Burkitt Lymphoma and Atypical Burkitt or Burkitt-like Lymphoma: Should These be Treated as Different Diseases?

Abstract

Burkitt lymphoma (BL) is a mature B-cell non-Hodgkin lymphoma with an aggressive clinical course. Since the advent of short, intensive, multiagent chemoimmunotherapy regimens, it has carried a favorable prognosis. BL has been rather well characterized, whereas the other lymphomas morphologically resembling it are more heterogeneous. The cases classified as atypical BL/Burkitt-like lymphoma by the 2001 World Health Organization (WHO) Classification of Tumors of Hematopoietic and Lymphoid Tissue were thought to represent a continuum between BL and diffuse large B-cell lymphoma (DLBCL). The optimal therapeutic strategy for this provisional entity was not definitively established. However, recent incorporation of molecular genetic data into the 2008 WHO Classification has allowed further refinements with significant therapeutic implications, including the designation of a new provisional entity, “B-cell lymphoma, unclassifiable, with features intermediate between BL and DLBCL.” This review presents a comprehensive overview of the previously designated provisional entity of atypical BL/BLL in conjunction with a detailed comparison with BL and DLBCL.

Introduction

The pathological classifications of non-Hodgkin lymphomas (NHL) provide a framework for the categorization, prognostication, and therapeutic eradication of defined entities based on morphology, immunophenotype, and clinical behavior. Provisional categories have been devised for those lymphomas that appear to represent a continuum between well-defined NHL entities such as Burkitt lymphoma (BL) and diffuse large B-cell lymphoma (DLBCL). The recent incorporation of molecular genetic data into the classification has allowed further refinement of previously defined entities, with extremely important therapeutic implications, notwithstanding the additional levels of complexity. The optimal therapeutic strategy for a few of these overlapping entities has been somewhat elusive, partly because of their rarity and partly because of the successive modifications to the pathological classification systems. This article provides a comprehensive overview of the provisional entity of atypical BL/Burkitt-like lymphoma (BLL), as defined by the 2001 World Health Organization (WHO) classification of tumors of hematopoietic and lymphoid tissue [1], in conjunction with detailed comparisons with BL, DLBCL, and the new provisional entity of B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL listed in the 2008 revision to the WHO classification [2].

Classic Burkitt Lymphoma

BL represents a subtype of high-grade mature B-cell NHL characterized by a rapid proliferative rate and generally aggressive clinical course. In the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissue, three clinical variants of BL are noted: endemic, sporadic, and immunodeficiency-associated [1]. Endemic BL refers to cases observed in African children that generally involve the bones of the jaw and other facial bones in addition to extranodal sites such as the gastrointestinal tract and kidneys [3]. Nearly all cases of endemic BL are associated with the Epstein-Barr virus (EBV). Sporadic BL accounts for 1–2% of NHL in adults and up to 30–50% of NHL in children in the United States and Western Europe [4]. In sporadic BL, the abdomen (especially the ileocecal region) is the most common site of disease involvement. In contrast to the endemic form of BL, detection of EBV is limited to 15–30% of the cases [5]. Immunodeficiency-associated BL refers not only to cases associated with HIV but also to those occurring in individuals with congenital immunodeficiency and in allograft recipients.

Childhood sporadic BL tends to be a distinct, mature B-cell NHL with classic BL morphology, consistent immunophenotype, and classic translocations between the immunoglobulin heavy chain (IgH) and MYC genes (Table 1) [1]. The morphology of classic BL is characterized by monotonously uniform and medium-sized neoplastic cells with round nuclei similar in size to macrophage nuclei and two to five multiple basophilic nucleoli. The cytoplasm is moderately abundant and highly basophilic, with multiple vacuoles on Giemsa or Wright stain owing to the presence of lipid. Characteristically, the mitotic rate is unusually high, with a “starry sky” pattern resulting from numerous intermixed tangible body macrophages phagocytosing apoptotic debris. The immunophenotype of BL involves B cell–specific expression of monotypic surface IgM, CD19, CD20, CD22, and CD79a, in addition to the germinal center (GC)-associated markers CD10 and BCL6, in conjunction with abnormal TP53 expression (discordance between TP53 and P21 expression). Typically CD5, CD23, BCL2, CD138, and terminal deoxynucleotide transferase (TdT) are not expressed. The proliferative index (Ki-67) is nearly 100% (at least 95% of tumor cells), related to the short doubling time of BL tumor cells.

