Introduction

Substantial clinical and laboratory advances have improved our understanding of both the pathogenesis and treatment of the malignant lymphomas of childhood and adolescence, which comprise Hodgkin lymphoma and the non-Hodgkin lymphomas (NHLs).1–5 These are the third most common type of cancer in children in the United States, comprising approximately 15% of newly diagnosed cases in this age group each year.^6–11 Among children less than 15 years of age, the NHLs account for approximately 60% of cases. However, when children up to age 18 are included, there is a slight predominance of Hodgkin lymphoma.^10,11

The NHLs of childhood are markedly different from those of adulthood.4,12 Diffuse high-grade extranodal subtypes account for the majority of pediatric cases, whereas low- and intermediate-grade lymphomas are predominant in adults. These differences are probably due in part to age-related maturational changes in the immune system and consequently in the types of cells susceptible to malignant transformation.⁵ The differences between adults and children in histologic subtype underlie the differing clinical features, staging, and treatment strategies in these age groups.⁴

The clinical presentation, staging, histologic subtypes, and treatment strategies in children and adults with HL are less dissimilar; however, some differences are worth noting.¹³ Epidemiological studies have suggested three distinct forms that depend on age: the childhood form in patients 14 years of age or younger, the young adult form (ages 15 to 34 years), and an older adult form in individuals 55 to 74 years of age.¹⁴ Among the four histologic subtypes of HL described in the Rye classification system,¹⁵ the nodular sclerosing subtype is most common in children, occurring in 40% of younger children and 70% of adolescents.¹⁶ Mixed-cellularity HL occurs in approximately 30% of cases and is more common in children with HIV infection and in those less than 10 years of age—it is frequently associated with an advanced stage (with extranodal extension) at presentation.¹⁶ Lymphocyte-predominant HL accounts for approximately 10% to 15% of pediatric cases, is usually associated with localized disease at presentation, and occurs more commonly in younger patients and in boys. The fourth subtype of Hodgkin lymphoma, lymphocyte depletion, is very rare in children.¹³

Despite excellent event-free survival rates in HL, there are well-recognized challenges related to the sequelae of therapy, which include endocrine dysfunction, chemotherapy-induced sterility, radiation-induced abnormalities in bone growth, chemotherapy- and radiation-related second cancers, and late cardiac deaths.¹³ Most current studies are exploring strategies that maintain an excellent treatment result while reducing late effects. For example, a combined-modality approach with low-dose radiation and combination chemotherapy may reduce the bone growth abnormalities associated with high-dose extended-field radiation therapy.¹³ However, as therapy is modified in an attempt to reduce the risk of late effects, the rates of event-free survival may be compromised.^19–19 For example, attempts at reducing alkylating agent–related late effects in patients with advanced-stage disease by substituting other chemotherapeutic agents have lowered event-free survival rates.^17,20 There have been some recent clinical trials that have also attempted to reduce toxicity without compromising outcome. Metzger et al. reported that among children with favorable-risk HL and a complete early response to vincristine, doxorubicin, methylprednisolone (VAMP) chemotherapy, a high rate of 2-year event-free survival could be achieved with limited use of radiotherapy.²¹ The results of the CCG 5942 study, which examined the benefit of adding involved field radiation therapy (IFRT) for patients who achieved a complete response to chemotherapy, were recently published.²² They reported a statistically significant improvement in EFS for those who received IFRT; however, there was no associated improvement in overall survival. The refinement of a risk-adapted treatment approach to the management of pediatric HL remains a challenge.²³ These and other issues regarding the management of children with HL will be discussed in Chapter 105. The remainder of this chapter will focus on the NHLs of childhood.

