Diffuse large B-cell lymphoma is the most commonly diagnosed subtype of lymphoma worldwide. The current World Health Organization (WHO) classification includes several subtypes, based on a combination of clinical, immunohistochemical, and genetic differences. Immunohistochemical staining is essential in evaluating diffuse large B-cell lymphoma and many related large B-cell lymphomas and aggressive B-cell lymphomas.
To address different immunohistochemical features used for identification, subclassification, prognosis and in some cases, therapy, of diffuse large B-cell lymphoma and related lymphomas.
The information outlined in this review article is based on our experiences with routine cases, on the current WHO classification of hematopoietic and lymphoid tumors, and on a review of English-language articles published throughout 2014.
Features and diagnostic criteria of diffuse large B-cell lymphoma, aggressive variants of B-cell lymphomas, including Burkitt lymphoma and “double-hit” lymphomas, are discussed. Identification of cell of origin (germinal center type versus activated B-cell type) is discussed at length. Finally, practical approaches for diagnosis are discussed.
Diffuse large B-cell lymphoma (DLBCL) is one of the most common diagnoses in hematopathology, typically including approximately 30% to 40% of non-Hodgkin lymphoma cases. In spite of being a common diagnosis, it represents a heterogeneous group of disorders that are related by the identification of a diffuse proliferation of B cells, which are typically large. However, from this simple starting point, distinguishing DLBCL from related disorders and proper determination of prognostic markers can add a degree of complexity to this diagnosis, which can be daunting to the practicing pathologist.
In all cases, to confirm a diagnosis of DLBCL, one must confirm the B-cell lineage of the lymphoma cells. Therefore, a de facto standard for B-cell identification is expression of CD20. In untreated lymphomas, CD20 is expressed in the most mature B-cell neoplasms, with some notable exceptions discussed below. Since CD20 is a target for a successful and commonly used monoclonal antibody therapy (rituximab) for most B-cell lymphomas, the use of this antibody should be a first consideration. Rituximab-treated B-cell lymphomas may no longer express CD20 and they require additional immunostains, such as PAX5, CD79a, CD19, or CD22, to identify B cells.
Confirming the presence of CD20+ large B cells with an appropriate histologic pattern could be considered a minimal standard for the diagnosis of DLBCL. It is however incumbent on the practicing pathologist to recognize that several variants and subtypes of DLBCL, and related disorders, rely on a more extensive evaluation of clinical, histopathologic, immunohistochemical, and genetic features for their accurate identification (Table 1). The current classification of DLBCL and related disorders from the 2008 World Health Organization (WHO) classification is highlighted in Table 2. Further, some groups have established guidelines for lymphoma diagnosis and/or reporting, including the College of American Pathologists (CAP) (Table 3).1–3
Currently, nationally recognized clinical guidelines for the treatment of patients with lymphoma emphasize the use of the 2008 WHO Classification of Tumors of Hematopoietic and Lymphoid Tissues.1,4 Based on this guidance, it is appropriate to provide subclassification and as specific a diagnosis as possible assuming that one has arrived at a diagnosis of “diffuse large cell lymphoma.” While this review emphasizes immunohistochemical studies, where relevant, other ancillary tests to identify subtypes of DLBCL and variants will also be discussed.
DIAGNOSIS, PROGNOSIS, AND THERAPY
As pathologists, our role in the care of patients is expanding rapidly. While in the past our focus was mainly on diagnosis, currently we are seeing an expansion of our role in establishing prognosis for patients, which may play an important role in directing therapy. Many prognostic factors may directly drive selection of the type of therapy (more or less aggressive) that is recommended (Table 4).
The 2008 WHO classification includes at least 26 types of large B-cell lymphomas, guiding classification of clinicopathologic entities based on histologic features, immunohistochemical features, and genetics.4 Evaluation of this large group of diagnoses often requires careful correlation of several clinical and pathologic findings. Immunohistochemistry is a keystone in identifying many of the different types of large B-cell lymphomas.
In almost all cases, primary identification is made by a histologic pattern along with the presence of pan–B-cell antigens. In the appropriate context, the combination of large cells in a diffuse pattern, with clear expression of CD20 may be “adequate” for identification of DLBCL, not otherwise specified (NOS). While this could be considered as a minimal standard of care, evaluation should take other morphologic, immunophenotypic, genetic, and clinical findings into account to completely classify DLBCL and related lymphomas. It is important to note that there are several specific subtypes of DLBCL that do not have distinctive histologic findings and as such can only be distinguished from DLBCL, NOS, by considering clinical, genetic, or immunophenotypic findings. Several markers may be expressed by specific subtypes of DLBCL but are not necessarily associated with specific morphologic features, including Epstein-Barr virus (EBV), CD5, anaplastic lymphoma kinase (ALK), CD30, and CD138.