Table 1.

Clinicopathological features of Burkitt lymphoma, unclassifiable B-cell lymphoma with intermediate features between Burkitt and diffuse large B-cell lymphomas, and diffuse large B-cell lymphomas

Parameter	Burkitt lymphoma (BL)	B-cell lymphoma, unclassifiable, with intermediate featuresa	Diffuse large B-cell lymphoma (DLBCL)
Age groups	Younger	Older	Older
Histology
Size	Medium	Medium	Large
Nuclei	Round	Round to oval, irregular	Round to oval, irregular
Nucleoli	Prominent, multiple	Prominent, single	Prominent
Cytoplasm	Basophilic with vacuoles	Basophilic with vacuoles	Less basophilic
Mitotic activity	Very high	Very high	Lower
Starry sky pattern	Nearly all	Yes	Less common
Derivation	Early germinal center	Early germinal center	Germinal center
Ki-67 proliferative index	>95%	<95%	<90%
Immunophenotype
B-cell	CD19+, CD20+, CD22+, CD79a+,  sIgM+	CD19+, CD20+, CD22+, CD79a+	CD19+, CD20+
GC markers	CD10+, BCL6+, TCL1+, MUM1−,  CD44−, CD138−	CD10±, BCL6±, MUM1−	CD10±, BCL6±, TCL1−, MUM1+,  CD44+, CD138±
Karyotypes	Simple	Complex	Complex
	t(8;14)(q24;q32)	t(14;18)(q32;q21)b	t(14;18)(q32;q21)
	t(2;8)(p11-12;q24)
	t(8;22)(q24;q11)
Oncogenes	BCL2−	BCL2+	BCL2+
	Ig-MYC rearrangement	Ig-MYC rearrangement	MYC-negative
		Non-Ig-MYC rearrangement	BCL6 rearrangement
		MYC+BCL2 rearrangements	BCL2 rearrangement
EBV-encoded RNAs (EBER)	LMP1−	Variable	LMP1+
	EBNA2−		LMP2+
	EBNA1+		EBNA2+

^aMorphologically typical DLBCL with MYC rearrangement or otherwise typical BL without MYC rearrangement should not be classified in the intermediate category

^bCan be associated with transformation of antecedent follicular center cell lymphoma

EBV Epstein-Barr virus; GC germinal center

The defining feature of BL is the presence of the classic translocation involving the IgH and MYC genes [(t8;14) (q24;q32)], which juxtaposes the MYC gene on chromosome 8 with IgH enhancer elements on chromosome 14, thereby inducing MYC overexpression [6]. The variant translocations between MYC and the kappa (locus 2p11-12) or lambda (locus 22q11) light chain (IgL) loci [t(2;8)(p11-12; q24) or t(8;22)(q24;q11), respectively] also result in MYC activation. The classic translocation of (t8;14)(q24;q32) occurs in approximately 80% of the cases, with the variant translocations in the remainder.

MYC Rearrangements

The MYC rearrangements are more readily detectable by techniques other than routine karyotyping, such as fluorescent in situ hybridization (FISH) or polymerase chain reaction (PCR) [7]. The break points within the MYC and IgH genes vary depending on BL subtype, further supporting differences in pathogenesis with respect to neoplastic transformation of B cells at different maturation stages [8]. In the sporadic type of BL, the break point in MYC occurs between exons 1 and 2, and the break point for IgH occurs in the switch (Sμ) region, whereas in endemic BL, the break point is upstream of MYC and within or near the IgH joining region. The MYC rearrangement in BL usually occurs in a background of relatively simple karyotype in the absence of rearrangements of BCL2 (locus 18q21) or BCL6 (locus 3q27). In these cases, the MYC rearrangement appears to be an early event in lymphomagenesis, leading to perpetually proliferative states owing to effects on cell cycle progression, cellular differentiation, apoptosis, and cell adhesion [6, 9]. Interestingly, up to 30% of cases with a classic BL immunophenotype may lack MYC rearrangements [10•].

