Burkitt Lymphoma

Modern classification systems⁴¹ define Burkitt lymphoma as a mature B-cell neoplasm composed of monomorphic medium-sized cells (“small noncleaved cell”), with an extremely short doubling time, resulting from c-myc gene translocations. Burkitt lymphoma and leukemia (classically known as acute lymphoblastic leukemia (ALL) L3 in the French American British classification) form a biological continuum. B-cell lymphomas with a similar proliferation rate and genetic features, previously termed “small noncleaved cell lymphoma, Burkitt-like,” have also been incorporated in this clinicopathological entity as one of its morphologic variants (see later).

Histologically, Burkitt lymphoma classical variant, present in endemic cases, as well as in many of the sporadic cases, is characterized by a diffuse growth pattern, uniform medium-sized cells (typically equal in size to the nuclei of adjacent histiocytes), with coarsely clumped nuclear chromatin and one to three nucleoli. When observed in Wright-Giemsa-stained smears, these cells are large, with moderate amounts of deeply basophilic cytoplasm and prominent cytoplasmic vacuoles. These tumors typically contain numerous mitotic figures, and, matching the high cell turnover rate, they have numerous “tingible-body” macrophages with pale cytoplasm that contain ingested apoptotic cellular debris and impart a characteristic “starry-sky appearance” on low magnification. Burkitt lymphomas that are otherwise typical, but show a greater degree of pleomorphism in nuclear size and shape, and one to two large nucleoli in the neoplastic cells, were classified as the atypical Burkitt/Burkittlike variant in the previous WHO Classification (2001). These lymphomas represent a subset of the tumors previously designated as “small noncleaved cell lymphoma, Burkitt-like.” This morphologic variant can be seen in sporadic cases and in immunodeficiency-associated Burkitt lymphoma. Finally, Burkitt lymphomas with plasmacytoid differentiation (eccentric nuclei with a single central nucleolus, basophilic cytoplasm that contains abundant monotypic immunoglobulin on immunohistochemical staining) represents a third morphologic variant. This latter variant can also be seen with immunodeficiency-associated lymphoma. The most recent WHO Classification (2008)⁴² no longer defines morphologic variants of Burkitt lymphoma, but rather recognizes the wide morphologic spectrum of a single disease.

The immunophenotype of Burkitt lymphoma, regardless of its morphologic variants, is that of a mature B cell of germinal center or postgerminal center type.⁴¹ Burkitt lymphoma cells strongly express CD19, CD20, CD22, CD24, as well as CD10 and BCL6, and are negative for BCL2, CD5, CD23, CD34, and TdT. The neoplastic cells also express surface immunoglobulin (typically IgM, less commonly IgA or IgG) with light-chain (kappa or lambda) restriction. Staining for proliferation markers (Ki-67) highlights a growth fraction of nearly 100% in all the histologic variants. Recent studies have demonstrated that MYC expression detected by immunohistochemical staining highly correlates with MYC gene rearrangements when it has nuclear localization and is present in a high percentage (>70%) of the lymphoma cells.^43,44 Demonstration of this high proliferation rate is required for the diagnosis of Burkitt lymphoma. Rare cases may exhibit a precursor B-cell immunophenotype associated with the characteristic Burkitt chromosomal translocations.^45,46 These cases show morphologic and immunophenotypic features intermediate between a classical Burkitt lymphoma and a precursor B cell. The latter includes expression of CD34 and TdT and lack of surface immunoglobulin expression. Recognition of these cases as Burkitt lymphoma/leukemia typically requires integration with the cytogenetic findings.

Genetically, Burkitt lymphoma is characterized by several translocations that rearrange the c-myc gene on chromosome 8q24, bringing it under the controlling sequences (enhancers) for immunoglobulin heavy- or light-chain genes and leading to overexpression of this gene in the neoplastic B cells.47–52 These translocations include the t(8;14)(q24;q32) seen in 85% to 90% of the cases, that involves the c-myc and IgH gene loci, and the less common variants, t(2;8)(p11;q24) and t(8;22)(q24;q11.2), that juxtapose c-myc to the immunoglobulin light-chain genes, Ig kappa (on 2p11) and Ig lambda (on 22q11), respectively. The breakpoints involving c-myc and IgH are highly variable, and cluster in regions different between endemic and sporadic Burkitt lymphoma. The IgH breakpoints most often involve the VDJ region in endemic Burkitt lymphoma, and the switch region Sµ in the sporadic cases, suggesting that the neoplastic transformation occurs at different stages of B-cell development in these two disease subtypes.³³ Similarly, the c-myc breakpoints appear to cluster in two different regions that correlate with the location of the IgH breakpoints.⁵³ This high variability in breakpoints leads to difficulty in generating polymerase chain reaction (PCR) assays that can detect this translocation. Long-range PCR combined with nested PCR strategies have allowed detection of the t(8;14) and of the variant translocations.^54,55 Approximately 70% to 80% of the sporadic cases have additional chromosomal abnormalities, the most common of which involve chromosomes 1 (1q), 6 (6q), 13 (13q), 17, and 22.⁵⁶ Some of these alterations (such as those of chromosome 13q), appear to have a negative impact on prognosis in pediatric Burkitt lymphoma.⁵⁶