Epidemiology and Pathogenesis

NHLs may occur at any age in childhood, but are unusual in children younger than age 3 years; the median age at diagnosis is approximately 10 years.4,24 There are approximately 500 new cases of pediatric NHL in the United States each year.^7,9,10,25 In contrast to Hodgkin lymphoma, which has a bimodal age distribution with peaks in early and late adulthood, the incidence of NHLs increases steadily with age.¹⁰ NHL occurs almost twice as commonly in whites as in blacks, and two to three times more often in boys than in girls; the explanation for these differences has yet to be elucidated.⁵

There are specific populations at increased risk for the development of NHL.5,26–29 These include individuals with primary immunodeficiency syndromes,^5,28,29 including ataxia-telangiectasia (A-T), Wiskott-Aldrich syndrome, X-linked lymphoproliferative syndrome (XLP), common variable immunodeficiency, Nijmegen syndrome, autoimmune lymphoproliferative syndrome (ALPS). It is important that these syndromes be recognized so that appropriate therapy can be designed. For example, in the management of children with A-T who develop a malignancy, involved field irradiation and radiomimetics such as bleomycin should be avoided. Children with A-T are also at increased risk for the development of late-onset hemorrhagic cystitis following exposure to cyclophosphamide. Boys with XLP are at increased risk for developing fatal infectious mononucleosis and/or B-cell lymphomas. XLP should be considered in any boy with a high-grade B-cell lymphoma whose brother has had either fatal infectious mononucleosis or B-cell lymphoma, or in any boy who has had two primary B-cell lymphomas. Children who have received immunosuppressive therapy (e.g., recipients of bone marrow or organ transplants) and those with the acquired immunodeficiency syndrome (AIDS) are also at a higher risk for developing NHL.²⁷ The overall prevalence of lymphomas among children with HIV infection is approximately 1.6%.²⁷ Among pediatric HIV-positive hemophiliacs, NHL is 36 times more frequent than in HIV-negative children with factor 8 deficiency. The majority of the HIV-associated NHLs have a B-cell immunophenotype with either aggressive B-cell/Burkitt or large-cell morphology. Proliferative lesions of mucosa-associated lymphoid tissue (MALT), which may be either benign or malignant, have also been described in children with HIV infection.²⁷ Although deficient T-cell function has been implicated in these congenital and acquired immunodeficiency states, further study is required to fully clarify the mechanisms of pathogenesis.

There are well-recognized geographic differences in both the incidence and the distribution of histologic subtypes of NHL.5,6,30 For example, NHLs are very rare in Japan but are very common in equatorial Africa. More specifically, Burkitt lymphoma accounts for approximately one-half of all childhood malignancies in equatorial Africa and is the predominant NHL subtype in Northeastern Brazil and areas of the Middle East.³⁰ In contrast, lymphoblastic lymphomas are the predominant histologic subtype in southern India.³¹ In some parts of the world, the distribution of histologic subtypes of childhood NHL has yet to be firmly established.

There are also geographic differences in the clinical and biological features of some NHLs.5,31,32 For example, Burkitt lymphoma in equatorial Africa (endemic Burkitt lymphoma) frequently involves the jaw, abdomen, orbit, paraspinal area, and central nervous system, whereas the common sites of involvement associated with Burkitt lymphoma in the United States and Western Europe (sporadic Burkitt lymphoma) include the abdomen, bone marrow, and nasopharynx.⁵ The predominant chromosome 8 breakpoints, as well as the breakpoints within the immunoglobulin heavy-chain gene (on 14q32) differ between sporadic and endemic cases.³³ In addition, sporadic cases have more complex karyotypic abnormalities than endemic cases in addition to the classic translocations, suggesting distinct mechanisms of malignant transformation.³⁴ The endemic cases are associated with IgM expression, whereas the sporadic cases generally are not.⁵ The sporadic and endemic cases also differ with respect to Epstein-Barr virus association.³² The overlap of the lymphoma belt in equatorial Africa with the malaria belt prompted speculation that an infectious agent might be involved in lymphomagenesis. This led to the discovery of the Epstein-Barr virus (EBV) and its association with African Burkitt lymphoma.⁵ Although a direct role in pathogenesis has not been demonstrated, the circumstantial evidence for its involvement is compelling. It has been suggested that as a B-cell mitogen, EBV increases the target pool of cells potentially susceptible to malignant transformation.³² Supporting this hypothesis is evidence that Rag gene expression can be induced by EBV, theoretically increasing the likelihood of a translocation occurring in immature B cells that are about to rearrange their immunoglobulin genes.³⁵ The potential role of Epstein-Barr nuclear antigen (EBNA)-1 in pathogenesis has been suggested by experiments demonstrating that lymphomas develop in mice transgenic for EBNA-1.³⁶ Moreover, an identified EBNA-1 variant has been shown to be associated with the majority of Burkitt lymphoma cases studied, prompting investigators to speculate that this tumor-associated mutation alters EBNA-1 function in a way that directly or indirectly provides a growth advantage for the lymphoma cell.³⁷ A more direct role for EBV in lymphomagenesis is suggested by studies of the EBV-positive Burkitt lymphoma cell line, Akata, which loses its malignant phenotype with spontaneous loss of EBV; however, the malignant phenotype is regained with EBV reinfection.³⁸ EBV association has been reported in approximately 90% of the endemic (i.e., African) Burkitt tumors and in approximately 15% of sporadic cases (United States and Western Europe).⁵ Aberrant and disrupted expression of the EBV genome has recently been reported in cases of sporadic Burkitt lymphoma that were EBV-negative by conventional EBNA screening.³⁹ This observation, coupled with the 50% rate of EBV association in Burkitt tumors in other parts of the world (including Brazil, Russia, Argentina, and Chile), suggest a widespread role for this virus in lymphomagenesis.

