Histologic transformation of follicular lymphoma: pathologists’ viewpoint

Abstract

Outcomes of patients with histologic transformation (HT) of follicular lymphoma (FL) have been believed to be poor. The most common histologic subtype of transformation from FL is diffuse large B-cell lymphoma (DLBCL), which accounts for 90% of the cases, and the remaining 10% of the cases include classic Hodgkin lymphoma, high-grade B-cell lymphoma, plasmablastic lymphoma, B-acute lymphoblastic leukemia/lymphoma, histiocytic/dendritic cell sarcoma, and anaplastic large cell lymphoma-like lymphoma. Because the histologic criteria for the diagnosis of DLBCL transformed from FL are unclear, convenient histopathological criteria for HT are required. One of the proposed criteria of HT from our institute is the presence of diffuse architecture with a proportion of large lymphoma cells of ≥20%, and for challenging cases, Ki-67 index ≥50% is used as a reference. Patients with HT to non-DLBCL have poorer outcomes than those with HT to DLBCL; thus, rapid and accurate histologic diagnosis is desired. In this review, we discussed the recent literatures describing the histopathologic variety and proposal of definition of HT.

INTRODUCTION

Follicular lymphoma (FL) accounts for approximately 20% of all lymphoma cases.¹ The rate of transformation to more aggressive forms of lymphoma is approximately 2%–3% per year.² Diffuse large B-cell lymphoma (DLBCL) is the most common manifestation of histologic transformation (HT) from FL, representing 93% of cases.^3-5 However, the pathological criteria for HT are unclear.

Transformation to high-grade B-cell lymphoma (HGBL) with MYC and BCL2 and/or BCL6 rearrangements (i.e., double-hit lymphoma, HGBL-DH) or HGBL, not otherwise specified;^6-10 classic Hodgkin lymphoma (CHL);^11-15 B-lymphoblastic leukemia/lymphoma (B-ALL);^16-19 B-prolymphocytic leukemia;²⁰ plasmablastic lymphoma;^21-24 anaplastic large-cell lymphoma-like lymphoma;^25-27 and histiocytic/dendritic cell (H/DC) sarcoma^28,29 is also rarely observed. This review focused on the histopathologic variety and proposal of definition of HT.

Definition of HT from each low-grade B-cell lymphoma to DLBCL in the World Health Organization classification

In the revised 4th edition of the World Health Organization Classification of Tumours of Haematopoietic and Lymphoid Tissues published in 2017 (WHO-HAEM4R),¹ definitions of transformation from low-grade B-cell lymphomas to DLBCL are unclear. In the chapters of FL and extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT) lymphoma, some comments regarding HT are present. In the chapter of FL, the following two sentences are found. “The presence of diffuse areas composed entirely or predominantly of large centroblasts (that would fulfill the criteria for grade 3 FL) in FL of any grade is equivalent to DLBCL and a separate diagnosis of DLBCL should be made.” “The presence of a diffuse component with grade 3 cytology always warrants an additional diagnosis of DLBCL.” Therefore, diffuse proliferation of FL grade 3A (>15 centroblasts per high power field [HPF] and centrocytes present) should be diagnosed with DLBCL. However, in the summary of the WHO HAEM5,³⁰ the following sentences were found: Rare cases of classic FL grade 3A may show a focal or extensive diffuse growth pattern. In the WHO-HAEM4R, the recommended diagnosis in such cases was “DLBCL with FL,” although sheets of large cells are not often present. Currently, it is uncertain whether such cases should better be classified as classic FL or DLBCL. Therefore, treatment decisions in individual patients should not be based on pathology information alone but rather should be made in multidisciplinary conference settings and await studies to define more objective criteria to predict clinical course. Recently, biopsy material is becoming very small, and only a small part of the tumor can be observed. Considering clinical course, comprehensively assessing the tumors and deciding for therapeutic strategies are required.