The biology of Burkitt lymphoma has been extensively characterized. MYC, a widely studied oncogene, initially discovered because of its involvement in the t(8;14), appears to play a central role in malignant transformation and the biological behavior of Burkitt lymphoma.50,51 MYC overexpression has been described in up to 50% of all human cancers,⁵⁷ occurring through both epigenetic and genetic mechanisms (that include chromosomal translocations and genomic amplifications). In Burkitt lymphoma, MYC is deregulated primarily by mutation/translocation and by overexpression.⁵⁸ Its oncogenic properties have been demonstrated both in vitro and in vivo, by using a variety of animal models.^52,57

MYC is a nuclear transcription factor. It is the founding member of a family of transcription factors of the basic helix-loop-helix-leucine zipper (BHLH-LZ) classes. MYC operates as a heterodimeric complex with a cofactor named MAX to bind specific DNA sequences, thereby transcriptionally regulating hundreds to thousands of target genes.⁵⁹ These genes are involved in diverse programs, including cell cycle, cell growth, apoptosis, protein translation, cell adhesion, various metabolic pathways, angiogenesis, and DNA repair. MYC can promote cell proliferation through cyclins D, B1 and A, and CDK4, that lead to G1 to S progression. It also inhibits differentiation, increases protein synthesis, and suppresses genes that encode cytoskeletal and cell adhesion molecules, thus contributing to neoplastic transformation. An interesting aspect of MYC biology, with major implications in neoplasia, is its role in apoptosis. MYC sensitizes cells to apoptosis and, therefore it appears that mechanisms that would block this downstream effect are necessary in the process of malignant transformation, allowing for cell proliferation to take precedence over cell death.⁶⁰ Apoptosis can be inhibited by a variety of proteins, such as BCL2, BCL-X_L, cFLIP, mTOR, Mdm2/Hdm2, Twist, Bmi1, and Cul7. Apoptosis promoters include a variety of proteins typically regarded as tumor suppressors, such as P53, ARF, PUMA, ATM, BAX, BID, BIM, Fas/CD95, and 4EBP1. Extensive studies of tumor cell lines and tumors arising in animal models,^57,61 as well as of primary human tumor samples, have shown that in most cases, MYC overexpression is accompanied by either the overexpression of one of the apoptosis suppressors (e.g., BCL2) or by inactivation of one of the pro-apoptotic factors (e.g., P53 or ARF).⁶² In animal models, alterations in P53-related pathways seem to play a major role in the biology of Burkitt lymphoma. The relevance of these findings to human Burkitt lymphoma is currently under study. Limited studies performed in pediatric sporadic Burkitt lymphoma suggest that these findings may be applicable to primary tumor samples as well.⁶³

Lymphoblastic Lymphoma

Modern classifications designate these neoplasms as B and T lymphoblastic leukemia/lymphoma, in an attempt to reflect the morphologic, immunophenotypic, genetic, and possibly biological continuum between the lymphomatous and the leukemic presentation in these blastic tumors.⁴¹ Indeed, at the present time, the separation between these two presentations is largely arbitrary, with lymphoblastic neoplasms involving less than 25% of the marrow cellularity being treated as lymphoma with marrow involvement, with those exceeding this cut-off being considered ALLs.^10,64,65

Histologically, lymphoblastic lymphomas efface the underlying lymph node architecture totally or partially, have a diffuse growth pattern, and may occasionally show a patchy “starry sky” appearance because of the presence of interspersed phagocytic histiocytes. The malignant lymphoblasts are characteristically small to intermediate in size, with scanty basophilic cytoplasm, homogeneous, finely dispersed nuclear chromatin and very small, inconspicuous nucleoli. A subset of cases may manifest with more abundant cytoplasm, vesicular cytoplasm, and prominent nucleoli. Although complex nuclear convolutions were initially described as a feature unique to T-lymphoblastic lymphomas, similar features may be seen in precursor B-cell neoplasms.

