Definition and characteristics

Follicular lymphoma (FL) is one of the most common indolent non-Hodgkin lymphomas in Western Europe and the USA. The median age of onset at first diagnosis is between 60 and 65 years, with women being affected slightly more often than men. Follicular lymphoma arises from follicular center B cells (centroblasts), or from malignant transformation of mature B cells and aberrant proliferation of germinal center-like B cells in lymphoid organs. It has as a genetic characteristic the t(14;18)(q32;q21) translocation between the locus IGH and the gene BCL2 (Swerdlow et al. 2017), resulting in constitutive expression of the anti-apoptotic gene BCL2 at all stages of B cell development. Follicular lymphomas are often initially asymptomatic, which is why they are often diagnosed at an advanced stage. Follicular lymphomas are mainly nodal diseases, but can also affect the spleen or the Waldeyer's pharyngeal ring (Nadel 2014).

Classification

According to the WHO classification 2017, follicular lymphoma is a mature B-cell neoplasm. Currently, four variant forms of follicular lymphoma are distinguished in addition to the classic follicular lymphoma (see Table 1).

Table 1: Follicular lymphoma WHO classification 2017 (Swerdlow et al., 2017)

Diffuse follicular lymphoma variant

 

In situ follicular neoplasia

Partial or total colonization of germinal centres by clonal B cells carrying the BCL2 translocation characteristic of FL; low risk of progression

Primary intestinal follicular lymphoma

IGH/BCL2/t(14;18) positive, in the mucosa/submucosa, low risk of progression

Testicular follicular lymphoma

No BCL2 translocation

Diffuse variant of follicular lymphoma

No IGH-BCL2/t(14;18) translocation, similarities to expression profil of FL

WHO classifies follicular lymphomas histologically based on their centroblast content into grades 1, 2, 3A, and 3B, with grades 1-3A belonging to indolent lymphomas and grade 3B being classified as aggressive lymphomas. Only about 15-20% of patients have grade 1-2 follicular lymphoma at the time of diagnosis. Grade 3B follicular lymphomas are biologically more similar to diffuse large B-cell lymphomas (DLBCL) than other follicular lymphomas. They rarely carry BCL2 translocations and have a high centroblast content (Nadel 2014, Swerdlow et al. 2017). The spread of lymphoma is assessed using the Ann Arbor classification (see Table 2).

Table 2: Staging of follicular lymphoma according to the Ann Arbor classification (Onkopedia guideline FL 2019) 

Stage

Disease Involvement

I

Single lymph node region (I) or one extralymphatic site (IE)

II

Tow or more lymph node regions, on the same side of the diaphragm (II) or local extralymphatic extension plus one or more lymph node regions on the same side of the diaphragm (IIE)

III

Lymph node regions on both sides of the diaphragm (III), which may be accompanied by local extralymphatic extension (IIIE)

IV

diffuse involvement of one or more extralymphatic organs or sites

In addition - A: – without systemic symptoms” and B: – with systemic symptoms:

  • unexplained fever  >38°C
  • recurrent night sweat
  • unexplained weight loss > 10% baseline during 6 months prior to staging
 

    Another subtype of follicular lymphoma is cutaneous follicular lymphoma, which is the most common extranodal follicular lymphoma. It usually presents on the head or trunk, where it can be treated as a local disease. Extranodal Follicular Lymphoma can furthermore affect organs such as the thyroid or ovaries in addition to the skin, duodenum and testes. If these lymphomas arise primarily in the aforementioned extranodal organs and do not have BCL2 rearrangement but have a high histologic grade, they form a distinct subgroup of extranodal follicular lymphomas. In contrast, most low-stage extranodal non-intestinal follicular lymphomas likely reflect disseminated nodal disease (Chott 2014, Swerdlow et al. 2017).

    Diagnostics

    Cytomorphology

    In the diagnosis of the various lymphoid neoplasms, the cytomorphology and histology of blood, bone marrow and lymph nodes are directional for guiding downstream diagnostics. The assessment of the bone marrow smear provides an initial landmark statement as to whether lymphoma is present or possible. Cytomorphology and histology are also useful for assessing the degree of lymphoma maturity.