In the chapter of MALT lymphoma in the WHO-HAEM4R,¹ “MALT lymphoma, by definition, is a lymphoma composed predominantly of small cells. Transformed centroblast-like or immunoblast-like cells may be present in variable numbers, but when solid or sheet-like proliferations of transformed cells are present, the tumor should be diagnosed as DLBCL.” From the definition, high ratio of large lymphoid cells is assumed for HT diagnosis.

WHO-HAEM5 includes a section with the description of “high-grade transformation of indolent B-cell lymphomas”.³⁰ In FL cases, genetic predictors of transformation include alterations of TP53, BTG1, MKI67, and XBP1. Genetics present at transformation include co-occurrence of MYC, BCL2, and BCL6 translocations; deletions of CDKN2A/B and TP53; deletions and gains (del1q, del6q, +2, +3q, +5); mutations in TP53, B2M, CCND3, GNA13, S1PR2, P2RY8, PIM1, EBF1, MYD88, and TNFAIP3.³¹ Regarding HT of low-grade B-cell lymphomas, including FL, MALT lymphoma, and chronic lymphocytic leukemia/small lymphocytic lymphoma, the common criteria for HT are required.

General eligibility of HT from FL

FLs are confirmed to have transformed when the FL grades 1–3A and DLBCL components are present in the same initial biopsy material or both components are present at the different sites synchronously at the initial presentation.³² An example of the former is a case with FL grades 1–3A and DLBCL components present in the same lymph node. An example of the latter is a case with DLBCL in the lymph node and FL grades 1–3A found in the bone marrow and/or gastrointestinal tracts at staging procedure. Cases with FL grades 1–3A are observed at initial presentation, then DLBCL diagnosed at relapsed phase is also HT. In our institute, tumors with DLBCL corresponding to diffuse proliferation of FL grade 3A, namely, tumors comprising admixture of CD10⁺ small and large B cells, are considered HT of FL. CD10⁺ DLBCL without a FL component, DLBCL with IGH::BCL2 fusion, or cases with FL grade 3B and DLBCL are not classified as HT of FL. The former two are insufficient for HT. Cases with FL grade 3B largely differ from BCL2-rearranged grade 1–3A FL at the point of genetic abnormality and clinical course. Thus, cases with FL grade 3B and DLBCL should not be classified as HT of FL; the term “transformation” should be used only when there is morphological evidence of simultaneous or prior low-grade FL.³² Cases with pure DLBCL that relapse as FL might be HT, but they are usually not registered as HT.

Histologic definition of transformation from FL to DLBCL: rate of large lymphoid cells and Ki-67 index

In our institute, biopsies are performed routinely before treatment for every relapsed/refractory disease to analyze the presence of HT and loss of CD20 expression. If the tumor is composed of sheet of large lymphoid cells, it must be DLBCL. However, if the tumor is composed of admixture of small and large lymphoid cells, establishing the diagnosis of HT is challenging because the definition of HT is unclear. If diffuse proliferation of FL grade 3A (>15 large cells/HPF) is defined as DLBCL according to the WHO-HAEM4R,¹ the proportion of large lymphoma cells varies greatly, and at a minimum, this proportion may be as low as <10% in some cases. Although there are no definite criteria for HT, a cutoff value of 30% for the proportion of large lymphoma cells has been used previously for FL⁵ and MALT lymphoma³³ in our institute. Recently, being suitable for clinical course and adapting the value between the two (<10% and 30%), HT is defined as the presence of diffuse architecture with a proportion of large lymphoma cells of ≥20% (Fig. 1A and 1B), and for the diagnosis of challenging cases, Ki-67 index ≥50% is used as a reference.³⁴ However, judgment of the size of lymphoma cells is also unclear. Simple and objective criteria for HT are required in routine examination in the future.

Fig. 1.

In our institute, histologic transformation is defined as the presence of diffuse architecture with a proportion of large lymphoma cells of ≥20%, and for the diagnosis of challenging cases, Ki-67 index ≥50% is used as a reference. A and B were challenging cases but were diagnosed as diffuse large B-cell lymphoma (A, hematoxylin and eosin [H&E], ×400; B, H&E, ×400).