The surrogate for MYC translocation in the absence of fresh tissue for karyotyping (until FISH and PCR techniques were readily available) was the proliferative index (estimation of the number of tumor cells positive for Ki-67 as assessed by immunohistochemical staining for MIB-1). However, in a clinicopathologic study of atypical BL/BLL conducted by the Mayo Clinic, the proliferative tumor fraction did not correlate with the presence or absence of IgH/MYC translocations [11]. About 5–15% of non-BL mature B-cell NHL cases will harbor an MYC rearrangement, more likely associated with one of the variant translocations with a non-IgH partner (such as IgL), and a tendency for the neoplastic cells to have complex karyotypic aberrancies. In clinicopathological series of other subtypes of B-cell NHL, such as follicular and mantle cell lymphoma, the presence of Burkitt-like cells on morphologic evaluation in conjunction MYC amplification was associated with a worse prognosis [12]. Indeed, the presence of 8q24 (MYC) has been associated with worse prognosis in essentially all histologic NHL subtypes, not just those with morphology resembling BL [13]. The MYC rearrangements noted in these cases are often considered secondary or acquired events and are associated with a particularly worse survival in DLBCL, compared with cases without evidence of MYC rearrangements, even in the era of modern therapy [14•].

Atypical Burkitt Lymphoma/Burkitt-like Lymphoma

It has been well recognized that certain cases of aggressive, mature B-cell NHL have some of the morphologic features of BL but have greater nuclear and cytoplasmic variability than typical BL and overlap with the morphologic spectrum of DLBCL. The distinction among these subtypes is not precise, with agreement among expert hematopathologists averaging only 53% [15]. Cases of aggressive mature B-cell NHL that exhibited Burkitt-like morphology but lacked some of the characteristic immunophenotypic findings of classic BL were previously classified as BLL in the 1994 Revised European-American Lymphoma classification [16]. Other terms previously used for BLL included “undifferentiated non-Burkitt lymphoma” in the modified Rappaport classification, and “small non-cleaved non-Burkitt’s lymphoma” (SNC-NB) in the National Cancer Institute’s working formulation [17]. In the 2001 WHO classification, aggressive mature B-cell NHL included BL and two provisional variants: BL with plasmacytoid differentiation and atypical BL/BLL [1]. In the latter variant, there is greater nuclear pleomorphism with a smaller number of prominent nucleoli than in BL, but similar features of nearly 100% growth fraction and “strong presumptive evidence” of MYC translocation.

The question arises whether atypical BL/BLL is a similar disease entity to BL, or whether it lies within a continuum between BL and DLBCL, with sufficiently different clinical behavior and morphologic, immunophenotypic, or genetic findings to warrant segregation from classic BL [18, 19]. These distinctions have significant therapeutic implications, particularly since a high proportion of cases of BL can be cured with short, intensive, multiagent chemotherapy (usually in combination with the anti-CD20 monoclonal antibody rituximab) and central nervous system prophylaxis, whereas CHOP-R (cyclophosphamide, doxorubicin, vincristine, prednisone, rituximab) is considered inadequate therapy for BL but represents a standard of care regimen for DLBCL. Thus, an accurate pathological diagnosis is imperative [20•].

A comparative study contrasted DLBCL with high-grade features (such as elevated proliferative index) versus classic BL with respect to immunophenotyping panels including germinal center (GC) markers (CD10, BCL6) and activated B-cell (ABC) differentiation markers (BCL2, CD44, CD138, MUM1). Segregation of these two histologic subtypes was possible with hierarchical clustering [21]. The group with high GC/low ABC scores tended to represent cases morphologically classified as BL, and the group with low GC/high ABC scores included the DLBCL cases. However, there was a continuum of expression of the GC and ABC markers, suggesting that a true biologic continuum exists intermediate between DLBCL and BL.

Gene Expression Profiling of High-Grade Mature B-Cell NHL

Gene expression microarray technology has allowed the characterization of a molecular signature that represents classic BL and distinguishes cases from DLBCL by identifying high levels of expression of MYC and normal germinal center B-cell target genes in conjunction with low levels of expression of major histocompatibility complex I and target nuclear factor-κB genes (such as FLIP, CD44, NFKBIA, and STAT3) [22, 23]. In 2006, two separate large studies of gene expression profiling of BL, atypical BL/BLL, and DLBCL were reported. In a report by Dave et al. [22] for the Lymphoma/Leukemia Molecular Profiling Project, 17% of cases that carried the molecular signature of BL had morphologic diagnoses of DLBCL or unclassifiable high-grade B-cell NHL. In the report by Hummel et al. [23] for the Molecular Mechanisms in Malignant Lymphomas Network Project of the Deutsche Krebshilfe, this figure was 34%.