    Immunophenotyping

    Immunophenotyping allows clear determination of lineage affiliation to the T- or B-lineage in lymphomas. Furthermore, multiparametric flow cytometry is often indispensable for differentiating a reactive change from a lymphoid neoplasm, e.g. in EBV infections.

    Follicular lymphomas show strong surface expression of immunoglobulins and usually express CD10, BCL2 and BCL6, whereas CD5 and CD43 are not expressed.

    Table 3: Characteristic findings in follicular lymphoma (after Béné et al. 2011)

    Antigene

    Finding

    CD19

    +

    CD20

    +

    CD22

    +

    CD23

    -

    CD25

    -

    FMC7

    +/-

    CD79b

    low/-

    CD5

    -

    sIg

    +

    CD10

    +

    CD11c

    -

    CD103

    -

    CD43

    -

    Chromosome analysis

    Translocation t(14;18)(q32;q21) is characteristic in follicular lymphoma

    The most common cytogenetic alteration in follicular lymphoma is the balanced translocation t(14;18)(q32;q21). This abnormality occurs in approximately 90% of patients with grade 1 and 2 follicular lymphoma and is associated with a rearrangement between the immunoglobulin heavy chain locus IGH and the BCL2 gene, resulting in overexpression of BCL2 and consequent inhibition of programmed cell death (Rowley 1988, Horsman et al. 1995). Variant BCL2 translocations that rearrange with immunoglobulin light chain loci occur rather rarely. Similarly, BCL2 rearrangements can be observed less frequently in grade 3B follicular lymphomas (Ott et al. 2002, Swerdlow et al. 2017). BCL2 translocations represent a characteristic genetic alteration but are not specific to follicular lymphoma, as they also occur in other mature B-cell neoplasms, albeit with lower frequency.

    Cytogenetic abnormalities in follicular lymphoma

    Balanced translocations:

    t(14;18)(q32;q21)/IGH-BCL2

    rarely t(8;14)(q24;q32)/IGH-MYC or other 8q24/MYC rearrangements

    3q27/BCL6 rearrangements

    Gains:

    1(q), 2p, 6p, 7, 8, 12(q), 17, 18/18q, 21, X

    Losses:

    1p, 6q, 7q, 9p, 10q, 13q, 17p

    Most patients with follicular lymphoma have additional abnormalities

    Additional genetic abnormalities occur in approximately 90% of patients with follicular lymphoma. Aberrations of the long arm of chromosome 3 (3q27) and/or BCL6 rearrangements are observed in 5-15% of cases. 3q27 rearrangements occur predominantly in grade 3B follicular lymphomas resembling DLBCL. In addition, other unbalanced events may occur. Loss of 1p, 6q, 7q, 9p, 10q, 13q, and 17p and/or gains of chromosome 1(q), 2p, 6p, 7, 8, 12q, 18/18q, 21, or an additional X chromosome are common (Offit et al. 1993, Tilly et al. 1994, Ott et al. 2002, Höglund et al. 2004, Katzenberger et al. 2004, Mamessier et al. 2014, Swerdlow et al. 2017, Qu et al. 2019).

    Additional abnormalities are associated with unfavorable prognosis

    In rare cases, the t(8;14)(q24;q32)/IGH-MYC translocation occurs in addition to the t(14;18), which is associated with an unfavorable prognosis. A complex karyotype, deletions in the long arm of chromosome 6 (6q23-26), 17p deletions, mutations in the TP53 gene, losses of the short arm of chromosome 1, trisomy 12, gains of chromosome 18 and the X chromosome are also associated with a less favorable prognosis and a short transformation time into diffuse large B-cell lymphoma (DLBCL) (Levine et al. 1988, Tilly et al. 1994, Höglund et al. 2004). In 30-40% of patients with follicular lymphoma, progression and transformation to high-grade lymphomas such as DLBCL or highly malignant B-cell lymphoma with 8q24/MYC and 18q21/BCL2 and/or 3q27/BCL6 rearrangement (HGBL), also called "double-hit" or "triple-hit" lymphoma, occurs. In such transformation, additional genetic abnormalities, especially MYC rearrangements, are usually involved. The combination of BCL2 and MYC rearrangements is associated with particularly aggressive disease progression (Lee et al. 1989, Fiedler et al. 1991, Bouska et al. 2017, Swerdlow et al. 2017). The absolute number of abnormalities also influences prognosis. Patients with more than 12 abnormalities show more rapid disease progression and shorter progression-free survival and overall survival (Qu et al. 2019).