Using the abovementioned criteria, the Ki‐67 index of a DLBCL component transformed from FL ranges from 16.8% to 98.4%, with a median of 60.6%, and a Ki‐67 index of ≥70% is an adverse prognostic factor for patients with HT of FL (P = 0.024; 5-year overall survival rates, 53% and 85%, respectively).³⁴ Broyde et al.³⁵ reported that the appropriate cutoff value for the Ki‐67 labeling index (LI) to distinguish between low‐grade B‐cell lymphoma and DLBCL was 45%, and among the de novo DLBCL cases, patients with a Ki‐67 LI of >70% had a poor outcome (3‐year overall survival rate, 75% vs. 56%; P = 0.015). The Ki‐67 LI and FL grade are mildly correlated: the Ki‐67 LI of FL grades 1–2 is usually ≤40%, and that of FL grades 3A–3B is 30%–60%.^36,37 Thus, it is reasonable that DLBCL should be considered with Ki-67 index ≥50%. However, Ki-67 LI might become low in necrotic tumors, and DLBCL with low Ki-67 LI exists. Hence, Ki-67 should be used with caution. In addition, disruption of follicular dendritic cell meshwork (defined by CD21 and/or CD23 staining) could also be helpful for the sign of transformation.

Ambiguous cases

When pathologists is uncertain whether HT exists, discuss with hematologists. As summarized in Fig. 2, if patients are diagnosed with FL with suspicious clinical transformation, they should be treated as transformed FL. While, for patients histologically diagnosed with DLBCL but with clinically indolent tumors, a discussion between hematologists and pathologists is required. Pathologists should mention whether the tumor is definite DLBCL or a challenging case between HT and diffuse FL.

Fig. 2.

Combination of histologic and clinical transformations. For patients diagnosed with follicular lymphoma with suspicious clinical transformation, they should be treated as transformed FL. However, for patients histologically diagnosed with DLBCL but are clinically indolent, a discussion between hematologists and pathologists is required to select optimal treatment.

Immunophenotypes of DLBCL transformed from FL

Using Hans algorithm,³⁸ majority of DLBCLs transformed from FL show germinal center B-cell (GCB) phenotype, but minority of DLBCLs (9%³⁹ and 16%⁵) show non-GCB phenotype. This result is primarily attributed to lost of CD10 expression and change to be positive or retain MUM1 expression through HT. Based on the clonal evolution studies, transformation specimens were generally composed of clones that were rare or absent in initial FL specimens, consistent with dramatic clonal expansions that came to dominate the transformation specimens.⁴⁰ Simultaneous occurrence of HT and CD20 negative change after rituximab therapy has been reported.⁴¹ However, in our previous study, CD20 negative change after rituximab therapy and HT were not strongly correlated.²⁶

HT to CHL

Non-DLBCL transformation is observed in 10% of patients with HT from FL (Table 1).²⁴ CHL is the second common histologic subtype of HT (Fig. 3A and 3B).²⁴ Majority of CHL cases are a mixed cellularity type, and minority of the cases are a nodular sclerosis type.^15,42 EBER1 is positive in 25% of CHL cases.⁴² An FL component is sometimes detected in the bone marrow or duodenum during the staging procedure of CHL. Using fluorescence in situ hybridization (FISH) analysis or Fluorescence Immunophenotyping and Interphase Cytogenetics as a Toll for the Investigation Of Neoplasm, namely, combination of FISH and immunohistochemistry for CD30,⁴³ the composite lymphomas of CHL and FL are cytogenetically related.^24,42,44

Table 1. Histologic subtypes of transformation from follicular lymphoma.

Histologic subtype	Incidence
Diffuse large B-cell lymphoma	90%
GCB type	90%
Non-GCB type	9% Ref.38, 16% Ref.5
Non-diffuse large B-cell lymphoma	10%
Classic Hodgkin lymphoma	Ref.11-15
High-grade B-cell lymphoma, NOS or DH	Ref.6-10
B-acute lymphoblastic leukemia/lymphoma	Ref.16-19
Histiocytic/dendritic cell sarcoma	Ref.28, 29
Plasmablastic lymphoma	Ref.21-24
Anaplastic large cell lymphoma-like lymphoma	Ref.25-27

GCB, germinal center B cell; DH, double hit (with MYC and BCL2 rearrangements); NOS, not otherwise specified

Fig. 3.