Pathology and Biology

The classification systems applied to pediatric NHL are similar to those used for lymphomas occurring in adults. In the National Cancer Institute (NCI) Working Formulation for clinical use, published in 1982,¹² which subclassified NHLs based on their morphologic appearance and clinical aggressiveness into three grades (low, intermediate, and high), most of the childhood NHLs would be designated as high-grade lymphomas (Figure 97-1). The Revised European-American Lymphoma (REAL) Classification introduced in the mid-1990s⁴⁰ and the World Health Organization (WHO) Classification initially published in 2001 and updated in 2008^41,42 define the different categories of NHL as distinct clinicopathological entities, with well-defined morphologic, immunophenotypic, and genetic features, named, where feasible, according to their postulated counterparts in the normal lymphoid system. This has led to the definition of several lymphoma subtypes within the high-grade lymphoma category previously assigned to most pediatric lymphomas. These include Burkitt lymphoma, diffuse large B-cell lymphoma (DLBCL, including a mediastinal or thymic subtype), B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and Burkitt lymphoma,⁴² precursor B or T lymphoblastic lymphomas, and anaplastic large-cell lymphoma (ALCL). Some low-grade lymphoma subtypes, including follicular center cell lymphoma and marginal zone lymphoma may also occur in children where they appear to have distinct clinicopathological features, albeit with less frequency. The WHO classification of pediatric NHLs is summarized in Table 97-1.

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.

Clinical Presentation

The primary sites of involvement and the extent of disease spread determine the clinical features of NHL at presentation (see Figure 97-1).^4,5,25 Although adults usually present with nodal disease, children with NHL usually present with extranodal disease, most frequently involving the abdomen (31% percent of cases), the mediastinum (26% of cases), or head and neck region (29% of cases).⁴ These are rapidly growing tumors associated with hematogenous disease spread, and the majority of children with NHL present with locally invasive or advanced-stage disease. Involvement of the central nervous system is characterized by the presence of cranial nerve palsies and/or cerebrospinal fluid pleocytosis. When the bone marrow is involved, the distinction between NHL and leukemia is somewhat arbitrary: if more than 25% of the marrow is replaced by lymphoblasts, the patient is considered to have leukemia; if less than 25%, the patient is considered to have advanced-stage NHL with marrow involvement.

There is a striking relationship between the histologic subtype and the presenting site of disease in the lymphoblastic and Burkitt lymphomas.⁴ Among the lymphoblastic lymphomas, the typical primary site of involvement is the mediastinum and/or head and neck region, but rarely the abdomen, whereas the Burkitt lymphomas typically present in the abdomen or head and neck region, but rarely the mediastinum. In contrast to patients with lymphoblastic and Burkitt lymphomas, children with large-cell lymphomas may present with disease at almost any location. Involvement of the central nervous system at diagnosis is associated with both the Burkitt and lymphoblastic histiotypes, but rarely with large-cell histology.¹⁷¹ In contrast, spread to the bone marrow may occur in any of these three histologic subtypes.^4,172

Primary involvement of the abdomen, as is typical of Burkitt lymphoma, may be associated with nausea, vomiting, or abdominal pain at presentation. These tumors usually arise from the distal ileum and result in obstruction of the bowel by either intussusception or direct compression of the lumen. Other primary sites of involvement in the abdomen include the appendix and/or large bowel. Abdominal tumors may be associated with malignant ascites; involvement of kidney, liver, or lymph node; and invasion of adjacent structures, including the abdominal wall.4,5 Involvement of the pelvis may include ureteral compression with associated hydronephrosis.

