The 2016 updated WHO classification of lymphoid neoplasias
2017-06-01

The World Health Organization (WHO) is preparing a revised and updated edition of the 2008 classification of tumors of the hematopoietic and lymphoid tissue to be released in 2017.1 The aim of this revised version of the 4th edition of the WHO classification is to incorporate the new scientific and clinical information to refine diagnostic criteria for previously described lymphomas, in some cases, change nomenclature to convey better the clinical features of the disease, and to introduce newly recognized disease entities. Much has been learned about non-Hodgkin lymphomas (NHL) after the 2008 WHO classification and monograph was published, as a consequence of new information coming from translational and basic research and improved techniques used for routine diagnosis. The list of genetic aberrations that are present in NHL and that are useful either for diagnosis or for understanding the pathogenesis of different diseases has been growing continuously. Some discoveries found using molecular techniques have been rapidly incorporated into daily diagnostic practice such as immunohistochemical stains for SOX11 or BRAF used to help in the diagnosis of mantle cell lymphoma (MCL) or hairy cell leukemia (HCL), respectively. Molecular detection of the recurrent MYD88 and RHOA or IDH2 mutations are helping to delineate the morphological spectrum of lymphoplasmocytic lymphoma (LPL) and angioimmunoblastic T-cell lymphoma (AITL), respectively. Nevertheless, the prognostic and diagnostic value of mutational analysis in daily practice and its role in targeted therapy remains to be determined. Although the goals of the WHO classification are to identify well-defined entities, as we move forward some challenges in the WHO classification still continue. The borders between some of the disease entities remain ill-defined for example nodular lymphocyte predominant Hodgkin lymphoma with diffuse growth pattern versus T-cell/histiocyte rich large B-cell lymphoma. Small B-cell lymphomas are composed mainly of small lymphocytes and are often referred as "low-grade" B-cell lymphomas. The WHO classification intentionally does not divide lymphomas by grade, and because they are not necessarily indolent, the preferred name used is "small B-cell lymphomas" (SBL). They include chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), follicular lymphoma (FL), nodal marginal zone lymphoma (MZL), MALT lymphoma, HCL, LPL, and MCL.1 The changes in the revised WHO classification are summarized in Table 1. Chronic lymphocytic leukemia/ Small lymphocytic lymphoma Chronic lymphocytic leukemia/ Small lymphocytic lymphoma - defined as >5 × 109/L PB CLL cells - TP53, NOTCH1, SF3B1, and BIRC3 mutations of potential clinical relevance -low count MBL <0.5 × 109/L PB CLL cells -high count MBL >0.5 but <5 × 109/L PB CLL cells - no major changes - remains a provisional entity Lymphoplasmacytic lymphoma/ Waldenström macroglobulinemia Lymphoplasmacytic lymphoma/ Waldenström macroglobulinemia - disease-defining mutation, MYD88 L265P - this mutation is not specific for LPL - ~50% of MGUS IgM carry this mutation Nodal marginal zone lymphoma -Pediatric marginal zone lymphoma Nodal marginal zone lymphoma -Pediatric marginal zone lymphoma Follicular lymphoma (FL) -in situ follicular lymphoma Follicular lymphoma (FL) - in situ follicular neoplasiaa - duodenal-type FLa - predominantly diffuse FL with 1p36 deletiona - molecular landscape better understood, CREBBP, EZH2 and KMT2D (MLL2) possible early driver mutations. - change in nomenclature. - considered a precursor lesion with low risk of progression. - localized variant with low risk of dissemination - new recognized variant, often localized inguinal mass, lacks BCL2 rearrangement - change in nomenclature. - usually occurs in children and young adults, rarely in older individuals. - excellent prognosis even with conservative approach. - recognized as a definite disease entity - frequent TNFRSF14 and MAP2K1 mutations - new provisional entity to distinguish from PTFL and DLBCL - usually occurs in children and young adults - involves mainly Waldeyer ring and cervical lymph nodes Mantle cell lymphoma - classical MCL - in situ mantle cell lymphoma Mantle