Conversely, few cases (0.4% in the report by Dave et al. [22] and 4% in the report by Hummel’s group [23]) that were pathologically deemed BL or atypical BL/BLL failed to harbor the BL molecular signature, suggesting that cases classified as atypical BL/BLL by morphology according to the 2001 WHO classification should be considered BL. This methodology also identified a subset of aggressive, mature B-cell NHL that did not fulfill the WHO criteria for the diagnosis of BL (morphologically classifiable as BL with absence of MYC translocation) but nonetheless manifested the molecular signature of BL [22]. These gene expression profiling studies therefore confirmed the homo-geneity of classic BL. The cases classified morphologically as atypical BL/BLL with a molecular signature of BL did poorly when treated with CHOP or CHOP-like regimens rather than the intensive regimens designed for BL [22].

The study by Hummel and colleagues [23] also identified cases with a molecular signature intermediate between the signatures of BL and DLBCL, further supporting the contention that a true continuum between these entities may indeed exist. These investigators also noted that within the cases of aggressive, mature B-cell NHL that were studied, a MYC rearrangement in the absence of a BL molecular signature was associated with an adverse clinical outcome independent of other adverse factors such as older age, advanced stage, or classification as ABC-like instead of GC-like [23]. In a subsequent study of aggressive, mature B-cell NHL in childhood, molecular profiling identified that up to 30% of cases morphologically diagnosed as DLBCL actually had a classic BL gene expression profile [24]. In contrast to the adult experience, no significant differences in outcome were noted between these disease entities because the pediatric BL and DLBCL cases were treated similarly.

Therapeutic Approach to Atypical Burkitt Lymphoma/Burkitt-like Lymphoma

A question remains regarding the optimal therapy for adults with high-grade, mature B-cell NHL classified as atypical BL/BLL (with or without MYC rearrangements). Should they be treated as having BL or DLBCL? Several retrospective studies suggest poorer outcome if atypical BL/BLL is treated with regimens designed for DLBCL instead of those typically used for BL. The prognosis for classic BL has improved significantly with the advent of short, multiagent, intensive systemic and intrathecal chemotherapy programs (Table 2) [10•, 25–34, 35•, 36]. Approximately 60–75% of adults with BL can be cured with these regimens, depending on age and other prognostic factors at presentation [4, 37]. The incorporation of rituximab into these intensive regimens has further improved outcome for subsets of classic BL [32–34, 35•]. Misdiagnosing the disease as a less aggressive entity, which potentially results in suboptimal or inadequate treatment (e.g., using CHOP-rituximab regimens designed for DLBCL), can lead to rapid emergence of resistance and poor outcome in what otherwise would be highly curable lymphoma with appropriate therapy. Conversely, overtreating typical DLBCL with high-intensity regimens may be harmful owing to the potential for significant toxicity.

Table 2.

Short, intensive multiagent chemotherapy regimens for adult Burkitt lymphoma/leukemia