Primary involvement of the mediastinum, as is typical of advanced-stage lymphoblastic lymphoma and mediastinal large B-cell lymphoma, may be associated with respiratory symptoms ranging from mild cough to severe respiratory distress caused by direct tumor compression of the airway, requiring emergency attention (see Emergency Situations).^4,5 Associated pleural effusions may further complicate the respiratory status. Compression of the superior vena cava by the tumor may obstruct venous blood return, resulting in swelling of the neck, shoulder, and face (“superior vena cava syndrome”); this condition may predispose the patient to the development of deep venous thrombosis. In rare cases, children with mediastinal masses may also have cardiac irregularities or tamponade.⁵

Involvement of the central nervous system may cause symptoms resulting from increased intracranial pressure, including nausea, vomiting, headache, and vision changes; there may also be neurological abnormalities on physical examination resulting from palsies of cranial nerves innervating the face or extraocular muscles. Involvement of the bone marrow may be associated with bone pain, pallor, neutropenia, and/or thrombocytopenia with associated bruising and bleeding. Involvement of the skin occurs in approximately 4% of children with newly diagnosed NHL. When present, it is usually associated with CD30+ anaplastic large-cell histology (ALCL)175–175; however, lymphoblastic lymphomas (often non-T-cell immunophenotype) may also involve the skin.^176,177

Diagnosis and Differential Diagnosis

The differential diagnosis of NHL comprises both benign and malignant conditions. If there is no mediastinal mass, and if blood counts and physical examination are within normal limits except for an isolated painless enlarged peripheral lymph node, a 10- to 14-day trial of antibiotics is permissible to treat presumed bacterial adenitis. Serologic and skin testing may be helpful in the diagnosis of histoplasmosis, tuberculosis, and Epstein-Barr virus infection, which may also cause adenopathy simulating lymphoma.

The NHLs of childhood grow very rapidly; therefore an expeditious diagnostic workup in consultation with a pediatric oncologist is indicated if NHL is suspected (Figure 97-2). The diagnosis of NHL is most readily established by examination of tissue obtained by open biopsy of the involved site. A comprehensive characterization of the biological features of the tissue, including histologic, immunophenotypic, cytogenetic, and molecular studies, will help to distinguish NHL from the small round blue cell tumors, including Ewing sarcoma, neuroblastoma, and rhabdomyosarcoma. When patients (such as those with large anterior mediastinal masses and associated airway compression) are too unstable to undergo anesthesia for open biopsy, the diagnosis may be established by parasternal fine-needle aspiration or biopsy with local anesthesia.¹⁷⁸ Among children who present with an associated pleural effusion, thoracentesis with cytologic examination of pleural fluid is usually diagnostic. For children with large abdominal Burkitt tumors, either percutaneous aspiration of the mass or paracentesis to obtain ascitic fluid often yields diagnostic cytologic and cytogenetic findings. A bone marrow and cerebrospinal fluid examination should be performed early in the workup of a child with suspected NHL, because these studies may be diagnostic and may preclude the need for more invasive procedures.

Prognostic Factors

Tumor burden at diagnosis, reflected by both the disease stage and serum LDH, is an important predictor of outcome for children.⁴ In one large, single-institution study of childhood NHL, the treatment era, disease stage, and serum LDH level all emerged as independent and significant prognostic indicators.⁴ Serum IL-2 receptor levels, which reflect tumor burden, have also been shown to predict outcome.^195,196 Of interest, stage was not found to be predictive of outcome among children with ALCL treated on the recently completed ALCL study.

