Morphological variants of Diffuse Large B cell lymphoma

Centroblastic:
The centroblastic variant represents the most common type of diffuse large B-cell lymphoma and comprises both monomorphic and polymorphic variants as defined by the Kiel Classification.¹¹  In the monomorphic variant the medium to large-sized cells are typical centroblasts (at least 60%), giving a monotonous appearance. Centroblasts are characterized by medium to large vesicular nuclei with fine chromatin and two or more membrane-bound nucleoli. The cytoplasm is scanty and can be amphophilic to basophilic.

In the polymorphic subtype there is a mixture of cells and immunoblasts are present in variable numbers (>10%-<90%).

Immunoblastic:
The immunoblastic variant is relatively rare and by definition >90% of the cells are immunoblasts with a single centrally located nucleolus and a discrete amount of basophilic cytoplasm. Centroblasts can still be present, but represent <10% of the cells. The reproducibility of diagnosing this variant has always been low due to the lack of objective immunophenotypic or genetic criteria. Immunoblasts may also show plasmacytoid differentiation, confounding diagnosis of this variant even more.

Although some studies have reported immunoblastic lymphoma to have a worse prognosis than centroblastic,^12,13  others have failed to confirm this.^14,15 

T cell/histiocyte rich:
This variant is composed of <10% large neoplastic B cells in an inflammatory background of T lymphocytes and non-epithelioid histiocytes. The presence of a prominent histiocytic component is considered the hallmark of this variant by some authors. ^{16, 17}  The large cells may resemble centroblasts, L&H cells or occasionally Reed-Sternberg cells. ¹⁸ Usually, small B cells are absent. A fine reticular fibrosis is often present. Immunophenotypic studies are essential to distinguish this variant from peripheral T-cell lymphoma and Hodgkin lymphoma.^16-19 

Anaplastic:
This rare variant is characterized by large pleomorphic cells which preferentially occupy the sinuses and exhibit a cohesive growth pattern. However, these cases are biologically distinct from anaplastic large cell lymphoma of T/null phenotype expressing ALK-1 protein. The majority of cases express CD30. ²⁰ 

Rare morphologic variants/ subtypes with distinctive immunophenotypic features:

Plasmablastic:
This variant is heterogenous and includes multiple entities. It comprises cases, which on morphology alone are indistinguishable from immunoblastic lymphoma, or plasma cell neoplasms. Moreover, "plasmablastic lymphoma of the oral cavity" occurring in HIV patients is considered as separate entity. Cytologically , these lesions lack plasma cell differentiation and are associated with EBV in about 50% of cases. (CD45-, CD20-; CD79a+/-, CD138+). ²¹ 

Diffuse large B-cell lymphoma with full length ALK-1 expression:
In this variant, the tumor cells are immunoblast-like cells with round pale nuclei, central nucleoli and abundant eosinophilic cytoplasm with occasional evidence of plasmablastic differentiation. Immunophenotyping is mandatory to identify this rare variant

(CD20-, CD30-, CD45weakly +, EMA+, cIgA+, ALK-1+(cytoplasmic), CD4+, CD57+, negative for other T and B cell associated antigens). (22)

Immunophenotyping in diffuse large B cell lymphoma
Accurate immunohistochemical studies are of paramount importance in the diagnosis of several variants of diffuse large cell lymphoma. The use of specific panels that address the putative normal cellular counterparts allows a better definition of these lymphomas.

Similarly to what has been described in HIV–related lymphomas,²³  a limited panel consisting of CD20, CD79a, CD10, BCL-6, MUM-1 (IRF-4) and CD138 helps in segregating these lymphomas according to the pattern of differentiation.

Germinal center-derived lymphoma will be positive for CD20, CD10, BCL6, and negative for MUM-1 and CD138. A subset of cases may be CD10 negative.^24,25  Post-germinal center cell lymphoma will be positive for CD20, MUM-1, variably positive for BCL-6, and negative for CD138. Plasmablastic lymphomas should be negative for CD20 and positive for CD79a, MUM-1 and CD138.

When diffuse large B-cell lymphomas are defined using both morphology and immunophenotype, cases with immunoblastic characteristics are mainly represented in the postgerminal center cell group, while cases expressing CD10 are almost never associated with immunoblasts. ²⁵ CD10 expression has been associated with a centroblastic morphology and the presence of t(14;18) translocation. ^26,27 

BCL-6 protein expression is often seen in diffuse large B-cell lymphomas with either germinal center or early post germinal center derivation. ^28,29 The prognostic significance of BCL-6 expression is controversial, but it is usually associated with a better prognosis.^25,30,31  Although in normal B cells expression of BCL-6 and MUM-1 are mutually exclusive, the same is not true for diffuse large B cell lymphoma and co-expression is frequently noted.²⁵ 

Several markers (BCL-2, MIB-1 and P53), although not specific to any morphologic variant, have independent prognostic significance in diffuse large B cell lymphoma and are important predictors of therapeutic responses. ⁴  In contrast to follicular lymphoma, BCL-2 expression does not correlate with the presence of t(14;18) and its expression has been associated with treatment failure.^3-8 

Based on distinctive gene expression profiles three subgroups of diffuse large B-cell lymphoma have been identified– germinal center B cell-like, activated B-cell like and type 3 -. ¹⁰ These groups show marked differences in clinical behavior and outcome. However, they are not strictly related to the morphological variants described in the WHO classification. Centroblastic monomorphic and polymorphic variants were observed most frequently in the germinal center B cell-like group, but also were seen in 32% of the activated B-cell like group and in 27% of the third group. However, other microarray-expression profiling studies did not find that the cell of origin of diffuse large B-cell lymphoma was associated with outcome.³²  More extensive phenotypic studies are needed to further identify new discriminating markers.

Among the morphologic variants, Tcell/histiocyte rich large B cell lymphoma has some distinctive immunophenotypic characteristics. The large cells express CD20 and frequently BCL-6 (60-90%). The expression of CD30, CD10, EMA and BCL-2 varies in the literature, possibly due to differences in case selection or technical factors. CD15 is uniformly negative. ^17,19 

Diffuse Large B cell lymphoma subtypes
The following four lymphomas are considered as distinctive clinical, immunophenotypic and genetic entities in the WHO classification.

Mediastinal (thymic) large B cell lymphoma
These lymphomas arise in the mediastinum. The cell of origin is a putative thymic B cell. Most patients are females in their third to fifth decade of life. The tumor is usually composed of medium-sized cells that show a diffuse pattern of infiltration, variably associated with interstitial fibrosis. ³³ In most cases the tumor cells have abundant clear cytoplasm. Although the cells express CD20, CD45 and variably CD30, they lack immunoglobulin, HLA Class I and II and CD5. ^34-35 The expression of BCL-6 and CD10 has been demonstrated by immunohistochemistry in a subset of cases suggesting either a germinal center ³⁴ or a post-germinal center derivation.³⁵  Overexpression of MAL gene has also frequently been observed.³⁶ 

Intravascular large B cell lymphoma
This entity is an extremely rare subtype of extranodal diffuse large B cell lymphoma which was originally described involving the skin and central nervous system. The neoplastic cells express B cell associated antigens and are exclusively intravascular. A defect in homing receptors on the neoplastic cells has been postulated for the intravascular growth pattern. ³⁷ 

Primary Effusion Lymphoma
PEL usually occurs in immunodeficiency settings and originally was described as neoplastic serous effusion without associated tumor masses. It has unique morphologic, phenotypic and genotypic characteristics.(³⁸ See Immunodeficiency section).

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41. Jonathan Said, M.D., David Geffen School of Medicine at UCLA, Los Angeles, CA