Highlights in Pathology: Hematopathology (May 2019)

Dr. Yi Daniel Li, Credit Valley Hospital, Trillium Health Partners

Classification of large B-cell lymphoma has undergone some significant revision under the Revised 4th Edition of the WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues in 2017.

Here, selected updates in diagnosis of large B cell lymphomas are highlighted.

1. High-grade B-cell lymphoma (HGBL) with MYC and BCL2 and/or BCL6 rearrangements (also known as “double/triple-hit lymphoma”) is a new diagnostic entity that requires correlation with cytogenetic/FISH finding

Swerdlow SH, Campo E, Pileri SA, et al. The 2016 revision of the World Health Organization classification of lymphoid neoplasms. Blood. 2016;127(20):2375-90.

One of major changes is the creation of two new diagnostic categories to replace the confusing category known as “B-cell lymphoma unclassifiable with features intermediate between DLBCL and Burkett lymphoma” from the previous WHO Classification:

1. High-Grade B-Cell Lymphoma (HGBL) with myc and Bcl2 and/or Bcl6 Rearrangements
2. HGBL, not otherwise specified (HGBL, NOS)

The former is also known as “double/triple-hit lymphoma” and requires cytogenetic/FISH studies for its diagnosis. The latter diagnosis is a very uncommon entity and can only be made after exclusion of the former by cytogenetic/FISH studies.

In cases where a classic DLBCL morphology is present, a diagnosis of “DLBCL, NOS” should be made after a double/triple-hit lymphoma genotype is excluded by genetics studies.

However, there is no consensus on when cytogenetic/FISH studies should be performed.

At our institution (Trillium Health Partners), we performed cytogenetic/FISH studies for MYC, BCL2 and BCL6 rearrangement in all newly diagnosed aggressive B-cell lymphomas. Other institutions only perform genetic studies when MYC overexpression (40% of lymphoma cells) is detected and/or in cases with a germinal center B cell (GCB) phenotype based on immunoperoxidase studies.

Of note, overexpression of MYC and BCL2 proteins defines “double-expressor lymphoma,” which is not a diagnostic category; nor is it equivalent to “double-hit lymphoma.”    

2. DLBCL, NOS needs to be divided into two clinically-important subtypes based on the Cell-of-Origin. Several methods have been developed to allow this classification

Scott DW, Wright GW, Williams PM, et al. Determining cell-of-origin subtypes of diffuse large B-cell lymphoma using gene expression in formalin-fixed paraffin-embedded tissue. Blood. 2014;123(8):1214-7.

Another modification is the based on the concept of “cell-of-origin.” DLBCL, NOS is a diverse group of lymphomas that requires further classification into more clinically-actionable diagnostic categories for improved personalized and tailored prognostication and treatment.

The previous WHO Classification separates DLBCL into two molecular “cell-of-origin” subgroups, namely “GCB” and “activated B-cell (ABC)” subtypes, as they differ significantly in gene expression pattern, biology and clinical outcome.

However, molecular diagnostics using gene expression profiling is not practical for most laboratory. Therefore, an immunohistochemistry (IHC) algorithm, known as “Hans Criteria” was developed as a surrogate predictor of cell-of-origin classification.

The new WHO Classification requires the identification of these two molecular subtypes and currently accepts Hans algorithm as a predictor, despite its imperfect reproducibility.

A recently developed parsimonious digital gene expression–based test that can be performed on formalin-fixed paraffin-embedded tissue (FFPET) shows improved accuracy and is highly concordant (>95%) with gene expression profiling by microarray in predicting cell-of-origin groups. Therefore, this method holds great promises in clinical subclassification of DLBCLs.

3. Epstein-Barr virus (EBV)-positive mucocutaneous ulcer is a new provisional diagnosis, biologically and clinical different from EBV-positive DLBCL

Willemze R, Cerroni L, Kempf W, et al. The 2018 update of the WHO-EORTC classification for primary cutaneous lymphomas. Blood. 2019;133(16):1703-1714.

A new provisional entity in the current WHO Classification is “Epstein-Barr virus (EBV)-positive mucocutaneous ulcer”.