A poor early response to therapy is considered by some to confer a poorer prognosis, as has been demonstrated in children with ALL.¹⁹⁷ In the LMB-89 protocol, poor early response is a criterion for placement in the most intensive arm of therapy.¹⁹⁸ The current COG study for children with advanced-stage lymphoblastic lymphoma is examining the impact of early response as determined both by diagnostic imaging and flow cytometric MRD technology. The use of flow cytometric MRD technology in children with T-lymphoblastic lymphoma demonstrated a poorer treatment outcome for those with a higher level of minimal disseminated disease (MDD) in the bone marrow at the time of diagnosis.¹⁹⁹ Specifically, those with greater than 1% level of lymphoblasts had a poorer event-free survival compared to those with a lower level of detectable disease. Of interest, this study also demonstrated a comparable level of MDD in the bone marrow and peripheral blood—a similar observation as made in patients with T-ALL.²⁰⁰

Prognostic factors have also been identified with respect to specific histologic subtypes. For example, among adults with ALCL, ALK protein expression is a favorable prognostic factor.203–203 It is unclear if ALK status has prognostic significance in children, although one study suggested that it did not influence outcome.²⁰⁴ Additional data are needed on this subtype to be certain of its prognostic significance. In a multivariate analysis of clinical features in childhood ALCL, mediastinal, visceral (lung, liver, spleen), and skin involvement were associated with adverse risk.²⁰⁵ There are certain pathological features that have been shown to have prognostic significance in pediatric ALCL.²⁰⁶ For example, in the ALCL99 study, those with lymphohistiocytic–small cell variant components had a significantly poorer event-free survival compared to those lacking this feature. Another study suggested that those children with B-lineage ALCL may have a poorer prognosis.¹³⁴ A poorer event-free survival has also been associated with a leukemic presentation.¹⁰² Among children with ALCL, the presence of circulating tumor cells in the peripheral blood or bone marrow as detected by either quantitative or qualitative PCR has been reported to be associated with a poorer outcome.²⁰⁷

Primary Treatment

Treatment of the NHLs of childhood has advanced dramatically over the past 30 years through the incremental development and clinical testing of effective multiagent chemotherapeutic regimens.4,5,25,171,198,205,208–246 In a randomized trial comparing two of the first successful regimens for treating childhood NHL (the cyclophosphamide-based COMP regimen and the multiagent LSA₂L₂ regimen designed for ALL), the Children’s Cancer Group clearly demonstrated that children with limited-stage disease fared well regardless of histology or treatment arm assignment, whereas treatment outcomes for children with advanced-stage disease varied with histology and treatment arm.²⁰⁸ Children with Burkitt lymphoma had a better outcome with the COMP regimen,²⁰⁸ whereas those with lymphoblastic NHL fared better with the LSA₂L₂ regimen.^210–210 However, among those who presented with advanced-stage large-cell disease, neither treatment arm emerged as superior. In contrast to the stage- and histology-directed therapeutic approach that has been historically used in the United States, a stage- and immunophenotype-directed approach is predominant in Europe. Recent studies in both the United States and Europe suggest that the immunophenotype-directed approach has advantages for treating children with advanced-stage large-cell NHL, a family of lymphomas whose prognoses vary with immunophenotype.²¹¹

Surgery and radiation therapy have minimal roles in the management of childhood NHLs.4,5 Surgery is indicated only for diagnostic purposes and in cases where an ileocecal mass, associated with mesenteric nodes only, can be completely resected (resulting in downstaging from III to II, which requires less intensive systemic therapy; see Staging).²⁴⁷ Otherwise, aggressive debulking procedures should not be undertaken. Prospective randomized trials have demonstrated that the incorporation of IFRT into a multiagent chemotherapy regimen does not improve outcome.^214,244 The use of radiation therapy in the management of relapse, central nervous system disease, and certain emergency situations will be discussed in other sections.

Treatment Complications

As the survival of children with malignant lymphomas has improved, there has been increased attention focused on treatment-related late effects.²⁷⁶ The most serious treatment sequelae in survivors of childhood NHL include anthracycline-related cardiomyopathy, cyclophosphamide-related sterility, second cancers, and transfusion-related hepatitis. Attempts to reduce or eliminate these complications have included the dose reduction or elimination of both specific chemotherapeutic agents as well as involved field irradiation. Improving the process of blood product screening has also been an important area of focused research.