Prognosis in DLBCL has been a fertile area of research for several decades. A wide range of individual markers may be associated with prognosis in DLBCL and related lymphomas, supporting the idea that this is a complex and heterogeneous group of neoplasms, and each year additional markers are put forward for evaluation5–18 (Table 5). While a plethora of unique aspects of DLBCL is prognostic, only those markers that have been vetted by extensive clinical study and can be routinely applied are the primary focus of this report.
DIFFUSE LARGE B-CELL LYMPHOMA, NOS
Most cases of DLBCL, NOS, express pan–B-cell markers CD20, PAX5, and CD79a, although the plasmablastic types may lack CD20 and PAX5 in many cases.19 Most cases also strongly express the B-cell transcription factors BOB1 and OCT2.
Several markers are associated with general evaluation of large cell lymphomas and may be of prognostic value, therapeutic relevance, or are used in combination with other markers or studies for prognosis or subclassification.
Some older studies20 have shown that proliferation rate, as measured by Ki-67, is associated with prognosis in DLBCL. However, more recent studies have not shown these same associations. Nonetheless, Ki-67 is a valuable stain in the diagnostic approach to lymphomas and can provide considerable additional information besides prognosis.21 Importantly, in cases with a near 100% proliferation rate, it may raise the diagnostic consideration of Burkitt lymphoma (BL) or other aggressive large B-cell lymphomas, but it is not useful alone in identifying double-hit lymphomas (DHLs).22
BCL2 expression in DLBCL is an adverse prognostic marker, particularly in cases of activated B-cell (ABC) type or with coexpression of MYC protein.23–25 Its expression is not necessarily related to the presence or absence of the t(14;18) IgH/BCL2 genetic abnormality seen in follicular lymphomas and a subset of DLBCLs. The role of BCL2 in germinal center (GC) and ABC types differs and this may have an impact on therapeutic choices for these patients.
MYC Protein Expression in Large B-Cell Lymphomas
Because of its association with high proliferation, the MYC gene has been of considerable interest in DLBCL. In most cases, the presence of a MYC translocation, identified by classical cytogenetics, fluorescence in situ hybridization (FISH), or other molecular studies, has been of benefit in evaluating lymphomas.26 The presence of MYC translocations is most often associated with a diagnosis of BL. However, DLBCLs may also have MYC translocations (8%–16%), as do a subset of DLBCLs with features intermediate between BL and DLBCL, DHLs (lymphomas with the combination of IGH/MYC translocations and either IGH/BCL2 and/or BCL6 translocation) (Table 6), and rare B lymphoblastic lymphomas.27 MYC amplifications have also been associated with poor prognosis.28
MYC immunohistochemistry is associated with prognosis in DLBCL, especially in association with BCL2 protein expression (Figure 1, a through c).29,30 High nuclear expression of MYC protein by immunohistochemistry (>40%–70%) in aggressive B-cell lymphomas may provide an appropriate triage step to indicate the need for additional genetic studies to evaluate for DHL or MYC translocations.31–33
DLBCL and Cyclin D1 Expression
Cyclin D1 expression can be seen in approximately 2% (30 of 1435) of DLBCLs.34 However, in contrast to the staining seen in mantle cell lymphoma, it is only weakly positive and often seen in only a subset of lymphoma cells. It is not associated with differences in prognosis or pathologic features. Its primary use in the evaluation of DLBCL is to exclude the possibility of the blastoid or pleomorphic variants of mantle cell lymphoma, which may at times appear similar to DLBCL.35
DLBCL and CD30 Expression
CD30 expression can be seen in 10% to 21% (35 of 167) of cases of typical DLBCL, and the most recent studies suggest CD30+ DLBCL may be associated with an overall better prognosis, with a unique gene expression profile.36,37 It is seen with a high frequency in specific lymphoma types such as lymphomatoid granulomatosis, primary mediastinal large B-cell lymphoma (PMLBCL), plasmablastic lymphoma (PBL), and gray zone lymphomas. At present, its evaluation is especially relevant because brentuximab vedotin, an anti-CD30 monoclonal antibody, is currently in clinical trials and may be useful in the treatment of some cases of DLBCL.36,38
COMMON AND RARE MORPHOLOGIC VARIANTS
The common and rare morphologic variants in DLBCL do not require a specific evaluation distinct from DLBCL, NOS. Studies39 have attempted to correlate immunoblastic morphology with prognosis in DLBCL. However, reproducibility of morphologic subtyping can be difficult in routine practice.