    FISH

    Fluorescence in situ hybridization (FISH) is an informative test for the detection of t(14;18) in the diagnosis of follicular lymphoma when lymphoma cells are detectable in blood or bone marrow. Especially in case of unclear findings, differentiation to other indolent lymphomas can be performed.

    Molecular genetics

    BCL2 rearrangement also detectable by molecular genetics

    In 85-90% of patients with follicular lymphoma a BCL2 rearrangement (IGH-BCL2) can be detected. The translocation t(14;18)(q32;q21) results in a BCL2 rearrangement in 18q into the region of the immunoglobulin heavy chain locus 14q32. However, since this rearrangement also occurs in 20-30% of all large B-cell lymphomas, it is not specific to follicular lymphoma. Regardless of the breakpoint, the BCL2 rearrangement results in overexpression of the BCL2 gene. In most cases, the rearrangement is detectable by PCR, which can be performed in addition to FISH analysis. Increased expression of BCL2 leads to a decreased apoptosis rate in the germinal center, favoring genomic instability and thus the accumulation of secondary oncogenic aberrations. BCL6 also plays a role in the germinal center. Normally, BCL2 is inhibited by BCL6, but in follicular lymphoma, both genes are co-expressed. Furthermore, when follicular lymphoma transforms into high-grade lymphoma such as DLBCL, mutations in the KMT2D (MLL2), CREBBP, EZH2, and MEF2B genes, which play a role in epigenetic regulation, are increased in addition to BCL2 rearrangements (Horsman et al. 2003, Morin et al. 2011, Nadel 2014, Bouska et al. 2017, Swerdlow et al. 2017).

    Table 4: Frequency of alterations in follicular lymphoma at diagnosis (Swerdlow et al. 2017)

    Gene

    Frequency of alterations (%)

    Predominant type of alteration

    BCL2

    85-90%

    Translocation, mutation

    KMT2D (MLL2)

    85%

    Mutation

    TNFRSF14

    45-65%

    Deletion, mutation

    EZH2

    60%

    Mutation (Y641)

    EPHA7

    70%

    Mutation

    CREBBP

    33%

    Deletion, mutation

    BCL6

    45%

    Translocation, mutation

    MEF2B

    15%

    Mutation

    EP300

    10%

    Deletion, mutation

    TNFAIP3 (A20)

    20%

    Deletion, mutation

    FAS

    5%

    Mutation

    TP53

    <5%

    Deletion, mutation

    MYC

    <5%

    Translocation, gain

    Rearrangements of the MYC gene have a negative impact on disease progression. When they occur together with BCL2 rearrangements, there is an additive effect associated with transformation to an aggressive and chemo-refractory disease stage. Mutations in the TP53, MLL2, and EZH2 genes are also associated with a worse prognosis. At diagnosis, TP53 mutations are rare (<5%, see Table 4) but occur in up to 30% of transformed lymphomas (Smith 2013).

    Pathogenesis

    The IGH-BCL2 rearrangement could also be detected by nested-PCR or RT-PCR in peripheral blood or reactive lymph nodes in more than 50% of healthy individuals, with an increased prevalence up to the age of 50 years (Schüler et al. 2009, Nadel 2014). Therefore, this genetic alteration does not appear to be solely responsible for the development of follicular lymphoma (Summers et al. 2001, Roulland et al. 2003, Schmitt et al. 2006, Nadel 2014). The frequency also appears to be age-dependent; at all ages, it was a median of 5.8 x 10-6 cells in a study of 715 healthy subjects and increased to 13.2 x 10-6 in the 60-69 age range (Schüler et al. 2009). A longitudinal study suggests that the increased frequency in this is most likely due to the expansion of one or more persistent t(14;18)-positive clones rather than the occurrence of additional t(14;18) translocations (Roulland et al. JEM 2006, Nadel 2014).