Regimen	Cycles	Components
CALGB 921a	Cycle 1	CTX 200 mg/m2 IV days 1–5; prednisone 60 mg/m2 PO daily days 1–7.
	Cycles 2, 4, 6	Ifosfamide 800 mg/m2 IV daily days 1–5; Mesna 200 mg/m2 IV  daily 0, 4, 8 h after ifosfamide.
		MTX 150 mg/m2 IV over 30 min, then 1.35 g/m2 IV over 23.5 h.
		Leucovorin 50 mg/m2 IV 36 h after initiation of MTX, then  15 mg/m2 IV q6h.
		Vincristine 2 mg IV day 1; Dexamethasone 10 mg/m2 PO daily  days 1–5.
		Cytarabine 150 mg/m2 CI daily days 4, 5.
		Etoposide 80 mg/m2 IV days 4, 5.
	Cycles 3, 5, 7	CTX 200 mg/m2 IV daily days 1-5.
		MTX 150 mg/m2 IV over 30 min, then 1.35 g/m2 IV over 23.5 h.
		Leucovorin 50 mg/m2 IV 36 h after initiation of MTX, then  15 mg/m2 IV q6h.
		Vincristine 2 mg IV day 1; Dexamethasone 10 mg/m2 PO daily  days 1–5.
		Doxorubicin 25 mg/m2 IV daily days 4, 5.
	CNS prophylaxis (Cycles 2–7)	MTX 15 mg; cytarabine 40 mg; hydrocortisone 50 mg day 1. Cranial XRT 2400 cGy in 12 fractions after cycle 7 for prior BM disease.
Modifiedb	Regimen A	CTX 800 mg/m2 IV daily days 1–2 [800 mg/m2 day 1; 200 mg/m2  days 2–5].
[Originalc]	CODOX-M	Vincristine 1.4 [1.5] mg/m2 IV days 1, 10.
		Doxorubicin 50 [40] mg/m2 IV day 1.
		MTX 3 [6.7] g/m2 IV day 11 (with leucovorin).
Low-risk: AAA		G-CSF subQ daily days 1–6 & 12–13.
High-risk: ABABAB		Rituximab 375 mg/m2 IV day 3 cycle 1, day 1 remainder of cycles.
	Regimen B IVAC	Ifosfamide 1500 mg/m2 + Mesna 360 mg/m2 IV daily days 1–5.
		Etoposide 60 mg/m2 IV daily days 1–5.
		Cytarabine 2 g/m2 IV q12h days 1–2.
		G-CSF subQ daily days 6–13.
		Rituximab 375 mg/m2 IV day 1.
	CNS prophylaxis	A: Cytarabine 50 [70] mg day 1, 3; MTX 12 mg day 15.
		B: MTX 12 mg day 5.
Dose-adjusted EPOCH +  Rituximabd	Every 21 days for 6–8 cycles	Etoposide 50 mg/m2 CI IV daily days 1–4. Doxorubicin 10 mg/m2 CI IV daily days 1–4.
		Vincristine 0.4 mg/m2 CI IV daily days 1–4.
		CTX 750 mg/m2 IV day 5.
		Prednisone 60 mg/m2 PO q12h daily days 1–5.
		Rituximab 375 mg/m2 IV day 1.
		G-CSF 5 mg/kg subQ daily days 6–15.
	CNS prophylaxis	MTX (dose and frequency not specified).
Hyper-CVAD + Rituximabe	Induction + Consolidation  (Cycles 1, 3, 5, 7)	CTX 300 mg/m2 IV q12h×6, days 1–3 + Mesna 600 mg/m2 CI IV  daily days 1–3.
		Dexamethasone 40 mg IV or PO days 1–4, 11–14.
		Doxorubicin 50 mg/m2 CI IV over 2-24h day 4 (48 h if EF <50%).
		Vincristine 2 mg IV days 1, 11.
		Rituximab 375 mg/m2 IV days 1, 11.
	Consolidation (Cycles 2, 4, 6, 8)	MTX 200 mg/m2 IV over 2h day, then 800 mg/m2 IV over 22h day 1.
		Cytarabine 3 g/m2 (1 g/m2 if age≥60 y) IV over 2h q12h×4, days 2–3.
		Methylprednisolone 50 mg IV q12h×6, days 1–3.
		Leucovorin 50 mg IV 12h after end MTX, then 15 mg IV every  6h×8 or until MTX level <0.1 μmol/L; acetazolamide if urine pH <7.
		Rituximab 375 mg/m2 IV days 1, 8.
	CNS prophylaxis	MTX 12 mg (6 mg if Ommaya) day 2; cytarabine 100 mg day 7  or 8 (total 16).

^aFrom Lee et al. [28] and Rizzieri et al. [30]

^bFrom Lacasce et al. [31], Abramson et al. [36]

^cFrom Magrath et al. [25]

^dFrom Dunleavy et al. [33]

^eFrom Thomas et al. [32]

BM bone marrow; CALGB Cancer and Leukemia Group B; CI continuous infusion; CNS central nervous system; CODOX-M cyclophosphamide, vincristine, doxorubicin, methotrexate; CTX cyclophosphamide; EF ejection fraction; G-CSF granulocyte colony-stimulating factor; h hours; IV intravenous; IVAC ifosfamide, etoposide, cytarabine; MTX methotrexate; PO by mouth; subQ subcutaneous; XRT radiation therapy