The design of clinical trials for pediatric NHL has been strongly influenced by the desire to avoid anthracycline-induced cardiac toxicity. Although it has been demonstrated that adults may tolerate cumulative doses of doxorubicin of 550 mg/m², much lower cumulative doses have been shown to have clinically significant effects on ventricular contractility in children.²⁷⁷ Factors that have been shown to be predictive of cardiac dysfunction include cumulative anthracycline dosage, higher anthracycline dose intensity, younger age at time of treatment, female sex, time interval since completion of therapy, and combined-modality therapy that includes mediastinal irradiation.^278,279 Future trials that focus on the reduction of cumulative anthracycline dosage and the utility of cardioprotectant agents are indicated.²¹⁹

The preservation of fertility is another important concern in the development of optimal therapeutic strategies for children with NHL. A dose-related depletion of germinal cells is associated with the use of alkylating agents such as cyclophosphamide and ifosfamide; these agents tend to be more gonadotoxic in males. Studies thus far have suggested that sterility is likely at cumulative doses of cyclophosphamide greater than 7.5 g/m², whereas fertility is usually maintained at cumulative dosages less than 4 g/m². In this regard, a number of pediatric NHL trials have attempted to eliminate or reduce the dosage of cyclophosphamide.²³⁹

Pediatric NHL trials have also focused on the elimination of involved field irradiation. In the first of two POG trials for limited-stage NHL, it was demonstrated that IFRT could be safely eliminated without compromising outcome.²¹⁴ A similar observation was made for children with advanced-stage disease in a St. Jude study.²⁴⁴ Although involved field irradiation is not used in most current NHL trials, cranial irradiation is considered in the management of overt CNS involvement in children with lymphoblastic lymphoma.

Children with refractory or recurrent disease, particularly after receiving modern intensive therapy, are generally considered to have a poor prognosis. Thus, aggressive or novel approaches are often used to treat relapse. Current approaches generally comprise multiagent salvage chemotherapy followed by an intensification phase that may include autologous or allogeneic hematopoietic stem cell transplantation (HSCT). Multiagent salvage regimens that have been studied include DHAP (dexamethasone, high-dose cytarabine, and platinum),²²⁶ which has been shown to be active in childhood and adult recurrent large-cell lymphoma, and VIPA (etoposide, ifosfamide, and high-dose cytarabine),²⁸⁰ which has been shown to be active in children with relapsed Burkitt lymphoma. Other active salvage regimens for children with refractory or recurrent malignant lymphoma include ICE (ifosfamide, carboplatin, and etoposide).²⁸¹ The addition of rituximab to the ICE regimen has been shown to be active for patients with recurrent CD20+ high-grade mature B-cell lymphomas (e.g., Burkitt lymphoma, DLBCL). The MIED (high-dose methotrexate, ifosfamide, etoposide, and dexamethasone) has been shown to be active in children with recurrent nonlymphoblastic lymphoma.²⁶⁰ Children who are found to have chemosensitive recurrent disease are usually considered for an intensification phase of chemotherapy followed by an HSCT. In a recent report, however, the benefit of HSCT in this setting was questioned.²⁴³ Although published data on the use of HSCT in children with recurrent or refractory NHL is limited compared to that reported for adults, there are reported studies supporting its use.^{232,282–290} For example, European cooperative group trials demonstrated that some children with Burkitt lymphoma who had a poor early response to therapy could by successfully salvaged with high-dose chemotherapy followed by an autologous HSCT.^{232,285,286,288,291} Moreover, the Spanish Working Party for Bone Marrow Transplantation reported a 58% event-free survival rate following HSCT in children with either recurrent/refractory NHL or high-risk NHL in first complete remission.²⁸⁵ The SFOP reported that 8 of 24 children with NHL who failed initial therapy were long-term disease-free survivors following HSCT.²⁸⁶ In a review of 22 children at the St. Jude Children’s Research Hospital, approximately 45% were survivors following intensive chemotherapy and HSCT.²⁹⁰ It is difficult to make direct comparisons between the published studies of HSCT in children because they vary with respect to type of HSCT (autologous versus allogeneic), preparative regimen, numbers of patients, and histologic subtypes studied.