    The risk of developing follicular lymphoma is increased 23-fold if one in 10,000 cells is t(14;18)-positive. The number of cells with this abnormality may increase due to contact with pesticides and viral diseases, such as with the hepatitis C virus. The occurrence of this genetic alteration in healthy individuals may complicate the interpretation of measurable residual disease in patients with follicular lymphoma. It is believed that t(14;18)-positive cells may be present in healthy-appearing individuals for up to 15 years without developing follicular lymphoma and that precursors of follicular lymphoma develop over a long period of time. However, the distinction between healthy and subclinical individuals is in a gray area and the impact of t(14;18) in healthy individuals cannot be clearly assessed (Nadel 2014).

    Prognosis

    Follicular Lymphoma International Prognostic Index (FLIPI) is used for risk classification in follicular lymphoma.

    The Follicular Lymphoma International Prognostic Index (FLIPI) is used for risk assessment of follicular lymphoma (Solal-Celigny et al. 2004). This clinical risk score considers the following parameters:

    Follicular lymphoma risk factors:

    • >4 involved nodal areas
    • LDH elevation
    • Age >60 years
    • Ann Arbor stage III or IV
    • Hemoglobin <12 g/dL

    Table 5: Follicular lymphoma - FLIPI risk score (Onkopedia guideline FL 2019)

    No. of risk factors

    Risk group

    Outcome, 10-y-OS (%)

    0-1

    Low

    62-71

    2

    Intermediate

    48-51

    3-5

    High

    34-36

    When the FLIPI is expanded to include the mutation status of the 7 genes EZH2, ARID1A, MEF2B, EP300, FOXO1, CREBBP, and CARD11, as well as the Eastern Co-operative Oncology Group (ECOG) performance status, the result is the m7-FLIPI score, which is a clinical genetic model for risk calculation (Pastore et al. 2015).

    The presence of a TP53 mutation is associated with shortened progression-free survival and overall survival in follicular lymphoma, similar to other hematologic diseases (O'Shea et al. 2008, Qu et al. 2019). Based on multivariable analyses, TP53 mutation is nevertheless not accounted for in the m7-FLIPI score, whose development is based on the complex interplay between genetic and clinical factors, not every single genetic risk factor playing a role (Pastore et al. 2015). CDKN2A deletions are also associated with shortened progression-free survival (Qu et al. 2019). Because mutations can be present in progenitor cells that are not fully malignant long before disease onset, resident progenitor cells that migrate to lymphoid organs could be a cause of recurrence. However, to date, these progenitors have not been fully characterized (Nadel 2014).

    Follicular Lymphoma: Prognosis Calculation

    Click here to access the prognosis calculation of the FLIPI score and the m7-FLIPI score.

    Influence of tumor microenvironment on prognosis

    The development and progression of follicular lymphomas is influenced by complex interactions between neoplastic B cells and surrounding cells (tumor microenvironment). Analysis of gene expression profiles of infiltrating T cells has shown that they too - in addition to B cells - have an impact on the prognosis of follicular lymphomas. Here, increased expression of proliferation genes and decreased expression of T cell genes are indicative of a worse prognosis (Rimsza & Jaramillo 2014).

    Cytotoxic T cells and T helper cells influence anti-tumor immunity and thus disease progression. Regulatory T cells expressing CD25 and regulated by FOXP3 (forkhead box protein P3) have been implicated in suppressing the immune response and could be found increased in follicular lymphoma. Here, not only the number of cells seems to play a role, but also their specific localization. In patients with longer overall survival, FOXP3+ cells were found preferentially around follicles and not in the follicles (Rimsza & Jaramillo 2014).

    Macrophages in the tumor microenvironment have also been studied for their impact on follicular lymphomas, but the results are less clear compared to T cells. Thus, both positive and negative prognostic relevance in follicular lymphomas has been described for macrophages. Overall, however, the tumor microenvironment appears to have a presumably dynamic impact on the development of follicular lymphomas, depending on the distribution and interplay of T helper cells, regulatory T cells, and macrophages. However, translation of these findings into routine diagnostics remains a challenge (Smith 2013, Rimsza & Jaramillo 2014).

    Recommendation

    Important notes on the examination material

    The diagnosis of follicular lymphoma should be based on surgical lymph node extirpation, if possible, and also from peripheral blood and bone marrow in advance or in parallel. For initial screening in inaccessible, e.g. retroperitoneal lymph nodes, a lymph node biopsy can be performed alternatively. Fine-needle aspiration (cytology) alone is not sufficient because of possible focal heterogeneity of lymphoma tissue and the possible need for further immunologic and molecular genetic testing.