Clinicopathologic Studies of High-Grade Mature B-Cell NHL

McClure and colleagues [11] conducted a detailed, retrospective, comparative analysis of classic BL (n=9), atypical BL/BLL (n =31), and DLBCL (n= 87), correlating the immunophenotypic and molecular characteristics with outcome. Significantly more heterogeneity in the phenotypes and MYC rearrangements was observed for the cases of atypical BL/BLL and DLBCL than for classic BL. Approximately 40% of the atypical BL/BLL cases and 6% of the DBLCL cases were typical Ig/MYC positive (11% of those with atypical BL/BLL and 3% of those with DLBCL were concurrently IgH/BCL2 positive), although very few harbored the IgH/MYC translocation of classic BL. When accounting for treatment effects (by classifying therapy as optimal, suboptimal, or inadequate), worse survival was noted with atypical BL/BLL morphology, or with the presence of Ig/MYC translocation across the atypical BL/BLL/DLBCL morphologic subtypes. Specific details regarding the therapeutic regimens were not provided, so it is unknown whether the worse outcomes in the atypical BL/BLL subsets were related to treatment with regimens typically used for DLBCL.

Nomura and colleagues [38] conducted a retrospective, comparative clinicopathological analysis encompassing 72 cases classified as BL or atypical BL/BLL and 182 cases of DLBCL. Overall survival (OS) rates were significantly inferior for the BL and atypical BL/BLL cases treated with CHOP or CHOP-like therapy as opposed to short, intensive, multiagent chemotherapy regimens designed for classic BL (22% vs 75%, P< 0.01), regardless of the presence or absence of MYC rearrangements. These findings suggested that confirmation of MYC rearrangements is not essential for otherwise typical BL, and that atypical BL/BLL should be treated as BL rather than as DLBCL.

Thus, the aggregate data suggest that atypical BL/BLL more closely resembles BL than DLBCL, regardless of MYC expression, and should be treated as such, based on genetic characteristics (BL and atypical BL/BLL share the molecular signature of classic BL), immunophenotypic findings (GC vs ABC marker expression), and retrospective clinicopathological correlations with outcome. However, these studies antedated the 2008 reclassification of highgrade, mature B-cell NHL, which no longer formally recognizes atypical BL/BLL as a variant pathological entity.

WHO Reclassification of High-Grade Mature B-Cell NHL

The 2008 revisions to the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissue no longer formally recognize the atypical BL/BLL and plasmacytoid variants of BL [2]. Cases of high-grade, mature B-cell NHL not readily classifiable as BL or DLBCL are now designated “B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL” (Table 1) [39]. Cases previously classified as atypical BL/BLL for morphologic features slightly deviating from typical BL (such as slight nuclear pleomorphism and/or more prominent single nucleoli), are now classified as BL if the immunophenotypic and genetic criteria are otherwise fulfilled. Cases of otherwise typical BL in which an MYC rearrangement cannot be demonstrated should be still classified as BL [40].

Cases with classic BL morphology but strong positivity for BCL2 and/or lower Ki-67 proliferative index (less than 90%) would be classified in the new provisional intermediate entity, also loosely referred to as “gray zone lymphomas” [41]. This category also includes the cases coined as “dual hit” mature B-cell NHL; these harbor rearrangements of both MYC (8q24) and BCL2 [t(14;18) (q32;q21)], resulting in a concomitant enhancement of proliferation and inhibition of apoptosis. These entities are associated with a particularly aggressive clinical course: survival rarely exceeds 6 months despite treatment with regimens designed for BL [42–45]. Interestingly, some of these “dual-hit” NHLs have a molecular signature similar to classic BL, whereas others have a signature intermediate between BL and DLBCL [22]. In one small retrospective series of dual-hit mature B-cell NHL, decreased expression of surface CD20 was noted, suggesting a potential for increased resistance to rituximab [46]. Rarely, “triple-hit” B-cell NHL cases, which also harbor BCL6 rearrangements, have been reported [47].