CELL-OF-ORIGIN SUBGROUPS: MOLECULAR
Gene expression microarray studies have led to the recognition that most DLBCLs may be separated into GC and ABC/nongerminal center (NGC) types, by RNA expression patterns.40–43 Gene expression arrays are not currently applied in routine diagnosis, and as such, other immunohistochemical systems have been proposed to act as surrogate markers for gene expression arrays (see below).44
CELL-OF-ORIGIN SUBGROUPS: IMMUNOHISTOCHEMISTRY
Germinal Center Type Versus Nongerminal Center Type
When considering DLBCL, NOS, determination of GC versus ABC type is of prognostic relevance. In general, GC-type DLBCL is associated with a better prognosis compared to ABC-type DLBCL. In the pre-rituximab era, the average 5-year survival associated with GC type was 60% versus 35% for ABC type; with the addition of rituximab to standard therapy, 5-year survival is 87% to 92% for GC type and 44% for ABC type.42,45 Moreover, responses to newer therapeutic options may be significantly different for GC versus NGC types, making this classification even more important.46 However, in the “special types” of DLBCL, those with a poor prognosis are typically of ABC type, and determination of GC versus ABC may not be relevant. Testing of GC versus ABC type of DLBCL is best accomplished by gene expression profiling. However, since this technique is not available for routine practice, other surrogate systems have been and continue to be tested.44,45,47–51 While these immunohistochemical approaches are not without controversy, in general there is relatively good concordance with gene expression profiling results.44,51–54 There are several schemes that allow for immunohistochemical classification of DLBCL, NOS (Figure 2, a through f).
The first and most well known is the Hans classifier.47 It is based on the application of 3 antibodies (CD10, BCL6, and MUM1). The overall concordance with gene expression array signatures is 80% (122 of 152). Other systems use combinations of other markers ([Choi et al45 : GCET1, MUM1, CD10, BCL6, FOXP1], [Muris et al48 : BCL2, CD10, MUM1], [Natkunam et al49 : LMO2], [Nyman et al50 : MUM1, FOXP1], and [Meyer et al51 : CD10, GCET1, MUM1, FOXP1, LMO2]) to achieve a more accurate prediction of the gene expression profile results (Figure 3, a through n).
There are differences suggested in the prognostic significance of BCL6 protein expression versus BCL6 translocation. If GC versus ABC type is relevant to prognosis, then expression of BCL6 generally supports GC type. However, genetic abnormalities of the BCL6 gene may be seen in ABC-type DLBCL.
CD5+ Diffuse Large B-Cell Lymphoma
CD5+ DLBCL has been described in the 2008 WHO classification and represents 5% to 10% of DLBCLs.19,55,56 CD5+ DLBCL is noted to have some differences from other typical DLBCL. In general, these cases are more often associated with a more aggressive clinical course, poor outcome, and are seen at a higher frequency in patients with human immunodeficiency virus (HIV)/AIDS. They express BCL2, with most expressing MUM1 and one-half expressing BCL6.55 There are 4 recognized morphologic subtypes: common variant (centroblastic monomorphic) (76%; 91 of 120); giant cell–rich variant, with intravascular involvement (11%; 13 of 120); polymorphic variant (12%; 14 of 120); and immunoblastic variant (1%; 2 of 120).57 Approximately 25% (3 of 13) of the giant cell variant cases express CD30.57 The expression of CD5 raises the differential diagnosis of mantle cell lymphoma and chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). Conventional mantle cell lymphoma can be distinguished by morphology and in cases of blastoid mantle cell lymphoma, the uniform expression of cyclin D1 would distinguish this entity from DLBCL. The possibility of CD5 expression in a large cell transformation of CLL/SLL (ie, Richter syndrome) is dependent on an antecedent history of the low-grade disorder with subsequent transformation.
T-Cell/Histiocyte–Rich Large B-Cell Lymphoma
T-cell/histiocyte–rich large B-cell lymphoma (TCHRLBCL) has distinctive morphologic and clinicopathologic findings.58–60 It is a rare large B-cell lymphoma. It is most commonly seen in middle-aged men, and is associated with an aggressive clinical course. By morphology, scattered large abnormal B cells (10% or less of the overall cellularity) are seen in a background rich in small lymphocytes (T cells) or histiocytes. Because of this distinctive appearance, the differential diagnosis of classical Hodgkin lymphoma (CHL) is frequently a consideration.
