes anaplastic DLBCL. The designation “anaplastic” for a DLBCL is a cytologic descriptive term that should not be confused with anaplastic large cell lymphoma, now considered a separate lymphoma entity comprised of neoplastic T cells. Occasional DLBCL have further cytologic diversity that includes signet ring cells, cells with multilobate nuclei, or spindle-shaped cells.
 FIGURE 86.7. Lymph node: cytologic variability in diffuse large B cell lymphoma. A: Centroblastic. B: Immunoblastic. |
Immunophenotype
DLBCL cells express CD45 and a variety of pan-B-cell antigens, including CD19, CD20, CD22, CD79a, and PAX-5 in almost every case.200 Ninety percent express light chain restricted surface Ig. Following rituximab (R) therapy, CD20 may be lost from the tumor cells,201 in which case B-cell lineage is defined by expression of one or more of the other B-cell antigens. Variable subsets of DLBCL express CD10, BCL6, GCET1, LMO2,202 BCL2, IRF4 (MUM1), and FoxP1.203 The pattern of expression of these latter markers by immunohistochemistry variably correlates with germinal center (CD10, BCL6, GCET1, LMO2) or nongerminal center (IRF4 and FoxP1) B-cell derivation of the DLBCL as defined by gene expression profiling (Fig. 86.8). Germinal center B cell (GCB)-derived DLBCL are thought to have a better prognosis than non-GCB DLBCL, and so
for practical purposes, several immunohistochemistry algorithms, Choi,204 Hans,205 Muris,206 Nyman,207 and Talley,208 have been proposed by which GCB and non-GCB are assigned. All have been shown to predict prognosis of R-CHOP-treated DLBCL patients in some, but not all studies. Therefore, phenotyping to assign a DLBCL to the GCB vs. non-GCB subtype for the purpose of selecting optimal therapy has been inconsistently applied to routine practice. Approximately 5% of DLBCL cases express CD5.200,209 These cases are distinguished from the blastoid variant of mantle cell lymphoma because they are cyclin D1–. They tend to occur in older patients, are more frequent in women than in men, and tend to be disseminated at the time of diagnosis.209 They are biologically distinct by genetic and gene expression profiling studies,210,211 but are not recognized as a separate entity in the WHO classification. One other marker of interest in DLBCL is Ki-67, a marker of cells in cycle. The fraction of Ki-67+ cells in a tumor is a general indicator of its proliferative rate. Ki-67 positivity ranges from 30% to nearly 100% in DLBCL cases, and it too, if expressed in greater than 90% of the cells, has been suggested as an adverse prognostic indicator in DLBCL.212
for practical purposes, several immunohistochemistry algorithms, Choi,204 Hans,205 Muris,206 Nyman,207 and Talley,208 have been proposed by which GCB and non-GCB are assigned. All have been shown to predict prognosis of R-CHOP-treated DLBCL patients in some, but not all studies. Therefore, phenotyping to assign a DLBCL to the GCB vs. non-GCB subtype for the purpose of selecting optimal therapy has been inconsistently applied to routine practice. Approximately 5% of DLBCL cases express CD5.200,209 These cases are distinguished from the blastoid variant of mantle cell lymphoma because they are cyclin D1–. They tend to occur in older patients, are more frequent in women than in men, and tend to be disseminated at the time of diagnosis.209 They are biologically distinct by genetic and gene expression profiling studies,210,211 but are not recognized as a separate entity in the WHO classification. One other marker of interest in DLBCL is Ki-67, a marker of cells in cycle. The fraction of Ki-67+ cells in a tumor is a general indicator of its proliferative rate. Ki-67 positivity ranges from 30% to nearly 100% in DLBCL cases, and it too, if expressed in greater than 90% of the cells, has been suggested as an adverse prognostic indicator in DLBCL.212
 FIGURE 86.8. Lymph node: germinal center B cell lymphoma phenotype. By the criteria of the Hans algorithm this DLBCL has a germinal center phenotype. Using immunohistochemistry on paraffin sections the neoplastic cells express: A: CD20, B: bcl-2, C: CD10, and D: bcl-6. |
Genetics
There is no single genetic abnormality that typifies DLBCL. Rather, a complex karyotype with genetic imbalances occurs in two-thirds of cases.213 In decreasing frequency, subsets of DLBCL cases have recurrent translocations involving BCL6 (3q27), BCL2 (18q21), and MYC (8q24) usually, but not always, to Ig heavy or light chain gene loci. BCL6 and BCL2 translocations do not predict prognosis in DLBCL patients but MYC translocations do.214 FAS and BCL6215 hypermutation are relatively common in DLBCL cases as are amplifications of REL216 and BCL2. Finally, particularly in DLBCL that arises from a lower grade lymphoma, p53 deletion or point mutations are also observed.217 Together these mutations are pathogenetic by promoting entry of the neoplastic cells into the cell cycle, inhibition of apoptosis, promoting genetic instability, and/or producing maturation arrest in the neoplastic cells. Finally, there is a subset of cases of morphologically typical DLBCL that usually express CD10 and BCL2 that contain translocations involving both MYC and BCL2.218 Colloquially termed “double-hit” lymphomas, these tumors have a particularly aggressive clinical course and are resistant to most chemotherapy regimens.219,220,221,222 By convention, they are typically categorized among “B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and Burkitt lymphoma”.223
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