    If lymphoma cells are detected in peripheral blood, the diagnosis can initially be made with a high degree of confidence without bone marrow biopsy or lymph node sampling. If enlarged lymph nodes are clinically prominent, a lymph node should be removed and histologically and immunohistologically processed. Based on these findings, an extended sampling of material, e.g. bone marrow, may be useful in individual cases and if clinically relevant.

    PET/CT (positron emission tomography / computed tomography) can be used to determine the total tumor mass, the Ann Arbor stage and the response to therapy (based on tumor regression or metabolic activity). PET/CT can also provide evidence of transformation via metabolic activity. Thus, PET/CT can contribute to treatment planning and prognostic assessment, but it cannot replace bone marrow biopsy in initial diagnosis (Guidelines Program Oncology - S3 Guideline 2020).

    Therapy

    Many follicular lymphoma patients do not show any symptoms or complications even in advanced stages and if no cytopenias are present, a wait-and-see strategy with close monitoring can be applied here ("watch and wait"). Early therapeutic treatment shows no survival benefit.

    Rituximab and obinutuzumab are anti-CD20 antibodies and represent today an essential pillar in the therapy of follicular lymphoma. They are used in both induction therapy and maintenance therapy. While obinutuzumab is only used in combination with chemotherapeutic agents in induction therapy, treatment with rituximab can take the form of monotherapy or chemoimmunotherapy. Since the pivotal study showed an improvement in progression-free survival compared to rituximab + chemotherapy, particularly for patients with intermediate or high FLIP index, obinutuzumab-based chemoimmunotherapy is indicated for patients in stages III or IV, while rituximab alone or in combination with chemotherapy is a treatment option at any disease stage. When used as maintenance therapy, anti-CD20 antibodies show a favorable impact on progression-free survival, but whether there is an advantage in terms of overall survival has not yet been conclusively determined; however, the incidence of infections is increased with maintenance therapy (Onkopedia guideline FL 2019, Freedman & Jacobsen 2020, Guidelines Program in Oncology - S3 guideline 2020).

    Radiation therapy, which can be combined with chemo(immuno)therapy as needed, may also be considered. Combining radiotherapy with rituximab resulted in improved progression-free survival in one study, but showed no impact on overall survival. In patients with stage I-II and grade 1-2 follicular lymphoma, radiotherapy alone within the first year of diagnosis may prolong disease-specific survival as well as overall survival, or lead to cure (Pugh et al. 2010, Freedman & Jacobsen 2020).

    Autologous and allogeneic stem cell transplants represent a curative treatment option, but are necessary only in rare cases. Allogeneic stem cell transplantation may be a curative option after relapse for young patients with resistant follicular lymphoma. In the group of patients with early relapse, autologous stem cell transplantation resulted in improved overall survival.

    In relapse, lenalidomide in combination with rituximab is another therapeutic option and offers a chemotherapy-free alternative. In addition, treatment with the PI3 kinase inhibitor idelalisib as well as anti-CD20-based therapies can be given with or without chemotherapy.

    CAR T-cell therapies are still being evaluated in trials and may influence treatment strategies in second-line therapies in the future (Freedman & Jacobsen 2020).

    Choice of therapy for follicular lymphoma

    According to the S3 guideline of the Arbeitsgemeinschaft der wissenschaftlichen Medizinischen Fachgesellschaften e. V. (AWMF), German Cancer Society e. V. (DKG) and German Cancer Aid (DKH), treatment decisions are based on various parameters. These include the Ann Arbor stage, the patient's condition, age, comorbidity, and the goal of therapy. Treatment options include "watch & wait" as described above, radiation therapy, anti-CD20 antibodies, immunochemotherapy, and stem cell transplantation. The various options can also be combined differently, and maintenance therapies with anti-CD20 antibodies can be used when complete or partial remission is achieved. Detailed treatment regimens are mapped in the S3 guideline as well as the Onkopedia guideline for follicular lymphoma (Onkopedia guideline FL 2019, Guideline Program Oncology - S3 guideline 2020). In addition, an overview of the different options in first-line therapy is given in Figure 1. (Note: Click on the figure to enlarge it).

    Figure 1: First-line treatment options for follicular lymphoma (adapted Onkopedia guideline FL 2019). 

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