The large B cells are positive for pan–B-cell antigens including CD19, CD20, CD79a, PAX5, OCT2, and BOB1 with expression of CD45.60 CD30 expression is seen in a small subset of cases, but the large cells do not express CD15. They do not express CD5, CD43, or CD138. They are usually positive for BCL6 and BCL2, without expression of CD10. Epithelial membrane antigen (EMA) may show variable positivity in some cases. In contrast to many cases of CHL, staining for EBV is exceedingly rare in TCHRLBCL. If EBV is present, other diagnoses should be considered including CHL or “EBV-positive DLBCL of the elderly.”
The background small lymphocytes are usually T cells that are positive for pan–T-cell antigens, typically with more CD8+ T cells compared to CD4+, in contrast to nodular lymphocyte predominant Hodgkin lymphoma (NLPHL), which is typically rich in CD4+ T cells. The histiocytes are positive for CD68 and CD163. Staining with follicular dendritic cell markers (eg, CD21, CD23) may be of benefit; follicular dendritic cell networks are absent in TCHRLBCL but are present in cases of NLPHL. Immunoglobulin (Ig) D is expressed in many of the small lymphocytes associated with NLPHL. Recently, gene expression arrays comparing the microdissected large cells of TCHRLBCL and NLPHL were performed and no consistent differences were identified between these two entities. While not specific, it does lend some credence to the suggestions that at least some cases of TCHRLBCL may arise from previous NLPHL.61
Primary DLBCL of the Central Nervous System and Primary Cutaneous DLBCL, Leg Type
Primary central nervous system lymphoma and diffuse large B-cell lymphoma, leg type are lymphomas of activated B cells (ABC type), and are negative for CD10. The leg type diffuse large B-cell lymphoma characteristically has very strong BCL2 expression, often stronger than the admixed T cells. In these cases, because of independent poor prognosis of the special type, complete evaluation of cell of origin is not necessary, provided CD10 expression is excluded in leg type.
EBV-Associated DLBCL of the Elderly: DLBCL and Epstein-Barr Virus
In the 2008 WHO classification the new entity, EBV-associated DLBCL of the elderly, was added. These lymphomas are always associated with EBV infection and by definition, are seen in patients 50 years or older.62–66 These lymphomas are more common in Asian patients (8%–10%) and less common in Western populations (5% or less). The morphologic findings include monomorphous large transformed B cells with most having areas of geographic necrosis, although this is not a universal feature.66 These lymphomas are associated with a poor prognosis compared to typical DLBCLs. They are identified most reliably by in situ staining for EBV (Epstein-Barr virus–encoded small RNAs; EBER).64 In the diagnosis of EBV-positive DLBCL, exclusion of impairment of the immune system secondary to immunosuppression, transplantation, autoimmune disease, medications, or previous lymphoma is necessary.
The immunophenotype of EBV-associated DLBCL of the elderly is positive for the pan–B-cell antigens CD79a, CD20, CD19, and PAX5. It is negative for CD15 and CD10. Epstein-Barr virus should show positivity in greater than 50% of cases and is usually present in virtually all of the lymphoma cells.64 These lymphomas are usually of NGC B-cell type, lacking BCL6 and CD10, but expressing MUM1 and usually also CD30. The EBER marker consistently shows positivity, and EBV-latent membrane protein (EBV-LMP) also shows positivity in almost all cases.
Several other large B-cell lymphomas are occasionally or always positive for EBV, including PBL, primary effusion lymphoma, DLBCL associated with chronic inflammation, and lymphomatoid granulomatosis. Subsets of cases of BL are also positive for EBV.67 Most cases of posttransplant lymphoproliferative disorders, including those of large cell, monomorphic type (eg, DLBCL) also show positivity. In all cases, consideration of these specific subtypes, clinical history, and overall pathologic findings are necessary to render an accurate diagnosis.
Some types of large cell lymphomas that are associated with chronic EBV activity are positive for immunohistochemical staining for EBV-LMP; however, many types of EBV-positive lymphoproliferative disorders do not express EBV-LMP consistently. When possible, in situ stains for EBV (EBER) are recommended for evaluation of EBV expression in lymphoproliferative disorders.
OTHER LYMPHOMAS OF LARGE B CELLS
Primary Mediastinal Large B-Cell Lymphoma
Primary mediastinal large B-cell lymphoma is associated with a fairly unique clinicopathologic presentation.68 It is seen most frequently in young adults, with a slight female predominance and a rapidly growing mediastinal mass. The lymphoma cells are large, with round to slightly irregular nuclei, occasional pleomorphic multilobated nuclei, with moderate amounts of pale cytoplasm, imparting a clear cell appearance. This finding is not seen in all cases, and is only focal in some cases. In general, PMLBCL has an equivalent or slightly better prognosis than typical DLBCL.