It is separated from EBV-positive DLBCL because of its distinct biological behavior and clinical outcome.

Unlike the highly aggressive EBV-positive DLBCL, EBV-positive mucocutaneous ulcer is less aggressive with limited proliferation potential and shows excellent response to conservative treatment. It exhibits several characteristic clinical features. For example, it is a solitary and sharply demarcated lesion with ulceration, and can involve the skin, oropharyngeal mucosa and GI tract, often in immunosuppressed patients.

Histologically, it resembles Hodgkin lymphoma with atypical Hodgkin-like cells on a mixed inflammatory background. The atypical cells are positive for PAX5, CD20 (variable expression), CD30 and MUM1; and negative for CD10, BCL6. CD15 is expressed in 50% of cases.

In situ hybridization study for EBV RNA transcript (EBER) is characteristically positive. Clinical information is often required to make this diagnosis. In patients on immunosuppressive therapy, dose reduction may result in complete remission, and should be attempted before aggressive therapeutic approaches. 

4. EBV-positive DLBCL, NOS is a new diagnostic entity that seems to encompass two distinct diseases based on patient’s age

Nicolae A, Pittaluga S, Abdullah S, et al. EBV-positive large B-cell lymphomas in young patients: a  nodal lymphoma with evidence for a tolerogenic immune environment. Blood. 2015;126(7):863-72.

EBV-positive diffuse large B-cell lymphomas, not otherwise specified (EBV+ DLBCL, NOS) is a new diagnostic entity in the current WHO Classification that replaces the obsolete term “EBV+ DLBCL of the Elderly,” because a subset of these cases occur in young, immunocompetent patients.

EBV+ DLBCL, NOS most commonly arises in “elderly” (> 50 years of age) patients as a result of aging-related defective immune surveillance for EBV infection, and follows a very aggressive clinical course.

In comparison, EBV+ DLBCL NOS in young (< 45 years of age) patients arises from an immunocompetent background, shows predominant nodal disease and has better prognosis with higher overall survival.

The morphology of EBV+ DLBCL, NOS is diverse, with “T-cell/histiocyte-rich large B-cell lymphoma-like” being most common (78% of cases) in young patients.  In situ hybridization study for EBV RNA transcript (EBER) is characteristically positive.

5. A complex and evolving entity that occupies the gray zone between DLBCL and Hodgkin disease

Pilichowska M, Pittaluga S, Ferry JA, et al. Clinicopathologic consensus study of gray zone lymphoma with features intermediate between DLBCL and classical HL. Blood Adv. 2017;1(26):2600-2609.

“B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and classical Hodgkin lymphoma” or “gray-zone lymphoma (GZL)”, as its name implies, is a very confusing diagnostic entity that lingers in the new WHO Classification system.

It is mentioned here because it is very challenging to diagnose and has clinical significant implications.

Of 68 cases of GZL initially diagnosed at academic centers in North America, only 23 (38%) were confirmed as GZL by centralized expert consensus review.

More than 60% of cases is re-classified as cHL, nodular lymphocyte predominant HL (NLPHL), DLBCL and primary mediastinal large B cell lymphoma.

The diagnostic challenge is attributed to the diverse, often non-specific, morphologic features. Intratumoral heterogeneity can also occur. Nonetheless, the most important morphologic feature of GZL is the abundance of tumor cells with confluent areas and relatively few mixed inflammatory cells.

The neoplastic cells in GZL show greater pleomorphism and infrequent eosinophilic nucleoli. Similarly, the immunophenotype of GZL is also variable and divergent from morphology. For example, tumours with cHL-like morphology exhibits DLBCL immunophenotype and vice versa.

Most GZL with cHL-like morphology show expression of CD20, CD30 (weak) and MUM1, and lack CD15. In contrast, GZL with large cell-like morphology are strongly positive for CD30 and CD15, and lack CD20 and CD79a expression.

However, all GZL are of B-cell derivation with expression of at least one B cell marker.

It is suggested that a minimal panel including CD20, PAX5, MUM1, CD30 CD15 and EBER should be performed.

A diagnosis of GZL should not be made in core biopsy, FNA or bone marrow biopsy samples.