cell lymphoma - classical MCL - leukemic non-nodal MCLa - cyclin D1- - in situ mantle cell neoplasiaa - two MCL subtypes are recognized - mostly unmutated IGHV, mostly SOX11+ - mutated IGHV, SOX11-, PB, BM, and spleen - TP53 may occur and result in aggressive disease - ~ 50% CCND2 rearrangements - change in nomenclature Monoclonal B-cell lymphocytosis (MBL) is now divided in "low-count" and "high-count" defined as less than or greater than 0.5 × 109/L CLL cells in peripheral blood (PB). High-count MBL is now recognized as a precursor lesion of CLL/SLL. In FL and MCL, the—in situ—lesions have been renamed as "in situ neoplasias" to avoid a lymphoma diagnosis. These are considered precursor lesions with relatively low rate of progression. Follicular lymphoma is clinically and morphologically a rather heterogeneous disease with complex cytogenetic and molecular abnormalities. Several variants are now recognized including duodenal-type FL, a rather localized disorder with low risk of dissemination, and predominantly diffuse FL with 1p36 deletion that often presents as a localized inguinal mass and lacks BCL2 rearrangement. Pediatric-type FL (PTFL) is now a definite entity. It has been renamed because similar cases may occur in adults. The criteria for this diagnosis should be strictly applied to avoid misdiagnosis especially with conventional FL grade 3B that is considered a more aggressive disease, and with conventional FL grade 1-2, BCL2 negative. Pediatric-type FL has excellent prognosis and a conservative watch-and-wait approach is recommended.3 Large-B-cell lymphoma with IRF4 rearrangement is a new provisional entity that most commonly affects children and young adults. It involves mainly the Waldeyer's ring and cervical lymph nodes and is usually low stage disease at presentation. These lymphomas may have a follicular, follicular/diffuse, or diffuse growth pattern. They are characterized by the strong expression of IRF4/MUM1 and BCL6, and approximately 50% of the cases express BCL2 and CD10. Most cases have IGH/IRF4 rearrangement and BCL6 alterations (Figure 1). Despite the strong expression of IRF4/MUM1, these cases have a germinal center signature by gene expression profiling (GEP). Most cases have shown good response to chemotherapy.4 Mantle cell lymphoma usually presents with advanced stage and rapid progression, and historically, it has been considered an aggressive disease. However, now it is recognized that there are 2 pathogenetic ways to develop MCL. The classical MCL orginates from a B cell with unmutated IGHV and expression of SOX11, whereas the leukemic, nonnodal subtype develops from IGHV mutated, SOX11- B cells. The latter involves mainly PB, bone marrow and spleen. Although these cases have an indolent behaviour, secondary alterations in TP53 may occur and can result in very aggressive disease.1 The main changes in SBL are due to the impact of new molecular/cytogenetic information obtained mainly by next generation sequencing. Hairy cell leukemia is defined in almost all cases by the BRAF V600E mutation, which is not detected in HCL-variant.5 In contrast, 50% of HCL-variant carry mutations in MAP2K1, which encodes MEK1 downstream of BRAF. Another specific genetic alteration found in >90% of LPL/Waldenström macroglobulinemia is MYD88 L265P mutation.6 This mutation is also found in 50% of IgM monoclonal gammopathy of undetermined significance, 30% of diffuse large B-cell lymphoma (DLBCL) of nongerminal center type, 50% of primary cutaneous DLBCL, leg type, and rare cases of MZL both splenic and in lymph nodes. It has also been described in 3% of CLL cases defining a specific group of young patients with good prognosis. This mutation is not found in plasma cell myeloma. Another mutation found in 30% of LPL/Waldenström macroglobulinemia and 20% of IgM MGUS is CXCR4 gene mutation, which seems to impact the clinical presentation and overall survival. There is a plethora of mutations that are not disease-defining mutations but have prognostic and biological implications. These include TP53, NOTCH1, SF3B1, and BIRC3 in CLL or TP53, ATM, NOTCH 1 and 2 in MCL.7 In FL, next generation sequencing studies have shown frequent mutations in chromatin regulator/modifier genes.8 Early driver mutations seem to include