In most cases, the differential diagnosis is between PMLBCL, CHL, and other types of diffuse large B-cell lymphomas. Because of its distinct prognosis and a proposed origin from thymic B cells, it often has a distinctive immunophenotype. However, gene expression arrays in PMLBCL show considerable overlap with CHL, and overlapping diagnostic features can be seen.69,70 In rare cases, collision tumors of PMLBCL and CHL, and synchronous and metachronous presentations of both, have been noted.
The immunophenotype shares features with other typical large B-cell lymphomas. The lymphoma cells are positive for pan–B-cell antigens including CD20, CD19, CD22, and PAX5. Distinctive features include expression of CD23 and MUM1 in most cases. Most cases are also positive for p63.71 CD30 expression is present in most cases, but is usually weak and heterogeneous, as compared to uniform and strong expression seen in CHL. CD15 is not typically identified and if present should raise the possibility of an “overlap” diagnosis (see below). Expression of BCL2 and BCL6 is variable and CD10 is typically negative. In comparing CHL and PMLBCL, Hoeller et al72 suggest that expression of BOB1 favors PMLBCL and expression of cyclin E favors CHL. Although these immunohistochemical stains are not routinely evaluated, most cases of PMLBCL express MAL, CD54, CD95, and TRAF1.72
Intravascular Large B-Cell Lymphoma
Intravascular large B-cell lymphoma (IVLBCL) is an extremely rare lymphoma.73,74 It occurs slightly more frequently in men (male to female ratio: 1.1:1) with a median age of presentation of 67 years. This lymphoma has an unusual range of clinical presentations dependent on tissue types involved, and the diagnosis is often unexpected. It can be seen in several tissue types, including skin, bone marrow, spleen, lung, brain, prostate, and many other sites. Hemophagocytosis is seen in a subset of cases and may be associated with significant cytopenias. As implied in the name, clusters and aggregates of large atypical lymphoid cells are seen in an intravascular distribution. Although classically associated with a poor prognosis, the advent of rituximab therapy has resulted in improved outcomes for many patients.73
IVLBCLs express pan–B-cell antigens, including CD19 (85%), CD20 (96%), CD79a (100%), and PAX5. They express CD5 in 38% and CD10 in 12% of cases.73 Most cases are positive for MUM1 (95%), except those that are CD10+. BCL2 expression is seen in 91% of cases. IVLBCL likely has unique markers that account for its intravascular distribution (such as CXCR3), but surface proteins that are critical for endothelial transmigration or adhesion are absent (CD29, CD54); however, use of these markers are not required for lymphoma diagnosis.
DLBCL Associated With Chronic Inflammation
In patients with long-standing chronic inflammation, DLBCL may develop. The prototype of this type of lymphoma is pyothorax-associated lymphoma, although other clinical scenarios including cysts and pseudocysts, and association with joint replacement, have also been described.75–77
These cases usually express pan–B-cell antigens CD20, CD79a, and PAX5, although cases with plasmacytic or plasmablastic differentiation may lose expression of mature B-cell markers. In these cases expression of plasma cell–associated markers, including MUM1 and CD138, may be seen. These cases will express EBV, and while EBER in situ staining may be more accurate, most cases will express EBV-LMP. They express BCL2 and lack CD10 and BCL6.76 CD30 is expressed in a subset of cases.77
Lymphomatoid granulomatosis is a rare EBV-driven lymphoproliferative disorder involving extranodal sites in patients with inherited or acquired immunodeficiencies.78,79 Pulmonary involvement is most common, followed by brain, kidney, liver, and skin. The morphologic features include a polymorphous lymphohistiocytic infiltrate associated with angioinvasive and angiodestructive lesions. Within this background are variable numbers of atypical EBV-positive B cells. These may appear Hodgkin-like, or as pleomorphic large lymphocytes.
The large abnormal cells are positive for EBV (EBER, EBV-LMP) and CD20. They express CD30 in most cases but are negative for CD15, in contrast to CHL.