Based on the current 2008 WHO classification, aggressive, mature B-cell NHL with large cells or cases with marked cellular pleomorphism should be classified as DLBCL regardless of MYC status, proliferative index, or immunophenotype, as up to 15% of DLBCL cases will harbor MYC rearrangements and some may have proliferative indices approaching 100% [48]. With gene expression profiling, most of the latter cases have been shown to manifest the molecular signature of DLBCL rather than BL [22]. The diagnosis of BL is therefore now restricted to cases with expression of CD10 and BCL6, absent or very weak expression of BCL2 protein, homogeneously very high proliferation index, and a proven MYC translocation in the setting of simple karyotype without evidence of other chromosomal translocations typical for other lymphoma entities. A high number of nonspecific cytogenetic abnormalities should lead to reconsideration of the diagnosis of BL.

A prospective, clinicopathologic, therapeutic study (the MRC/NRCI LY10 trial) included cases of aggressive, mature B-cell NHL with a Ki-67 fraction at least 95% by immunohistochemistry that were uniformly treated with a dose-modified risk-stratified BL regimen, CODOX-M (cyclophosphamide, vincristine, doxorubicin, methotrexate), with or without IVAC (ifosfamide, etoposide, cytarabine) (Table 2) [10•]. The findings further corroborated the heterogeneity of the non-BL NHL observed in prior retrospective analyses. Classic BL morphology was observed only in a minority of cases despite the preselection for high proliferative index. MYC rearrangement as the sole cytogenetic abnormality by interphase FISH occurred exclusively in the cases with GC phenotype, absence of BCL2 expression, and abnormal TP53 expression (nuclear staining for protein in absence of P21), consistent with the findings of classic BL.

However, approximately 30% of the patients with typical BL immunophenotype did not demonstrate MYC rearrangement and were therefore not classified as having BL; instead, they were included in the category of DLBCL. Although the group designated as BL was accurately delineated in this study, the comparator group of DLBCL appeared to include cases that would have been classified as atypical BL/BLL in the 2001 WHO classification, or the provisional entity of “B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL” in the subsequent 2008 WHO classification. As expected, the BL group was younger and had a higher incidence of bone marrow involvement, B symptoms, and high-risk disease compared with the DLBCL group. OS rates for the BL and DLBCL groups were similar at 2 years (67% vs 59%, P= 0.38). However, within the high-risk group, OS and progression-free survival (PFS) rates were superior for the BL group, in part related to better treatment delivery (treatment completion rates were 76% for BL vs 46% for DLBCL), and in part related to inclusion of the worst prognosis subgroup—those harboring t(14;18) and/or 3q27—in the DLBCL group. Within the DLBCL histologic subtype, those with these particular karyotypic abnormalities had the worst 2-year PFS (27%) and OS (40%) rates. As previously reported in other series, the few cases with the dual t(8;14) and t(14;18) translocations had particularly poor outcomes, with survival times of less than 6 months from the start of therapy.

Conclusions

Whereas classic BL has been well characterized as a homogenous disease readily diagnosed by pathologists with high concordance in the setting of typical morphologic features and supporting immunophenotypic and genotypic data, other subtypes of mature B-cell NHL, such as DLBCL, are much more heterogeneous. Gene expression profiling has shown that a subset of cases of aggressive, mature B-cell NHL that were classified as atypical BL/BLL (slight morphologic variations but otherwise typical of BL) by the 2001 WHO Classification of Tumors of Hematopoietic and Lymphoid Tissue actually harbor the molecular signature of BL. Several clinicopathologic studies have also reported a worse outcome for the atypical BL/BLL subset after regimens typically used for DLBCL, as opposed to treatment with short, intensive, multiagent regimens designed for BL. In the 2008 revisions to the WHO classification, atypical BL/BLL is no longer formally recognized; many of these cases are now classified as BL, whereas others are relegated to the provisional category of unclassifiable B-cell lymphoma with features intermediate between DLBCL and BL. This category has been created to encompass the cases likely to truly represent the continuum between these two subtypes of mature B-cell NHL. This heterogeneous group includes the worst prognostic category, “dual-hit” mature B-cell NHL, which manifests both MYC and BCL2 gene rearrangements, resulting in an extremely aggressive clinical course and universally poor outcome despite treatment with intensive chemoimmunotherapy regimens [44]. Determining the optimal treatment for the lymphomas that fall into this provisional category created by the 2008 WHO classification represents yet another therapeutic challenge in managing aggressive, mature B-cell NHL.