It appears, however, that histologic subtype should be considered in determining the type of HSCT (autologous vs. allogeneic) for children with refractory or recurrent NHL.²⁹² Among children with Burkitt lymphoma, an autologous approach has been shown to be beneficial for those with a poor early response.^232,282,286 For those with disseminated recurrent Burkitt lymphoma involving the marrow, many would favor an allogeneic approach if a suitable donor can be identified; however, following RICE, some patients with high-grade mature B-cell lymphomas were in long-term clinical complete remission following autologous HSCT if they responded well to RICE.²⁹³ An autologous HSCT has been shown to be effective strategy for some children with recurrent ALCL.^284,290,292 The BFM study reported though that among those children who received an autologous HSCT for recurrent ALCL, those with either a CD3+ immunophenotype or an early relapse had a poorer prognosis.²⁹⁴ A recent study by the BFM identified an excellent outcome using an allogeneic HSCT strategy for those with high-risk ALCL relapses.²⁹⁵ Among children with recurrent lymphoblastic lymphoma, the results with autologous HSCT have been discouraging.^292,243 Most would consider an allogeneic HSCT for children with recurrent lymphoblastic lymphoma if a suitable donor can be identified.

Various preparative regimens have been successfully used in the salvage of children with refractory or recurrent NHL.232,282–290 Two of the earliest reported regimens are BACT (carmustine, cytarabine, cyclophosphamide, and thioguanine) and BEAM (carmustine, etoposide, cytarabine, and melphalan).^282,283,289 High-dose busulfan was reported as an important component of a successful French (SFOP) preparative regimen.²⁸⁶ Gordon et al. reported the successful salvage of children with recurrent peripheral T cell lymphoma using a preparative regimen that featured thiotepa²⁸⁴; however, it was associated with significant mucositis.

Currently, most would consider an autologous HSCT for children with chemosensitive recurrent ALCL, and in those with Burkitt lymphoma who have a poor early response or in some cases of recurrent disease. If a suitable donor is available, an allogenic HSCT approach would be considered for children with widely disseminated recurrent lymphoblastic lymphoma or Burkitt lymphoma involving the bone marrow. An allogeneic HSCT approach may also be considered for those with high-risk recurrent ALCL. It is clear that there is need for additional prospective clinical trials to evaluate the optimal preparative regimen and HSCT approach for children with recurrent or refractory NHL. In this regard, the European Lymphoma Bone Marrow Transplantation Registry has suggested that the potential graft versus lymphoma effect of allogeneic HSCT be studied.²⁸⁸

Future Directions

Despite dramatic improvements in the treatment of childhood NHL, approximately 20% to 30% of patients either do not achieve a complete remission or develop recurrent disease.4,5,25 Treatment-related late effects that place cancer survivors at risk are of additional concern.²⁷⁶ Therefore, the development of more effective and less toxic therapy remains an ongoing challenge. The identification of clinical and biological features that are predictive of treatment failure may help in the refinement of a risk-adapted treatment approach to children with NHL.

Improvement in treatment outcome may be achieved by the incorporation of new active agents, or the development of novel schedules for the delivery of currently used agents. Novel therapeutic approaches include the incorporation of immunotherapeutic approaches into multiagent chemotherapeutic regimens. In this regard, trials are underway for children with CD20+ B-cell lymphomas that incorporate the anti-CD20 antibody rituximab—an agent that has already been shown to be valuable in the treatment of some adults with CD20+ B-cell lymphomas.298,299 For example, the current BFM trial for high-grade mature B-cell lymphomas features a rituximab window.²⁵³ The results of this window were recently published and indicated that rituximab was active and generally well tolerated. Current studies in COG are piloting the incorporation of rituximab into both Group B and C arms of LMB-96-based therapy. A phase I/II study for children with CD30+ ALCL has been designed to examine the safety and efficacy of a newly developed anti-CD30 antibody.³⁰⁰ Antibody drug conjugates that target CD30 have been studied in adults with CD30+ lymphomas³⁰¹; studies in children are pending. Other potential therapies include the use of protein-specific cytotoxic T-lymphocytes, a strategy that has been successful in preventing and treating EBV-related posttransplantation lymphoproliferative disease.³⁰² Novel approaches may also include the incorporation of small molecule inhibitors, as well as antisense and anti-idiotype strategies. In this regard, the COG is currently studying a MET/ALK small molecule inhibitor in children with ALK-positive malignancies.³⁰³ Molecular characterization of the chromosomal abnormalities associated with the NHLs of childhood is providing us with tools that enhance diagnosis, disease classification, and monitoring of the response to therapy (detection of minimal residual disease). A clearer understanding of molecular pathogenesis may provide insights that permit the development of therapeutic strategies that target tumor-specific molecular lesions.