ALK-Positive Large B-Cell Lymphoma
Anaplastic lymphoma kinase–positive large B-cell lymphoma (ALK-LBCL) is exceedingly rare, is more common in men (male to female ratio: 5:1) and presents in a wide age range (14–85 years).80,81 It is composed of large cells with large round nuclei, with prominent nucleoli and often with large amounts of pale-staining cytoplasm. These cells express the ALK protein but lack T-cell antigen expression and T-cell receptor gene rearrangements seen in anaplastic large (T) cell lymphoma.82 Staining with ALK is typically granular and cytoplasmic, although rare cases with cytoplasmic and nuclear staining can be seen, depending on the specific translocation present. These lymphomas will often express plasma cell–associated markers, including CD138, CD38, IgA (88%; 29 of 33), and cytoplasmic restricted light chains.81 They will often lack expression of pan–B-cell antigens (CD19, CD20, and PAX5).81,83 CD45 expression is seen in most cases (82%; 19 of 23) and EMA expression is seen in almost all cases. CD30 is rarely expressed (6%; 2 of 36) and in contrast to plasmablastic lymphoma, there is no association with human herpesvirus 8 (HHV8) or EBV (EBER). ALK-LBCL may be susceptible to the relatively recently developed anti-ALK therapy, crizotinib.38,84
Plasmablastic lymphoma is an aggressive B-cell neoplasm, which has immunophenotypic features of plasma cells with large cell morphology, and varying degrees of associated, more typical mature plasma cells.85,86 Most cases are associated with immunodeficiency, particularly HIV/AIDS, although a subset of cases is seen in immunocompetent patients. Plasmablasts are large and round, with round nuclei, prominent nucleoli, and small or minimal amounts of cytoplasm.
The immunophenotypic features of PBL include expression of CD138, CD38, and CD79a (50%–85%), but PBLs usually lack expression of CD45, CD20, and PAX5. They will also express monoclonal cytoplasmic light chain in approximately more than one-half of cases. The proliferation rate by Ki-67 is greater than 90%. They lack expression of CD56, which is seen more frequently in high-grade transformation of plasma cell myeloma. The EMA and CD30 markers often show positivity. Importantly, most cases are positive for EBV by in situ staining (EBER). Human herpesvirus 8 is not seen, in contrast to some plasma cell neoplasms associated with HIV/AIDS.85 Expression of CD56, cyclin D1, or bone/bone marrow involvement would suggest plasmablastic plasma cell myeloma rather than PBL, while the presence of EBV positivity or a MYC translocation would favor PBL.18
Large B-Cell Lymphoma Arising in HHV8-Associated Multicentric Castleman Disease
An exceedingly rare lymphoma, this is a large B-cell lymphoma that arises in HHV8-associated multicentric Castleman disease.87–89 Most cases occur in HIV/AIDS patients or in areas with endemic HHV8 infection, including Africa and the Mediterranean area. These lymphomas have a plasmablastic appearance and start as clusters and cell aggregates in a background of multicentric Castleman disease (so-called microlymphomas).
The abnormal cells will have variable expression of CD20 and typically lack expression of CD79a and CD138. They show expression of HHV8-associated antigens (LANA-1) and viral interleukin 6. They also express cytoplasmic IgM and monotypic λ light chain. In contrast to some other plasmablastic lymphomas seen in HIV/AIDS, there is no evidence of EBV infection.
Primary Effusion Lymphoma
Primary effusion lymphoma (PEL) is another exceedingly rare B-lineage lymphoma, most often seen in immunocompromised patients, such as those with HIV infection.90–92 As its name implies, most cases are seen in fluid effusions of the pleural or peritoneal cavities and exhibit overlapping morphologic features of immunoblastic, anaplastic, or plasmablastic large cell lymphomas. Some rare cases of solid tissue–based PEL have been reported. Virtually all cases have evidence of HHV8 infection, and cases not associated with HIV/AIDS are typically seen in areas with endemic HHV8 infection. Coinfection with EBV (as identified by EBER staining) is seen in several cases (60%). Exceedingly rare cases of HHV8-negative PEL have been identified.93
The immunophenotype includes expression of CD45 (88%; 38 of 43) but lack of expression of most pan–B-cell markers (CD20: 67% [51 of 76]; CD19: 92% [45 of 49]; CD79a: 88% [21 of 24]; and CD22: 97% [32 of 33]).90,91 Plasma cell–associated markers including CD138 (77%; 17 of 22), CD38 (100%; 48 of 48), MUM1 (92%; 47 of 51), VS38c, and EMA show positivity.90,91 CD30 often shows positivity (69%–81%; 62 of 76). Primary effusion lymphoma cells lack T-cell–associated antigen expression, although exceedingly rare cases of PEL of T-cell lineage have been reported.90,91 Almost all cases show evidence of clonal B-cell gene rearrangements by polymerase chain reaction.
Burkitt lymphoma is a distinctive, but relatively rare, aggressive B-cell lymphoma.67 It can present in both extranodal and nodal sites. Patients are generally younger than 30 years, but BL may be seen in older patients as well. It has a distinctive morphology of intermediate-sized lymphocytes with round nuclei, mature chromatin, and small to moderate amounts of deeply staining cytoplasm. Mitotic figures and apoptotic bodies are frequently seen. Tingible body macrophages will often impart the classic “starry-sky” pattern at low magnification.