Immunophenotypically, the majority of lymphoblastic neoplasms designated as lymphomas (~90%) are of T lineage, whereas the remaining show B lineage differentiation (Table 97-2).^68–68 Very rare cases of NK cell origin have been described.^67,69,70 The antigen expression profile of lymphoblastic lymphomas is largely similar to that of their leukemic (ALL) counterparts. Most of the lymphoblastic neoplasms express markers of early lymphoid differentiation, including terminal deoxynucleotidyl transferase (TdT), CD34, and CD10. TdT and CD34 expression may be absent in a subset of cases, requiring careful correlation with morphology and other immunophenotypic features for the differential diagnosis with mature (peripheral) T-cell and B-cell lymphomas. T-lymphoblastic lymphomas additionally express T-lineage antigens, including CD1a, CD2, CD3, CD4, CD5, CD7, and CD8. By analogy with the stages of intrathymic T-cell differentiation, several subtypes of T-lymphoblastic neoplasms have been defined, including an early double-negative stage (CD1a–, cytoplasmic CD3+, CD7+, variably positive for other pan–T cell antigens such as CD2 and CD5), early cortical thymic stage (CD1a+, CD2+, cytoplasmic CD3+, often CD4+, CD8+, CD5+, CD7+, CD21+), and late cortical thymic stage (CD1a–, CD2+, surface CD3+, CD5+, CD4+ or CD8+). The majority of T-cell lymphoblastic lymphomas correspond to a late stage of intrathymic maturation.^71,72 Gene expression profiling studies⁷³ have shown that different genetic lesions correlate with malignant transformation at different stages of intrathymic T-cell maturation in T-cell ALL (see later). T lymphoblastic lymphomas are characterized by a more frequent expression of T-cell receptor αβ than γδ, as compared with T-ALL.⁷⁴ The immunophenotypic profile of true precursor B lymphoblastic lymphoma (without peripheral blood or bone marrow involvement) has been less extensively characterized, because of availability of paraffin-embedded tissue only, in many of the cases.^77–77 By immunohistochemistry, these neoplasms are positive for TdT, CD34, CD10, pan-B cell antigens (CD79a, CD22, PAX-5) and often negative for CD20, a mature B-cell antigen, as well as for immunoglobulin kappa and lambda light chains. These features, along with the morphology and clinical presentation, are helpful in the differential diagnosis with Burkitt lymphoma and DLBCL, as both mature B-cell neoplasms that are typically negative for TdT and CD34, and strongly positive for CD20.^78–81

Limited information is available regarding the cytogenetic abnormalities encountered in T-lymphoblastic lymphoma.⁶⁸ It is generally assumed that they resemble those of T-lineage ALL. Recurrent chromosomal abnormalities often include reciprocal translocations that disrupt developmentally important transcription factor genes and lead to their overexpression, as a result of rearrangements to loci for T-cell receptor (TCR) genes, most commonly TCRA (14q11.2) and TCRB (7q35). For example, the TAL1/SCL gene (1p32) is involved in the t(1;14)(p32;q11) present in approximately 3% of T-ALLs.⁷² The HOX11 transcription factor gene (10q24) is rearranged as part of the t(10;14)(q24;q11.2) and the t(7;10)(q35;q24).^82,83 The LMO1 (11p15) and LMO2 (11p13) oncogenes are disrupted by the t(11;14) and t(7;11), respectively.⁸⁴ Other less common translocations do not involve the TCR genes. For example, the t(9;17) translocation is more commonly detected in T-lineage lymphoblastic lymphoma than precursor T-ALL,⁸⁵ and is often associated with a mediastinal mass and aggressive disease course. The t(10;11)(p12;q14), leading to the AF10-CALM fusion gene, has been rarely reported in pediatric lymphoblastic lymphoma cases,^86,87 and limited studies suggest a poor prognostic significance.⁸⁸ The t(8;13)(p11;q11-14) has been reported in rare cases of T-cell lymphoblastic lymphoma that present with eosinophilia and myeloid hyperplasia.^91–91