mutations in genes such as CREBBP, KMT2D (MLL2), and EZH2. The 2008 WHO classification of lymphoid malignancies recognizes within the group of DLBCL, several subtypes characterized by unique clinical and pathological features including primary DLBCL of the central nervous system, primary cutaneous DLBCL, leg type, T-cell/histiocyte-rich large cell lymphoma, and EBV positive DLBCL of the elderly. Nevertheless, most cases of DLBCL fall into the "not otherwise specified" (NOS) category.1 The changes in the revised WHO classification are summarized in Table 2. Diffuse large B-cell lymphoma, NOS - germinal center B-cell typea - activated B-cell typea - cell of origin is required. Use of IHC algorithm is acceptable - coexpression of MYC and BCL2 (DE) is prognostically relevant. - change in nomenclature because it occurs also in younger patients - should be distinguished from other well characterized EBV-associated lymphomas - MYC rearrangement in ~50% of cases - 70% EBV+ with latency I or II -new provisional entity - resembles Burkitt lymphoma but lacks MYC translocation High grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangementsa High grade B-cell lymphoma, NOSa - new category for all double or triple hit lymphomas excluding transformed FL, lymphoblastic lymphoma and MCL - include cases with BCLU and blastoid morphology without gene rearrangements On the basis of GEP studies, DLBCLs have been divided into 2 main subgroups; germinal center B cell-like (GCB) and activated B cell-like (ABC)–DLBCL. These molecular subgroups reflect either the stage in B cell development from which the disease originates or the activity of different biological programs. Gene expression profiling, which is considered the gold standard to assign the molecular subtypes, is not routinely available and is not cost-effective in routine diagnosis. Several studies have attempted to recapitulate the molecular subgroups (GCB vs. non-GCB) using a limited panel of antibodies available in most pathology laboratories. The Hans algorithm has been the most widely used in clinical trials. Although most studies have found that immunohistochemical algorithms correlate with prognosis in DLBCL, everybody agrees that these algorithms are an imperfect substitution for GEP.9 Nevertheless, because of the potential prognostic value of cell of origin and the increasing efforts to tailor therapy on the basis of molecular characteristics of DLBCL, the revised WHO classification requires the identification of these 2 subtypes and the use of immunohistochemistry algorithms is now acceptable.1 The prognostic importance of simultaneous MYC and BCL2 protein expression, so called "double-expressor" (DE) has been stressed in the revised WHO classification. The recommended cutoff for MYC is >40% positive tumor cells, and the cutoff for BCL2 expression is >50%.1 MYC and BCL2 DE have been reported to occur in 19-34% of DLBCL patients, and to have a worse prognosis than patients who do not express any or only 1 protein, but better prognosis than double hit (DH) or triple hit (TH) DLBCL (see below), which have a dismal outcome. Interestingly, the DE cases appear more commonly in the ABC subtype, and it has been suggested that this may largely contribute to the known inferior survival of the ABC subtype.2 The EBV+ large B-cell lymphoma is now recognized as a definite entity; however, the term "elderly" has been substituted by NOS because these lymphomas can present in younger patients as well. It is important to distinguish this entity from other well-characterized EBV+ lymphomas.10 The EBV+ mucocutaneous ulcer has been added as a new recognized entity and is characterized by a limited growth despite the aggressive morphological features, and good outcome with conservative approach. It is usually associated with iatrogenic immunosuppression and age-related immunosenescence. The Burkitt-like lymphoma with 11q aberrations is a rare disorder that has been added as a provisional entity. Morphologically, these cases resemble Burkitt lymphoma but lack the MYC rearrangement. Instead, they have a very characteristic 11q chromosomal alteration with proximal gains and telomeric losses. In contrast to Burkitt lymphoma, they have a more common nodal presentation and broader morphological