The characteristic immunophenotype of BL shows pan–B-cell antigen expression (CD20, PAX5, CD19, CD22, CD79a), with strong expression of CD10, and a very high proliferation rate by Ki-67 (approximately 100%). Burkitt lymphoma lacks expression of terminal deoxynucleotidyl transferase (to exclude B lymphoblastic leukemia/lymphoma). BCL2 usually shows negativity, but may show weak positivity in approximately 25% of cases. Admixed CD3+ T cells are infrequent but CD68+/CD163+ macrophages are consistently seen. The BL cells consistently express germinal center–associated markers including BCL6, GCET1, and HGAL. Some cases will show expression of MUM1, and CD43 is expressed in about one-half of cases. Burkitt lymphoma lacks expression of cyclin D1, CD5, and CD23. Flow cytometry will often show strong expression of CD38, a reflection of the high proliferative cell fraction in BL.67,94 A classic immunophenotype can accurately diagnose BL in approximately 94% of cases (CD20+, CD10+, Ki-67 expression of almost 100%, CD38+; BCL2−).94 In situ staining for EBV (EBER) will be positive in some cases, more often in those associated with immunosuppression.
In the current classification, the presence of a MYC-associated translocation (t(8;14) MYC/IGH) or variants is necessary to confirm all but the most classic cases. In some cases, BCL2 expression may be seen, and when present, particularly when strongly positive, other diagnoses, including MYC translocation–positive DLBCL, “B cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL,” and DHL should be excluded.
In MYC translocation–positive lymphomas, a simple karyotype (IGH/MYC, with no or 1 other genetic abnormality) is supportive of a diagnosis of BL, while multiple genetic abnormalities support a diagnosis of either DHL or DLBCL.26
B-Cell Lymphoma, Unclassifiable, With Features Intermediate Between Diffuse Large B-Cell Lymphoma and Burkitt Lymphoma (BCLUWFIBDLBCLABL) and Double-Hit Lymphoma
Within the category of aggressive B-cell lymphomas, a subset of cases with the morphologic appearance of DLBCL, or cases that fall in the BCLUWFIBDLBCLABL provisional category, will have evidence of a combination of genetic findings, leading to a diagnosis of DHL. This is estimated to occur in 11% of cases of DLBCL and B-cell lymphoma, unclassifiable.27,95–99 Double-hit lymphoma is characterized by the presence of a translocation of MYC and another classic B-cell lymphoma “driver” translocation. The most commonly identified combination is the presence of an IGH/MYC translocation and IGH/BCL2. Other possible combinations are shown in Table 6. The immunophenotype of DHL is not characteristic. CD10 is generally expressed (76%–100%), as well as BCL2 (63%–96%), which usually shows strong positivity.95 BCL6 is expressed in 63% to 96% of cases. While Ki-67 expression is usually 90% or higher, several studies22,95 have shown wide variation (15%–100%) caused by biologic factors, suggesting that Ki-67 alone is not adequate for triaging cases for DHL workups.
B-Cell Lymphoma, Unclassifiable, With Features Intermediate Between Diffuse Large B-Cell Lymphoma and Classical Hodgkin Lymphoma
Occasional lymphomas have features that show considerable overlap in clinical, histologic, and/or immunophenotypic features between diagnoses of CHL and DLBCL.100 In general, these occur in the mediastinum. These have been included in a provisional diagnosis in the 2008 WHO classification, “B cell lymphoma, unclassifiable, with features intermediate between DLBCL and classical Hodgkin lymphoma” (BCLUWFIBDLBCLACHL). As mentioned, there are considerable molecular similarities between PMLBCL and CHL, suggesting that lymphomas with overlapping features might exist. These are often referred to as “gray zone lymphomas” when distinction between DLBCL and CHL cannot be resolved with immunohistochemical staining. Table 7 highlights immunohistochemical findings which would help to resolve the differential diagnosis. In cases with equivocal results, a diagnosis of BCLUWFIBDLBCLACHL would be appropriate. When studied as a group, gray zone lymphomas are more aggressive than either PMLBCL or CHL.68
PRACTICAL GUIDELINES FOR APPLICATION TO DIAGNOSIS
With this myriad of entities and possibilities, some guidance to the practical application of this information is appropriate. Clinical scenarios are discussed below.