Much progress has been accomplished in the recent years in the understanding of the biology of pediatric T-cell lymphoblastic neoplasms. Molecular studies combined with gene expression profiling have uncovered several oncogenes that appear to play important roles in malignant transformation and correlate with disease subgroups with potential prognostic and therapeutic implications. TAL1/SCL shows activating mutations in up to 50% of all T-ALL,73,92 independent of detectable translocations, and correlates with leukemias arrested at the late cortical stage of thymocyte maturation expressing αβ TCR, and with an inferior overall survival.⁷³ HOX11(TLX1) mutations are present in ~30% of all T-ALL, more commonly in adults than in children⁹² and in early cortical T-ALL expressing αβ TCR, and correlate with a superior survival in limited studies.⁷³ LYL1 is mutated in up to 22% of pediatric T-ALL, correlating with the double-negative early thymocyte stage of differentiation and an inferior survival.⁷³ The MLL gene is mutated in 4% to 8% of T-ALL cases, associated with maturation arrest at early thymocytes stages, expression of γδ TCR, and no impact on prognosis.⁹² Certain molecular alterations may provide opportunities for therapeutic targeting. Cryptic deletions of the INK4/ARF locus at 9p21, leading to alterations of the p14/p16 loci are present in up to 75% of all T-ALL,⁹² leading to defects in cell cycle control. Because these alterations likely lead to neoplasia through the retinoblastoma (Rb1) and P53 oncogenes, targeting the latter genes may prove effective in a significant proportion of T-ALL. In addition, 10% to 20% of T-ALL harbor constitutively activated tyrosine kinases (LCK, FLT3, ABL1), which may also provide targets for tyrosine kinase inhibitor drugs.⁹² Finally, NOTCH1-activating mutations are present in more than 50% of T-ALL.⁹³ This transcription factor activates a wide variety of cellular pathways related to cell growth, using c-MYC as an essential mediator. Inhibition of these signaling pathways has been shown to inhibit cell growth in vitro, in T-ALL cell lines.⁹⁴

Anaplastic Large-Cell Lymphoma

ALCL is a lymphoma of mature (peripheral) T cells, characterized by strong expression of CD30 (Ki-1)⁹⁵ and, morphologically, by the presence of large, “hallmark” cells.⁴¹ These lymphomas may present as systemic disease or as primary cutaneous neoplasms. A subset of the systemic lymphomas expresses the anaplastic lymphoma kinase (ALK) protein as a result of ALK gene rearrangements and have been designated by the most recent WHO classification⁴² as ALCL, ALK positive. These constitute the most common subtype of ALCL encountered in children. Only rare cases of primary cutaneous ALK + ALCL have been reported in both pediatric and adult patients.⁹⁶ Cases of ALK-negative primary cutaneous lymphoma have also been reported in children.⁹⁷

ALCL was initially described as a lymphoma composed of large anaplastic cells with pleomorphic nuclei (hence the designation), and sinusoidal lymph node involvement, that expressed CD30 (Ki-1).95,98 This histologic subtype of ALCL is now designated as the common (classic) subtype. The availability of an immunohistochemical assay for ALK expression has allowed the recognition of lymphomas with very heterogeneous morphology as ALCL. Consequently, the term ALK lymphoma or ALKoma was proposed for this category of tumors, to better accommodate morphologic variants that lack obvious anaplastic morphology.⁹⁹ ALCL typically involves the lymph node sinuses and interfollicular area, with subtotal effacement of the lymph node architecture. Regardless of their morphologic subtype, ALK-positive ALCLs consist of a mixture in variable proportions of large anaplastic “hallmark” cells with kidney- or horseshoe-shaped nuclei, and small atypical lymphoid cells with irregular nuclear outlines, as well as inflammatory cells that may include histiocytes and plasma cells. In the common (classical) type the hallmark cells predominate, sometimes forming cohesive clusters and sheets, whereas in the small cell type, these cells are sparse and the neoplastic infiltrate consists predominantly of small lymphoid cells.¹⁰⁰ In the lymphohistiocytic type, the neoplastic cells may be difficult to identify within an exuberant polymorphous inflammatory background that includes histiocytes and plasma cells, is usually poor in neutrophils and eosinophils, and sometimes shows associated fibroblastic proliferation, vascular proliferation, and fibrosis.¹⁰¹ Occasionally, the fibrotic component may impart a partial nodularity to the tumor. Within this infiltrate, immunohistochemical staining for CD30 or ALK will highlight large tumor cells that typically aggregate around blood vessels. In the monomorphic type, the hallmark cells may be difficult to find, with the tumor consisting predominantly of a monotonous, highly mitotic population of large immunoblastic cells. This latter variant may be associated with a “starry-sky” appearance because of frequent tingible-body macrophages. Rarely, ALCL may manifest with prominent (leukemic) peripheral blood involvement (at diagnosis or at relapse).^102–106 In such cases, the lymphoma cells seen on the peripheral blood smear include a mixture in variable proportions of small lymphoid cells with marked nuclear membrane irregularity (“cerebriform”) and large immunoblastic cells with deeply basophilic and occasionally vacuolated cytoplasm, and coarsely clumped nuclear chromatin. In most cases, there is a predominance of small cells, very similar to the cellular composition of the small cell variant. Notably, in such cases a significant proportion of the peripheral blood leukocytosis usually consists of granulocytes that may show prominent left shift and toxic changes. These findings, combined with the clinical picture of fever, malaise, and respiratory distress may distract from the atypical lymphoid population present. In the sarcomatoid type,¹⁰⁷ the tumor cells are sparse and present within an edematous stroma that contains atypical proliferating fibroblasts. In these cases, the tumor cells may have the classical morphology or a spindle cell appearance and may cluster around blood vessels. Rare histologic subtypes of ALCL include giant-cell rich,⁹⁸ signet-ring cell,¹⁰⁸ neutrophil-rich,^111–111 and eosinophil-rich.¹¹² The latter three have only been reported in ALK-negative ALCL.