All of the following cases are histologic transformation of follicular lymphoma. Mixed cellularity classic Hodgkin lymphoma with IGH::BCL2 fusion (A, hematoxylin and eosin [H&E], ×400; B, CD30, ×400), high-grade B-cell lymphoma with MYC rearrangement and IGH::BCL2 fusion (C, H&E, ×400; D, Ki-67, ×400), B-acute lymphoblastic leukemia with MYC rearrangement and IGH::BCL2 fusion (E, H&E, ×400; F, TdT, ×400), and histiocytic sarcoma with IGH::BCL2 fusion (G, H&E, ×400; H, CD68 [PGM1], ×400).

HT to HGBL or B-ALL

The histopathology of HGBL and B-ALL transformed from FL is shown in Fig. 3C–3F. Because patients with HT to HGBL or B-ALL show poor outcomes, accurate and immediate histologic diagnosis is required.²⁴ However, ambiguous cases between HGBL and B-ALL with both MYC translocation and IGH::BCL2 fusion have been observed.^24,45,46 Cases with mainly nodal disease, histologically suspected with HGBL but with (focally) TdT positivity, or with leukemic disease without TdT positivity are unclear. Nevertheless, to ensure optimal treatment, differential diagnosis should be established based on morphology (starry sky pattern, size of the nucleus and nucleolus, chromatin pattern, and cytoplasmic vacuole), presence or absence of leukemic presentation, and TdT, CD20, and sIg expressions. In general, lack of sIg expression, dim or lack of CD20 expression, and TdT expression support immaturity in neoplastic cells. However, there are several issues that need to be considered in the discrimination between HGBL and B-ALL. First, the presence of double hit does not confirm the diagnosis of HGBL-DH because B-ALL HT also shows double hit. Second, dim or negative CD20 expression is not a differential characteristic of B-ALL because approximately 20% of B-cell non-Hodgkin lymphomas after rituximab-containing treatment show complete or partial CD20-negative change. Third, TdT expression might not be a critical finding for the differential diagnosis between HGBL and B-ALL as TdT-positive HGBL HT from FL, not B-ALL, was previously reported in patients with a history of FL.⁴⁶ The presence or absence of IGH::BCL2 fusion can distinguish between therapy-related secondary B-ALL and B-ALL as HT of FL.

HT to H/DC sarcomas

HT from FL to H/DC sarcomas (Fig. 3G and 3H) is known phenomenon.^28,29 The presence of IGH::BCL2 fusion in both tumors suggests that those are same clone. The presence of BRAF_V600E mutation in H/DC sarcoma suggests that FL and H/DC sarcoma are different clones. According to a previous study for de novo H/DC sarcomas, 39% (9/23) of the patients showed clonal IGH (± IGK) gene rearrangements.⁴⁷ Among the nine patients, one had IGH::BCL2 fusion. In cases of post chimeric antigen receptor T-cell therapy (CAR-T), decrease or loss of pan-B-cell markers, CD19, CD20, CD79a, and PAX5, was observed in B-cell lymphomas. In addition, one each case transformed to HGBL with peripheral T-cell lymphoma-like phenotypes and HGBL with histiocytic differentiation.⁴⁸

CONCLUSIONS

The most common histologic subtype of transformation from FL is DLBCL, which accounts for 90% of the cases, and the remaining 10% of the cases include CHL, HGBL, B-ALL, and H/DC sarcoma etc. As the histologic criteria for the diagnosis of DLBCL transformed from FL are unclear, convenient criteria for HT are required. Patients with non-DLBCL HT have poor outcomes; thus, rapid and accurate histologic diagnosis is required. Recent therapies, such as anti-CD20 antibody, CAR-T, and bispecific antibody therapies, might cause more complicated morphologic and phenotypic changes in FL cases in the future.