Small-gauge needle core biopsies of lymphoma are common practice and may present inherent diagnostic problems. However, in cases of a limited sample, with the minimal acceptable morphologic evidence of a large cell lymphoma (eg, diffuse sheets of large lymphocytes), a minimal evaluation for diagnostic confirmation would consist of the combination of CD3 and CD20 staining. The presence of unequivocal positivity in the large cells with admixed rare CD3+ T cells would be minimally acceptable in the appropriate clinical setting. However, more than the absolute minimum staining should be considered if sample is available and specific entities are to be addressed. Stains that would be added to address specific entities, subtypes, or prognosis might include Ki-67, MYC, and BCL2. Additional stains, as available, would include CD30 and CD5, followed by stains for GC/ABC origin determination, EBV (as indicated, particularly in older patients), and cyclin D1 (as indicated to exclude mantle cell lymphoma).
When ample material is present for a diagnosis, then initial evaluation should use a balanced approach to address a broad range of diagnostic and prognostic studies in a timely manner. When there are diffuse sheets of large cells present, then testing could typically include the following stains: CD20, CD3, CD5, CD10, BCL2, BCL6, Ki-67, MUM1, MYC, cyclin D1, CD30, and EBER in situ hybridization (Table 8). This represents a slight departure from the recommended National Comprehensive Cancer Network testing panels (excluding CD45 staining, which may be added when the lymphoid nature of the cells may not be readily evident). It may also be of benefit to add staining for GCET1, FOXP1, and LMO2 for more accurate cell-of-origin determination using the Tally classifier. It is the opinion of one of the authors (D.P.O.) that when ample tissue is present, the Tally classifier should be performed.
When there is a predominance of plasmablasts or immunoblastic cells, a slightly different approach may be of benefit: CD45, CD20, CD3, CD30, CD138, κ/λ, EBV, HHV8, ALK, Ki-67, MYC, and BCL2. Cell-of-origin evaluation is not necessary unless other markers indicate a diagnosis of DLBCL, NOS, rather than plasmablastic or other special subtypes. This approach would be appropriate to address PEL and PBL.
Aggressive B-Cell Lymphoma
A panel consisting of CD20, CD3, CD43, BCL2, CD10, BCL6, MYC, and Ki-67 may be useful in assessing aggressive B-cell lymphoma, particularly when Burkitt lymphoma is in the differential diagnosis. It may also be appropriate to add cyclin D1 and terminal deoxynucleotidyl transferase to address blastoid MCL and lymphoblastic lymphoma, respectively. Cases with high MYC expression should be reflexed to FISH studies to evaluate for the possibility of a MYC translocation. Fluorescence in situ hybridization studies to rule out an IGH/BCL2 translocation should probably be obtained in cases with strong BCL2 expression. BCL6 FISH studies would address DHL with a combination of MYC and BCL6 abnormalities.
Rare Large Atypical Lymphocytes in Background of Small Lymphocytes/Histiocytes
In these cases, the primary differential diagnosis is different as is the immunohistochemical approach. In these circumstances, the differential diagnosis is biased toward CHL, NLPHL, TCHRLBCL, gray zone lymphomas, BCLUWFIBDLBCLACHL, and in some cases, anaplastic large cell lymphoma. A panel of immunohistochemical studies would typically include CD15, CD30, CD45, PAX5, CD20, and EBER to evaluate for possible CHL. In equivocal cases, the addition of the following stains may be of benefit: CD79a, OCT2, and BOB1. If NLPHL is in the differential diagnosis, a marker of follicular dendritic cells such as CD21 to evaluate for nodules, or a marker of follicular T cells such as PD1 to evaluate for rosettes around the large cells, may be helpful. CD30 and ALK stains may be of benefit if the lymphohistiocytic variant of anaplastic large cell lymphoma is being considered in the differential diagnosis.
It must be kept in mind that rituximab-treated lymphoma may lose CD20 expression in up to 37% to 60% (13 of 35; 6 of 10) of cases, and some of these cases may also lose expression of PAX5 or even CD79a.101,102 Application of a broader panel of B-lineage antigens, which may include CD19 and CD22, may be necessary to confirm a B-cell lineage.
Since recurrence may be associated with genetic changes, evaluation of MYC expression is appropriate because a DLBCL, NOS, may subsequently transform into a DHL or MYC translocation–positive DLBCL. Epstein-Barr virus studies may also be helpful in evaluating the possibility of a secondary lymphoma due to treatment.
The authors have no relevant financial interest in the products or companies described in this article.
This article is provided for educational purposes only and is not intended to suggest either a practice standard or the only acceptable pathway for diagnostic evaluation. The views presented reflect the authors' opinions. The application of these opinions to a particular medical situation must be guided by the informed medical judgment of the responsible pathologist(s), based on the individual circumstances presented by the patient. The College of American Pathologists has no responsibility for the content or application of the views expressed herein.