On immunohistochemical staining, ALCL cells are characteristically positive for CD30 and ALK (in the ALK-positive cases) and usually positive for CD45 (leukocyte common antigen). In the small cell variant (including the forms with leukemic presentation), the small cells may be CD30 negative and usually show a much lower level of expression of ALK than the large “hallmark cells” also manifest within the same tumors. The majority of ALK-positive ALCLs are also EMA positive (and cytokeratin negative).¹¹³ Many tumors express CD43. Although the majority of these tumors are of T-lineage when examined at the molecular level, many of them fail to express several pan T-cell antigens as detectable by immunohistochemistry, hence the apparent “null-cell phenotype.” More than 50% of these lymphomas fail to express CD3, CD5, CD7, and CD45RO.¹¹⁴ The majority of ALCL will express CD2 and CD4, whereas they are only exceptionally CD8 positive. Also, regardless of the CD4/CD8 expression status, the tumor cells are very often positive for cytotoxic T-cell antigens (e.g., TIA-1, perforin, granzyme B). Rare ALCLs also express CD56, which has been proposed as a factor of poor prognosis. Flow cytometric analysis usually shows a similar immunophenotype. In addition, 40% to 50% of the analyzed cases have been found to express antigens typically associated with myeloid differentiation, such as CD11b, CD13, CD15, and CD33.^117–117

All ALK-positive ALCLs show aberrant expression of ALK, a membrane-bound receptor tyrosine kinase that is not expressed by any normal lymphoid elements. Across normal tissues, ALK expression can only be found within the brain, in scattered neurons.¹¹⁸ The aberrant ALK expression seen in ALCL is the result of various chromosomal translocations that juxtapose the ALK locus (chromosome 2p23) to other partner genes, which in turn lead to the activation and determine the subcellular localization of the ALK (as seen by immunohistochemistry). The most common translocation manifest in up to 75% of ALCL is t(2;5)(p23;q35).^119–123 The result of this translocation is a fusion protein (p80) that includes the portion of the ALK protein with associated tyrosine kinase activity and the oligomerization domain of nucleophosmin (NPM) encoded in 5q35.^118,124,125 Normal NPM molecules dimerize and shuttle between the cytoplasm and the nucleus (nucleolus). This is why, in most ALCL that contain the NPM-ALK fusion, ALK expression can be detected immunohistochemically in the nucleus and the cytoplasm. Several other translocations have been described in ALK-positive ALCL (frequency 2% to 5%). They include t(1;2)(q21;p23) (fusion partner tropomyosin 3 gene, TPM3),^122,126 t(2;3)(p23;q21) (fusion partner tropomyosin receptor kinase-fused gene, TFG), t(2;22)(p23;q11) (fusion partner clathrin heavy-chain gene, CLTCL),¹²⁷ and inv(2)(p23;q35) (fusion partner Pur H gene, ATIC).¹²⁸ In all these cases, the fusion partners for ALK are proteins that dimerize (aspect that appears to be essential for activating the kinase function of ALK) but typically localize to the cytoplasm. Therefore in ALCL containing these translocations, ALK expression can be detected only in the cytoplasm. The presence of NPM-ALK can be explored for diagnostic purposes using PCR, RT-PCR, or fluorescence in situ hybridization (FISH). Aberrant ALK expression appears to play an essential role in tumorigenesis in ALCL.^129,130 ALK inhibitors are currently being studied as potential therapeutic agents for this type of lymphoma.