spectrum; however, they show similar aggressive clinical behaviour.11 The morphological distinction between BL and DLBCL has been problematic for pathologists. Gene expression profiling studies have shown that BL has a characteristic signature but that there are cases within the spectrum of DLBCL and aggressive B-cell lymphomas, which have a molecular signature similar to BL or fall into an intermediate category. The 2008 WHO classification recognized this problem an added a provisional category of B cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL (BCLU) (Table 2). The MYC rearrangements are detected in 30% to 50% of the cases and are usually associated with additional chromosomal aberrations. In this group, the incidence of DH/TH involving MYC and BCL2 and/or BCL6 has been reported to be high (32%-78%). Because the precise morphological boundaries of this category was not well defined, and therefore, lacked reproducibility among pathologists, it was decided to put all DH/TH in 1 group regardless of the morphology of the tumor cells and designate this group as high-grade B-cell lymphoma with DH/TH rearrangements. Nevertheless, the morphology should be described in the report (DLBCL vs BCLU vs blastoid). The majority of these cases have a GCB phenotype. Cases with high-grade morphology, BCLU or blastoid morophology but which lack MYC, BCL2, and/or BCL6 rearrangements should be grouped as high-grade B-cell lymphoma, NOS.1 High-grade B-cell lymphoma is a disease of older patients presenting with nodal or extranodal disease usually in an advanced clinical stage, high lactate dehydrogenase, and frequent bone marrow and central nervous system infiltration with a dismal prognosis. Which cases should be analyzed by fluorescence in situ hybridization (FISH) is a matter of debate. Whether all DLBCL should be analyzed for MYC, BCL2, and BCL6 or only those with GCB phenotype or whether to preselect the cases using a 2-step approach (>40% MYC expression, >50% BCL2 expression) should be decided in the different institutions. Nevertheless, only 6% of GCB DLBCL will have DH/TH. Using the 2-step approach will reduce the costs considerably and DH lymphomas missed by this approach will be enriched by cases harbouring MYC and BCL6 and those cases without dual expression, whose clinical significance is unclear. Mature T and natural killer cell neoplasms comprise a heterogeneous group of disorders that account for approximately 15% of all NHL. In contrast to B-cell lymphomas, most T-cell lymphomas lack defining genetic alterations, and its classification relies on a combination of morphological and immunophenotypical features.1 The recognition that T-cell lymphomas are related to the innate and adaptive immune system, as well as enhanced understanding of T-cell subsets such as follicular helper T-cells (TFH) has contributed to improve the classification of T and natural killer cell neoplasms (Table 3). We have learned that the morphological spectrum of AITL is broader than previously thought. The importance of the EBV+ lymphoproliferative disorders of childhood has resulted in the addition of chronic active EBV infection—systemic and cutaneous forms—and changes in nomenclature in the revised 2016 WHO classification.12 The better understanding of T-cell lymphomas with cutaneous presentation has resulted in new provisional entities—primary cutaneous acral CD8+ T-cell lymphoma—and change in nomenclature in primary cutaneous CD4+ small/medium LPD to stress the indolent behaviour of this disease. Furthermore, in the last years molecular studies have shed light onto the molecular signatures and chromosomal alterations in peripheral T-cell lymphoma (PTCL), NOS. All these findings add increasing evidence that cell lineage is a major determinant in mature T-cell lymphomas biology and help to better delineate established and new entities. - new provisional entity - NK-cell counterpart of T-LGL - change in nomenclature due to the fulminant clinical course and monoclonal proliferation - Hemophagocytic syndrome usually present Chronic active EBV infectiona - systemic form - cutaneous form - Hydroa-vacciniforme-like LPDa - severe mosquito bite allergya - new category to encompass the EBV+ T and NK-cell LPD in the pediatric age - can be monoclonal - change