Numerous studies have uncovered signaling pathways that are important in the biology of the ALK-positive ALCL, and especially in tumors that harbor the NPM-ALK fusion product. These pathways appear to be important in NPM-ALK-induced tumorigenesis and are attractive therapeutic targets. They include the Jak/STAT (specifically the Jak3 and STAT3 family members) and the PI3Kinase/Akt pathways. Inhibition of these pathways has been shown to induce apoptosis and inhibit tumor cell growth in studies performed on lymphoma cell lines. Inhibition of the Src kinase pp60^src and pharmacological blockade of Hsp90 have likewise been shown to induce lymphoma cell apoptosis in vitro.¹³¹

Diffuse Large B-cell Lymphoma

The WHO classification defines DLBCL as a mature B-cell neoplasm with diffuse growth pattern, composed of large lymphoma cells (i.e., larger than the normal macrophage nuclei).⁴¹ Several morphologic variants are accepted (all of which may be encountered in children).¹³² These variants include centroblastic, immunoblastic, T-cell/histiocyte rich, and anaplastic. DLBCL may manifest as a de novo lymphoma or as progression of a preexisting low-grade lymphoma. In children, de novo DLBCL is the more common occurrence. In a small percentage of cases, a preexisting follicular lymphoma component may be identified.

DLBCL is a tumor with diffuse growth pattern composed, in variable proportions, of several types of large lymphoma cells that include centroblasts, immunoblasts, and anaplastic cells. The predominance of one or another cell type leads to the various morphologic subtypes. Centroblasts are medium-sized to large cells with scanty cytoplasm, irregular nuclear outlines, vesicular chromatin, and several peripherally located medium-sized nucleoli. Immunoblasts are large cells with abundant basophilic cytoplasm; vesicular or clumped chromatin; eccentric nuclei; and large, single, centrally located nucleoli. Anaplastic cells are similar to those seen in ALCL and include giant cells with pleomorphic nuclei or Reed-Sternberg-like morphology. In the centroblastic variant (>80% of the pediatric DLBCL),¹³³ the tumor consists of a polymorphous mixture of centroblasts and immunoblasts, with <90% immunoblasts. In the immunoblastic variant (<10% of the pediatric DLBCL),¹³³ >90% of the lymphoma cells are immunoblasts. In the rare anaplastic variant, frequent anaplastic cells are admixed with centroblasts and immunoblasts. Occasionally, a cohesive growth pattern may be present in the latter subtype. In T-cell/histiocyte-rich B-cell lymphoma, the bulk of the tumor consists of inflammatory cells, whereas the neoplastic cells (centroblasts, immunoblasts, Reed-Sternberg-like) represent <10% of cellularity. Rare cases of DLBCL with plasmablastic morphology, immunohistochemical expression of ALK, and the presence of ALK fusion transcripts have been reported.^134,135

In all subtypes of DLBCL, the lymphoma cells strongly express CD45 and CD20, as well as other B-cell antigens, such as CD19, CD22, and CD24. A subset of DLBCL may express CD10, BCL-6, and BCL-2 (resembling follicle center cell lymphoma). Occasional DLBCLs are positive for CD5. CD30 expression is typically present in the anaplastic subtype and may be seen in the T cell–rich subtype. Light-chain restricted immunoglobulin expression may be demonstrated in many cases, although a significant proportion of these lymphomas may lack immunophenotypic evidence of immunoglobulin expression. Such cases show clonal rearrangements of the immunoglobulin genes when analyzed by molecular means.¹³⁶ Some of the DLBCL may show high proliferation indices when analyzed by Ki-67 immunohistochemistry, but they typically do not exceed 90%, a feature helpful for the distinction from Burkitt lymphoma. The ALK-positive plasmablastic B-cell lymphomas have unique immunophenotypic features, which reflect their distinctive morphologic differentiation. They lack expression of mature B-cell antigens such as CD20, and express weak CD79a, CD138, and abundant cytoplasmic immunoglobulin, typically IgA. They are CD30 negative in most cases.¹³⁴

When cytogenetic studies are performed, these tumors typically show complex chromosomal abnormalities. Immunophenotypic and genetic studies including gene expression profiling by several methodologies have demonstrated that DLBCL is a biologically heterogeneous group of neoplasms that includes at least two major subgroups: a germinal center (GC)-like group and an activated peripheral blood B-cell (ABC)-like group.139–139 The GC-like group is characterized by expression of CD10 and BCL-6, while negative for multiple myeloma oncogene (MUM1); contains ongoing Ig gene mutations; may show the t(14;18), 12q12 gains, amplification of c-rel gene (chromosome 2p); low levels of Blimp-1 gene mRNA expression; and a more favorable outcome. A significant number of the GC-like tumors occurring in adults also harbor the t(14;18), IGH/BCL2 translocation. The ABC-like group lacks expression of CD10 and BCL-6 as well as ongoing Ig gene mutations; may show trisomy 3 or gains on chromosomes 3q or 18q21-22 and losses at 6q21-22; has constitutive activation of nuclear factor kappa B (NF-κB), high levels of Blimp-1 mRNA expression, with lack of protein expression; aberrant IL-4-induced STAT6 signaling; and is associated with a poor prognosis.¹⁴⁰

Recent studies have shown that in pediatric DLBCL, the distribution and characteristics of these subgroups are different, with children showing a much higher frequency of the centroblastic variant (83% vs. 75%, respectively) and of the GC-like subtype (83% vs. 40% to 48%, respectively), when compared with the adult tumors, and with a much lower incidence of the ABC-like subtype in this age group.¹³³ In addition, the t(14;18) appears to be virtually absent in the pediatric GC-like DLBCL.¹³³ Furthermore, these disease subgroups do not appear to have an impact on prognosis in children.¹³³ All these insights offer interesting opportunities for differential, age-appropriate therapeutic targeting in the various disease subtypes in DLBCL. The most recent WHO Classification (2008)⁴² has introduced an additional category of aggressive B-cell lymphoma, designated as “B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and Burkitt lymphoma. This is a heterogeneous category of mature B-cell lymphomas, which includes cases of DLBCL with a gene expression profile similar to Burkitt lymphoma, many of which harbor dual translocations involving MYC and BCL2 (“double-hit lymphomas”), correlating with a particularly poor outcome. It is not yet clear if cases of DLBCL with high proliferation rate and immunophenotype resembling Burkitt lymphoma, but lacking a detectable MYC translocation, should also be included in the latter category.

Primary Mediastinal (Thymic) Large B-Cell Lymphoma (Med-DLBCL)

Med-DLBCL is a clinicopathologically and biologically distinct subtype of DLBCL that accounts for less than 10% of all large-cell lymphomas in children.40,141 It has also been designated as mediastinal clear-cell lymphoma of B-cell type and mediastinal diffuse large-cell lymphoma with sclerosis. It is currently thought to originate in the CD19+ CD21– B cells and asteroid B cells, that reside in the thymic medulla.¹⁴²

Histologically, Med-DLBCLs can present as any of the DLBCL variants. Approximately half of the cases can have prominent associated stromal fibrosis, adding to the difficulty in obtaining suitable diagnostic biopsy samples in many of these cases. Cytologically, the neoplastic cells may have the appearance of centroblasts or immunoblasts, or may have abundant pale cytoplasm and markedly lobated, “flowerlike” nuclear membrane outlines (so-called “clear-cell” appearance).41,143–148

Immunophenotypically, Med-DLBCL shows a mature B-cell immunophenotype, with expression of CD45 and of pan-B-cell antigens, including CD19, CD20, CD79a, and PAX-5. There is typically partial and variable coexpression of CD30.¹⁴⁹ Some cases may express CD23. Other antigens expressed include BCL-2 and BCL-6.¹⁵⁰ The neoplastic cells are typically negative for CD10, CD21, immunoglobulin, and HLA class I and II molecules.¹⁵¹

Little conventional cytogenetic data is available largely because of the small tumor samples typically available in these patients. Alternative approaches have shown consistent gains of material on chromosome 2p (seen in ~25% of the cases), with amplification of c-REL, a gene encoding for a transcription factor important in B-cell development, as well as gains of material on 9p (50% to 75% of the cases) with amplification of the JAK2 gene, whose product is important in the JAK/STAT signaling pathway. Other alterations that have been described involve the chromosomes 6p (MHC class I locus), Xq, 12q, and the P53 gene.142,152,153