in nomenclature. Umbrella term that covers the entire spectrum of the disease - new subgroup. - no major changes - change in nomenclature - lack association with celiac disease - mostly derived from γδ T cells - STAT5B mutations in 36% the cases - recurrent SETD2 alterations Primary cutaneous CD30+ LPD - Lymphomatoid papulosis (LyP) - anaplastic large cell lymphoma Primary cutaneous CD30+ LPD - Lymphomatoid papulosis (LyP) - anaplastic large cell lymphoma - new morphological subtypes - no major changes - differential diagnosis with LyP type D Angioimmunoblastic T-cell lymphoma Angioimmunoblastic T-cell lymphoma - Follicular T-cell lymphomaa - PTCL with TFH phenotypea - frequent TET2, RHOA and IDH2 mutations - new subgroups with THF phenotype - definite entity with different subgroups - 6p25 rearrangement (DUSP22) subgroup has good prognosis - new provisional entity - the non-invasive disease has good prognosis Peripheral T-cell lymphoma is a diagnosis of exclusion with broad morphological spectrum presenting mainly as a nodal disease. Gene expression profiling studies have discovered that there are 3 distinct molecular subgroups in PTCL, NOS defined by the overexpression of the transcription factors GATA3 and TBX21 or expression of cytotoxic genes.13 GATA3 and TBX21 are master regulators of T helper (TH) cells, skewing TH polarization into TH2 and TH1 differentiation pathways, respectively. Importantly, these subgroups have biological and clinical implications. The GATA3 group has an inferior prognosis and overall survival than cases with the TBX21 signature. GATA3 and T-bet antibodies are reliable surrogates to the molecular signatures. AITL, the prototype of this group, is characterized by recurrent mutations in TET2, RHOA, IDH2, and DNMT3A in a significant proportion of cases.14 These mutations have been found also in PTCL, NOS with TFH phenotype suggesting that all these lymphomas represent different morphological manifestations of the same disease. Follicular T-cell lymphoma (FTCL) is also included in this group, but often presents with localized disease and less symptoms. For designation of TFH phenotype, at least 2 to 3 TFH-related antigens should be expressed by the tumor cells including ICOS, CXCL13, CD279/PD1, CD10, BCL6, SAP, and CCR5.1 In contrast to the 2008 WHO classification, anaplastic large-cell lymphomas (ALCL) ALK- is recognized as a definite entity with different cytogenetic prognostic subgroups.15 The subgroup with DUSP22/IRF4 rearrangements on chromosome 6p25 usually lacks cytotoxic granules and seems to have a better prognosis (Figure 2). ALCL ALK- is also associated with breast implants. This subgroup has been incorporated in the revised classification as a provisional entity designated as breast implant–associated ALCL. In primary intestinal lymphomas, 2 distinct entities are now recognized; enteropathy-associated T-cell lymphoma (EATL), previously known as EATL type I and monomorphic epitheliotropic intestinal T-cell lymphoma (MEITL), previously known as EATL type II. MEITL is not associated to celiac disease and is characterized by a monomorphic proliferation of lymphoid cells with CD8 and CD56 expression and mostly derived from γδ T-cells. STAT5B mutations were reported in 36% of cases, all with a γδ T-cell phenotype.16 The 2016 revision of the WHO classification maintains the same principles of the 2008 edition, which is to recognize distinct entities on the basis of morphology, immunophenotype, genetic changes, and clinical features. The main changes in this revision include modification in the nomenclature of some diseases mostly to convey better the clinical features of the entity. Lymphoma designation was changed either to "neoplasia" or "lymphoproliferative disorder" to denote either the low risk of progression to a full-blown lymphoma in precursor lesions or to stress the indolent behaviour of the disease, respectively. New provisional entities have been recognized and new scientific and clinical research resulted in upgrading some provisional entities to definite entities. The major contribution of molecular studies that has shed light onto the molecular pathways and chromosomal alterations of many disease entities has also been incorporated. The author have no competing interest.