Biologically, Med-DLBCL has the profile of activated germinal center or postgerminal center cells. The lymphoma cells contain clonal Ig gene rearrangements with somatic mutations,¹³⁶ BCL-6 gene mutations, and express MUM1/IRF4, BCL6, and the B cell–specific transcription factors OCT-2, BOB.1, and PU.1. Overexpression of an IL-4-inducible gene, FIG1 (IL-4I 1), suggests the possible oncogenic activation of signaling pathways such as IL-4/IL-13 or Janus kinase 2 (JAK2). Gene expression profiling experiments have demonstrated a close resemblance of this lymphoma with that of classical HL, suggesting that at least in the mediastinal location, these neoplasms may share a common origin from a thymic B cell. Notably, both tumors show overexpression of genes involved in cytokine signaling pathways, such as IL-13, tumor necrosis factor (TNF), and NF-κB. These findings suggest that targeting of some of these overexpressed genes or of activated NF-κB could represent therapeutic possibilities in these tumors.¹⁴²

Uncommon Pediatric Lymphomas

Lymphomas that occur infrequently in children and adolescents include follicle center cell lymphoma,¹⁵⁴ hepatosplenic lymphoma,¹⁵⁵ panniculitislike T-cell lymphoma,¹⁵⁶ mycosis fungoides,^157–160 T/NK lymphoma, nasal-type natural killer,¹⁶¹ marginal zone lymphoma (including the extranodal MALT type),¹⁶² and HTLV-1-associated leukemia/lymphoma.^163,164 The clinical and biological features of these lymphomas in children are generally similar to those observed in adults; however, this conclusion is drawn from the very few reported pediatric cases. Pediatric follicular lymphomas and pediatric marginal zone lymphomas appear to have a spectrum of clinicopathological features distinct from the tumors seen in adults and will be detailed below.

Pediatric Follicular Lymphoma⁴²

Because of the relatively small numbers of cases reported, little is known about the genetics and biology of the follicular lymphomas occurring in children. It appears that in addition to rare cases of classic follicular lymphomas, resembling phenotypically and genetically those seen in adults, children may present with a second subtype, which is in fact more common in this age group.154,165–170 These follicular lymphomas, typically limited to one site (most commonly lymph nodes in the head and neck area or testis/epididymis) are characterized by effacement of the normal architecture by neoplastic follicles composed of large, expansive, irregular germinal centers with geographic appearance, and thin or absent mantle zones. Cytologically, the germinal centers are typically composed of monotonous predominantly large centroblasts, with occasional cases showing numerous mitotic figures and a “starry-sky” appearance because of numerous tingible-body macrophages. Most of these cases have morphologically the appearance of grade 2 or grade 3 (WHO) follicular lymphoma, albeit without the prognostic significance that this grading would have in classical adult follicular lymphoma.

Immunophenotypically, the neoplastic cells express CD20, CD10, BCL-6, and in some cases, CD43, and are typically negative for BCL-2. Staining for CD21 highlights the follicular dendritic cell meshwork that underlies the expanded neoplastic follicles.

Pediatric follicular lymphoma lacks, in most cases, the t(14;18) and BCL-2 rearrangements, features that distinguish it from the usual cases of adult follicular lymphoma. They typically have clonal Ig gene rearrangements and have been found to harbor BCL-6 gene rearrangements. Interestingly, they also lack P53 overexpression, another feature that distinguishes them from the high-grade adult-type follicular lymphomas. All of these features suggest an alternate molecular pathogenesis for pediatric follicular lymphoma, justifying alternate therapeutic options for these patients.

Pediatric Nodal Marginal Zone Lymphoma⁴²

These cases appear to have distinctive clinical and morphologic characteristics, based on very limited studies addressing this entity.¹⁶² These lymphomas present predominantly in males with typically asymptomatic disease, mostly stage I, involving most often lymph nodes in the head and neck area. Histologically, these lymphomas are largely similar to those seen in adults, with the distinctive feature of lymphoma cells often colonizing follicles with progressively transformed germinal centers.

Immunophenotypically, the lymphoma cells express CD20, often show aberrant expression of CD43, and are characteristically negative for CD5 and CD10. Flow cytometric analysis shows immunoglobulin light-chain restriction, and molecular clonality studies usually demonstrate clonal IGH gene rearrangements. The latter studies are sometimes necessary in order to differentiate between these indolent lymphomas and reactive lymphoid hyperplasia.